essay on urine formation

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Essay on Kidneys: Functions, Urine Formation and Hormones

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In this article we will discuss about the kidneys:- 1. Introduction to Kidney 2. Functions of Kidney 3. Urine Formation 4. Mechanism of Action of Diuretics 5. Renal Function Tests 6. Congenital Tubular Function Defects 7. Uremia 8. The Artificial Kidney 9. Hormones.

• Essay on the Hormones of the Kidney

Essay # 1. Introduction to Kidney:

A large number of waste products are produced in the body as a result of metabolic activities. The main waste products are carbon dioxide, water, and nitrogenous compounds. The retention of these products produces a harmful effect on the normal health.

Therefore, the removal of these products from the body is a must. Carbon dioxide is removed mainly through lungs and water as well as nitrog­enous compounds are removed through urogenital system. The kidneys are the most important com­ponent of this system.

The kidneys are two in number, usually bean shaped, and exist behind the peritoneum on either side of the vertebral column extending from the 12th thoracic to the 3rd lumbar vertebra. Each kid­ney weighs about 120-170 grams and is about 11-13 cms. long, the left being larger than the right one.

Each kidney is found to consist of two main parts by section. The outer part is called cortex and the inner one is medulla. The cortex consists of a large number of glomeruli and convoluted tubules. The medulla is composed of renal tubules project­ing into a cavity towards the inner region of the kidney called the pelvis, the region where the renal artery and vein enters and leaves the kidney re­spectively.

Nephron –Basic Unit od Kidney:

It is a functional basic unit of kid­ney. Each kidney is provided with about one mil­lion nephrons containing the glomerulus and the tubule. The glomerulus is a network of afferent and efferent capillaries.

Each glomerulus is surrounded by a double-walled epithelial sac known as Bow­man ‘s Capsule which leads to the tubule which is divided into three parts—proximal convoluted tu­bule, loop of Henle, and the distal convoluted tubule.

The Proximal Convoluted Tubule (PCT) is about 45 mm long and 50 mm in diameter. This lies in the cortex along with glomerulus. Its lumen is continuous with that of the Bowman’s Capsule. It consists of cells with scalloped outline and brush border. The brush border is formed by numerous microvilli which increases the surface enormously for absorption.

The loop of Henle consists of three parts—the descending limb, a thin segment, and an ascending limb. The proximal convoluted tubule opens into the descending limb which is continued into the thin segment from where the ascending limb arises. The whole loop of Henle is lined by a single layer of flattened epithelial cells.

The ascending limb of the loop of Henle con­tinues into the distal convoluted tubule (DCT) which finally opens into a collecting tubule or duct which carries the urine to the renal pelvis from where it is carried to the bladder by the ureter.

The distal convoluted tubule commences near the pole of the glomerulus and establishes a close proxim­ity to the afferent arteriole of its parent glomerulus. The DCT contains cuboidal epithelium.

Nephrons are mainly of two types—cortical and juxtamedullary. The loop of Henle of the juxtamedullary is long and dips deep into the sub­stance of the medulla. But the loop of Henle of cortical is short and only a very small part of it dips into the medullary tissue and the greater part re­mains embedded in the cortical substances.

Moreo­ver, the glomeruli of the juxtamedullary lie very close to the medulla while those of cortical lie close to the surface of the kidney. The juxtamedullary nephrons constitute 20 per cent of nephrons, while the cortical nephrons constitute 80 per cent of the total nephrons. These two types of nephrons have the same common function.

Blood Supply of the Kidneys:

The short renal artery arising from the abdominal aorta supplies the blood to the kidney. The renal artery after en­tering the kidney divides into a number of arterioles—the afferent arterioles which further branch into capillaries and enter into each glomeru­lus.

The capillaries then join to form another arteri­ole—the efferent arteriole which opens into another set of capillaries called peritubular capillaries sur­rounding the proximal tubule, the loop of Henle, and the distal tubule. Ultimately, the capillary set opens into a venule which joins with other venules to form the renal vein. The renal vein then opens into the inferior vena cava.

Blood Flow to Kidney through the Nephron:

The blood flows through both the kidneys of an adult weigh­ing 70 kg at the rate of about 1200 ml/mt. The portion of the total cardiac output (about 560 ml/ mt.) which passes through the kidneys is called the renal fraction. This is about 560/1200 ml per minute, i.e., about 21 per cent.

There are two sets of capillaries—the glomeru­lus and the peritubular. These two capillaries are separated from each other by the efferent arteriole which contributes sufficient resistance to blood flow. The glomerular capillary bed provides a high pressure of about 70 mm Hg, while the peritubular bed provides a low pressure about 13 mm Hg.

The pressures in the artery and vein are 100 mm of Hg. and 8 mm of Hg respectively. The high pressure in the glomerulus exerts the filtering of fluids con­tinually into the Bowman’s Capsule. The low pres­sure in the peritubular capillary system, on the other hand, functions in the same way as the usual ve­nous ends of the tissue capillaries with the fluid being absorbed continually into the capillaries.

Essay # 2. Functions of Kidney :

a. Kidney eliminates excess of certain nutri­ents such as sugar and amino acids when their concentration increases in the blood.

b. It removes certain non-volatile waste prod­ucts such as urea, uric acid, creatinine, and sulphates, etc. from the body.

c. It eliminates certain foreign or toxic sub­stances such as iodides, pigments, drugs, and bacteria, etc. from the blood.

d. It regulates hydrogen ion concentration of the blood by removing excess of non­volatile acids and bases.

e. It maintains the osmotic pressure of the blood by regulating the excretion of wa­ter and inorganic salts and thus preserves the constant volume of the circulating blood.

f. It regulates the arterial blood pressure by causing the secretion of the hormone renin.

g. It maintains the erythrocyte production by excreting the secretion of the hormone erythropoietin.

Essay # 3. Urine Formation in Kidney :

The regulatory activities of kidneys form urine as a by-product. Urine formation involves three main steps—the glomerular filtration, the tubular reabsorption, and the tubular secretion.

a. Glomerular Filtration (Ultrafiltration):

Glomerulus filters out substances of low molecular weight from the blood with the retention of substances of high molecular weight, especially the proteins. Therefore, proteins are retained in the glomeruli and are not normally found in urine. If protein is detected in the urine, it indicates the kidney damage or other disease which ef­fect the glomerular membrane.

In normal adult, two million nephrons filter one li­tre of blood each minute to give about 1200 ml of glomerular filtrate (primary urine) at Bowman’s Capsule. Therefore, the Glomerular Filtration Rate (GFR) in adult is about 120 ml per minute. The hydrostatic pressure of the blood in the glomerular capillaires (P g ) is the main force for driving the fluid (Water and sol­ute) out of the glomerulus.

The pressure is opposed by two forces:

(i) The hydrostatic pressure of the Bow­man’s Capsule fluid (P BC ).

(ii) The osmotic pressure of the plasma proteins (P pp ).

Therefore, the effective filtration pressure (P ef ) is calculated by the following rela­tion:

P ef = P g – (P PP + P BC )

. . . P ef = 74 – (30 + 20) mm of Hg

. . . P ef = 24 mm of Hg.

Thus, by substituting the normal values of the various forces, it has been found that the calculated effective (net) filtra­tion pressure (P ef ) is 24 mm Hg.

A fall in blood pressure may reduce the P ef which results in less amount of urine. When the aortic systolic pressure is re­duced to 70 mm Hg, the hydrostatic pres­sure of the blood in glomerular capillaries is reduced to 50 mm. Hg. This reduces the P ef to Zero [50 – 50] and thus filtration will be ceased. Under such circumstances, urine will not be formed (anuria) until the blood pressure is maintained.

b. Tubular Reabsorption:

The rate of forma­tion of the primary urine is 120 ml/minute, while the rate of urine passing to the blad­der under the same condition is 1-2 ml/ minute. Therefore, it indicates that about 99 per cent of the glomerular filtrate is reabsorbed during its passage through the different segments of the renal tubule.

Al­though, the glomerular filtrate contains nearly the same concentration of glucose as in plasma, the urine contains nil or very little glucose. Hence, glucose is also prac­tically completely reabsorbed in the tu­bules when the blood sugar level is nor­mal. The capacity of reabsorption depends on the renal threshold of that substance.

The reabsorption of different solids takes place at different sites in the renal tubules. Amino acids, glucose, and small amounts of protein that pass through the glomeru­lus are reabsorbed in the first part of the proximal tubule.

Sodium, chloride, and bi­carbonate are reabsorbed uniformly along the entire length of the proximal tubule and also in the distal tubule. Potassium is reabsorbed in the proximal and secreted in the distal tubule.

The glomerular filtrate produces about 170 litres in a day; whereas the tubules reabsorb about 168.5 litres of water, 170 gm of glucose, 100 gm of NaCl, 360 gm of NaHCO 3 , and small amounts of phosphate, sulphate, amino acids, urea, uric acid, etc. and excrete about 60 gm of NaCl, urea and other waste products in about 1.5 li­tres of urine. Most of these solids are reabsorbed by active transport mechanism, while some (e.g., urea) are reabsorbed by passive transport mechanism.

In diseases, the reabsorption mechanism is altered developing glycosuria, phosphaturia, and amino aciduria.

c. Tubular Secretion:

Although, most of the substances are reabsorbed by the tubular cells, some substances are actively trans­ported or actively excreted into the tubu­lar lumen. The secreted substance by the tubular epithelium in man are creatinine and potassium. The tubular epithelium also removes a number of foreign sub­stances that are introduced into the body for therapeutic and diagnostic purposes.

These foreign substances are penicillin, p-Aminosalicylic acid, phenosulphonphthalein (PSP), p-Aminohippuric acid, and diodrast. The hydrogen ions and ammo­nia formed in the distal tubular cells are also actively excreted into tubular lumen and thus pass to urine.

Hormonal regulation:

The function of kidney is regulated by three important hormones. These hormones are aldoster­one (from adrenal cortex), parathormone (from parathyroid), and vasopressin (from hypophyseal posterior lobe).

Aldosterone restricts the excretion of Na + and stimulates the excretion of K + . Parathormone stimulates excretion of phosphate. Vasopressin, the antidiuretic hormone, is held responsible mainly for the reabsorption of water. In the absence of this hormone, a large amount of very dilute urine is excreted.

Essay # 4. Mechanism of Action of Diuretics :

a. Diuretics, the drugs, enhance losses of water and salt via the urine through inter­ference with normal reabsorptive mecha­nisms.

b. Osmotic diuretics are nonreabsorbable substances which increase tubular osmolarity. The osmotic substances which limit the amount of water. Osmotic diuresis is responsible for the serious dehydration which accompanies diabetic ketoacidosis.

c. Diamox is the inhibitor of carbonic anhydrase. It blocks both HCO 3 − reabsorption in the proximal tubule and regeneration in the distal tubule.

d. Thiazide diuretics, furosemide, ethacrynic acid and mercurials all inhibit chloride rea­bsorption in the ascending limb.

Essay # 5. Renal Function Tests :

Clearance is measured to assess quantitatively the rate of excretion of a given substance by the kid­ney. This is a volume of blood or plasma which contains the amount of the substance which is ex­creted in the urine in one minute.

A. Inulin Clearance :

a. Inulin is a polysaccharide which is filtered at the glomerulus but not secreted or reabsorbed by the tubule. Therefore, it is a measure of glomerular filtration rate. Mannitol can also be used for the same purpose.

b. These clearances vary with the body size. The clearance is calculated on the basis of ml/1.73 m 2 .

c. To measure inulin clearance it is wise to maintain a constant plasma level of the test substance during the period of urine collections.

The clearance is measured ac­cording to the following formula:

C in = U x V/P

where C in = Clearance of inulin (ml/min)

U = Urinary inulin (mg/100 ml)

V = Volume of urine (ml/min)

P = Plasma inulin (mg/100 ml)

B. Endogenous Creatinine Clearance :

a. Creatinine is filtered at the glomerulus but not secreted or reabsorbed by the tubule. Its clearance is measured to get the GFR.

b. This method is convenient for the estima­tion of the GFR because it does not re­quire the intravenous administration of a test substance.

c. Normal values for creatinine clearance are in males: 130 ± 20 ml/mt and females: 120 ± 15 ml/mt.

C. The Phenolsulphonephthalein (PSP) Test :

a. The dye is almost completely eliminated within 2 hours.

b. If less than 25 per cent of the dye is not excreted in 15 minutes, it is an indication of impairment of renal function.

D. Other Functional Tests :

a. Dilution test (water excretion test)

b. Urine concentration test (specific gravity test)

c. Vasopressin (ADH) test

d. Urine acidification test

e. Blood NPN, urea and creatinine

f. Urea clearance test.

a. Dilution test :

(i) After emptying the bladder of the indi­vidual after overnight fast, he is advised to drink 1200 ml water in 30 minutes.

(ii) During four hours after drinking, the urine is collected at hourly intervals.

(iii) In normal individuals in cold climates, 1200 ml of urine is excreted in four hours.

(iv) This test is not applicable to warm climates since the greater part of the ingested water is lost in perspiration during summer.

(v) In case of impaired renal function, the amount of water eliminated in four hours will be less than 1200 ml depending on the degree of impairment and specific grav­ity of urine is often 1.010 or higher in con­ditions of oliguria.

b. Urine concentration test (specific gravity test):

(i) The bladder is emptied on the day of the test at 7 a.m. and the urine is discarded.

(ii) The urine is collected at 8 a.m. and the specific gravity is measured. If the sp. gr. is 1.022, the test may be rejected.

(iii) If the sp. gr, is below 1.022, another urine specimen should be collected at 9 a.m. and the sp. gr. is determined.

(iv) In case, the urine does not have a sp. gr. of 1.022, it is sure that the renal concentrat­ing power is impaired either due to tubu­lar defects or decreased secretion of ADH (diabetes insipidus). If the urine volume is large and the sp. gr. is below 1.022, the ADH test must be carried out. 3.

c. Vasopressin (ADH) test :

(i) The individual is not allowed any food or water after 6 p.m. on the night before the test. Vasopressin (5 units) is injected intramuscularly at 7 p.m. in the night.

(ii) The urine is collected at 7 a.m. and 8 a.m. and the sp. gr. is determined. If the sp. gr. is 1.022, it is quite confident that the indi­vidual suffers from diabetes insipidus and ADH injection is effective in controlling it.

d. Urine acidification test :

(i) This test should not be done on individu­als who have acidosis or poor liver func­tion.

(ii) No dietary or other restrictions are in­volved in carrying out this test. The blad­der is emptied at 8 a.m. Thereafter, hourly specimens of urine are collected until 6 p.m. At 10 a.m., ammonium chloride in a dose of 0.1 gram/kg body weight is given. A portion of each specimen is transferred to stoppered bottles and sent immediately to the laboratory for pH determination.

(iii) In normal individuals, all urine specimens collected after 2 hours from the time of administration of ammonium chloride should have a pH between 4.6 and 5.0 but in patients with renal tubular acidosis, the pH does not fall below 5.3.

v. Blood non-protein nitrogen :

(i) In acute nephritis, the NPN values are in­creased and range from a slight increase (NPN-45 mg, urea N-25 mg, creatinine-2 mg per 100 ml) to very high values (NPN- 200 mg, urea N-160 mg creatinine-25 mg per 100 ml).

(ii) NPN increase and retention are due to im­paired renal function and excessive pro­tein catabolism.

Essay # 6. Congenital Tubular Function Defects in Kidney :

a. Diabetes Insipidus :

(i) This disease is developed due to the non- production of ADHr. The individual passes large volume of urine (5-20 litres in 24 hours). The individual has to drink large amount of water to make up the loss.

(ii) The reabsorption of water in the distal tu­bules does not take place in the absence of ADH.

b. Vitamin D Resistant Rickets :

(i) The tubular reabsorption of phosphate does not take place under this condition.

(ii) Excessive loss of phosphate in urine leads to the development of a type of rickets which does not respond to usual doses of Vitamin D.

c. Renal Glycosuria :

In this condition, the tubular reabsorption of glu­cose is affected. Although the blood sugar is within normal level but glucose is excreted in urine due to defective reabsorption by the tubules.

d. Idiopathic Hypercalcinuria :

Calcium is not reabsorbed by the renal tubules in this condition. Hence, large amounts of calcium are excreted in the urine. Renal calculi may be de­veloped owing to the presence of large amounts of calcium in urine.

e. Salt losing Nephritis :

(i) Large amounts of sodium and chloride ions are excreted in urine in this condi­tion due to the defect in the tubular reab­sorption of these ions resulting in severe dehydration, hyponatremia and hypo-chloremia.

(ii) Blood urea is increased due to the reduced glomerular filtration rate.

(iii) This condition does not respond to aldos­terone administration but responds to parenteral administration of sodium chlo­ride solution.

f. Renal Tubular Acidosis :

(i) In this condition, the urine becomes alka­line or neutral due to the defect in the so­dium and hydrogen ion exchange mecha­nism in the distal tubules. There is a loss of sodium in the urine.

(ii) The acidosis is accompanied by excessive mobilization and urinary excretion of cal­cium and potassium.

(iii) These abnormalities led to clinical mani­festation of dehydration, hypokalemia, defective mineralisation of bones and nephrocalcinosis.

g. Fanconi Syndrome:

(i) A number of defects in tubular reabsorp­tion exist in this condition. The defects are renal amino acid in renal glycosuria, hyperphosphaturia, metabolic aciduria, with increased urinary excretion of Na, Ca and K.

(ii) In some individuals, cystinosis prevails due to the abnormality of cystine metabo­lism in which cystine crystals are depos­ited in macrophages in the liver, kidney, spleen, bone marrow, lymph nodes and cornea.

h. Hartnup Syndrome (Hard Syndrome) :

(i) In this condition, a number of amino ac­ids are not reabsorbed owing to the defect in tubular reabsorption mechanism.

(ii) Disturbances in tryptophan metabolism is suggested by the presence of increased amounts of tryptophan, indican and in­dole acetic acid in urine.

(iii) The clinical symptoms are of niacin defi­ciency—a pellagra like skin lesions and mental deficiency.

i. Nephrogenic Diabetes Insipidus (Water-Losing Nephritis) :

This condition is due to congenital defect in water reabsorption in the distal tubules and may, there­fore, resemble true diabetes insipidus.

Essay # 7. Uremia –Clinical Kidney Condition :

The renal failure develops the clinical condition uremia. This condition occurs both in the chronic renal failure and acute failure. The concentration of urea and other NPN constituents in plasma are increased depending on the severity of this condi­tion.

In chronic renal disease, excretion of acid (hy­drogen ion) and also of phosphate ion is impaired. This results in the steady development of acidosis in uremia.

In acute renal failure, the urine output is very low (300 ml or less in 24 hours). This leads to a steady increase in urea and NPN constituents and electrolytes (K + and Na + ) in plasma. There is rapid development of acidosis too.

The important findings of severe chronic uremia or acute uremia are:

a. High concentration of urea and other NPN constituents.

b. High serum potassium concentration.

c. – Water retention leading to generalised edema.

d. Acidosis.

Uremic coma occurs in serious cases :

i. Urea and NPN:

The concentration of urea and other NPN constituents of blood are very much increased (i.e., 10 times the nor­mal level) in severe renal failure.

ii. Potassium:

The potassium ion level may be slightly increased in chronic uremia. But in acute uremia, the concentration in serum is very much increased. Potassium is released from the cells due to the break­down of cellular proteins. This released potassium passes into the blood and in­terstitial fluid.

When the concentration of potassium ion increases to 8 m. Eq/litre, it exerts a cardiotoxic effect resulting in the dilatation of the heart and when potassium ion concentration reaches at 12 to 15 mEq/ litre, the heart is likely to be stopped. This happens in severe uremia.

iii. Water Retention and Edema:

If the uremic patient drinks water and consumes other fluids, the water is retained in the body. If salt is not consumed, water retention in­creases in both the intracellular and extra­cellular fluid resulting in extracellular edema.

iv. Acidosis:

The metabolic processes in the body produce daily 50 to 100 m mol of more metabolic acid than alkali. This ex­tra metabolic acid is excreted mainly through the kidneys. Acidosis develops rapidly in acute uremia. The patient faces ‘Coma’ due to severe acidosis.

Essay # 8. The Artificial Kidney :

During recent years, the artificial kidney has been developed to such an extent that several thousand patients with permanent renal insufficiency or even total kidney removal are being maintained in health for years.

The artificial kidney passes blood through very minute channels bounded by thin membranes. There is a dialyzing fluid on the other side of the membrane into which unwanted substances present in the blood pass by diffusion. The blood is pumped continually between two thin sheets of cellophane; the dialyzing fluid is on the outside of the sheets.

The cellophane is porous enough to allow all con­stituents of the plasma except the plasma proteins to diffuse freely in both directions—from plasma into the dialyzing fluid and from the dialyzing fluid into the plasma.

The rate of flow of blood through the artificial kidney is several hundred ml per minute. Heparin is infused into the blood as it enters the kidney to prevent clotting of blood. To prevent bleeding as a result of heparin, an anti-heparin substance, such as protamine, is infused into the blood as it is re­turned to the patient.

The Dialyzing Fluid:

Sodium, potassium and chloride concentrations in the dialyzing fluid and in normal plasma are identical; but in uremic plasma, the potassium and chloride concentrations are considerably greater. These two ions diffuse through the dialyzing membrane so rapidly that their concentrations fall to equal those in the dialyzing fluid within three to four hours, expo­sure to the dialyzing fluid.

On the other hand, there is no phosphate, urea, urate or creatinine in the dialyzing fluid.

When the uremic patient is dialyzed, these substances are lost in large quanti­ties into the dialyzing fluid, thereby removing major proportions of them from the plasma. Thus, the constituents of the dialyzing fluid are such that those substances in excess in the extracellular fluid in uremia be removed at rapid rates, while the es­sential electrolytes remain quite normal.

Utility of Artificial Kidney:

The artificial kid­neys can clear 100 to 200 ml of blood urea per minute which signifies that it can function about twice as rapidly as two normal kidneys together whose urea clearance is only 70 ml per minute. However, the artificial kidney can be used for not more than 12 hours once in three to four days be­cause of danger from excess heparin and infection to the subject.

Essay # 9. Hormones of the Kidney :

a. Not only the kidney performs excretory functions but it acts as an endocrine or­gan. It liberates many hormones which affect other organs and tissues and some hormones which locally act within the kid­ney itself. It also destroys several hor­mones which are liberated from other en­docrine organs.

b. The juxtaglomerular cells of the renal cor­tex produce the proteolytic enzyme rennin and secrete it into the blood. Rennin acts on a 2 -globulin which is normally present in blood plasma, although it is pro­duced in the liver.

Rennin splits off a polypeptide fragment called angiotensin I which is decapeptide containing 10 amino acids. Another enzyme of the lung acts on angiotensin I to split off 2 amino acids and thus form the octapeptide angiotensin II.

Angiotensin increases the force of the heartbeat and constricts the arterioles. It raises blood pressure and causes contrac­tion of smooth muscle. It is destroyed by the enzyme angiotensinases present in normal kidneys, plasma and other tissues. Recent studies suggest that rennin angi­otensin system is important in the mainte­nance of normal blood pressure.

c. Prostaglandins are the other hormones of the kidney. They cause relaxation of smooth muscles. They cause vasodilata­tion and a decrease in blood pressure. They also increase renal blood flow. Kininogen which is produced by the kidney has an antihypertensive effect.

d. The two hormones erythropoietin and erythrogenin have an effect on bone mar­row to stimulate production of red cells. Kidney plays an important role in the re­lease of erythropoietin and thus in con­trol of red cell production. Hypoxia stimu­lates production of erythropoietin.

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• Tissue Fluid: Formation and Functions | Plasma | Blood | Biology
• Formation of Urine in Human Body: 3 Processes

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Physiology of urine formation

May 6, 2017 Gaurab Karki Anatomy and Physiology , Excretion and Osmoregulation , Zoology 0

Physiology of Urine formation

There are three stages involved in the process of urine  formation. They are- 1. Glomerular filtration or ultra-filtration

2. Selective reabsorption

3. Tubular secretion

Glomerular filtration

• This takes place through the semipermeable walls of the glomerular capillaries and Bowman’s capsule.
• The afferent arterioles supplying blood to glomerular capsule carries useful as well as harmful substances. The useful substances are glucose, aminoacids, vitamins, hormones, electrolytes, ions etc and the harmful substances are metabolic wastes such as urea, uric acids, creatinine, ions, etc.
• The diameter of efferent arterioles is narrower than afferent arterioles. Due to this difference in diameter of arteries, blood leaving the glomerulus creates the pressure known as hydrostatic pressure.
• The glomerular hydrostatic pressure forces the blood to leaves the glomerulus resulting in filtration of blood. A capillary hydrostatic pressure of about 7.3 kPa (55 mmHg) builds up in the glomerulus. However this pressure is opposed by the osmotic pressure of the blood, provided mainly by plasma proteins, about 4 kPa (30 mmHg), and by filtrate hydrostatic pressure of about 2 kPa (15 mmHg in the glomerular capsule.
• The net filtration pressure is,

Therefore: 55-(30 +15) = 10mmHg.

• By the net filtration pressure of 10mmHg, blood is filtered in the glomerular capsule.
• Water and other small molecules readily pass through the filtration slits but Blood cells, plasma proteins and other large molecules are too large to filter through and therefore remain in the capillaries.
• The filtrate containing large amount of water, glucose, aminoacids, uric acid, urea, electrolytes etc in the glomerular capsule is known as nephric filtrate of glomerular filtrate.
• The volume of filtrate formed by both kidneys each minute is called the glomerular filtration rate (GFR). In a healthy adult the GFR is about 125 mL/min, i.e. 180 litres of filtrate are formed each day by the two kidneys

Selective reabsorption

• As the filtrate passes to the renal tubules, useful substances including some water, electrolytes and organic nutrients such as glucose, aminoacids, vitamins hormones etc are selectively reabsorbed from the filtrate back into the blood in the proximal convoluted tubule.
• Reabsorption of some substance is passive, while some substances are actively transported. Major portion of water is reabsorbed by Osmosis.
• Only 60–70% of filtrate reaches the Henle loop. Much of this, especially water, sodium and chloride, is reabsorbed in the loop, so that only 15–20% of the original filtrate reaches the distal convoluted tubule, More electrolytes are reabsorbed here, especially sodium, so the filtrate entering the collecting ducts is actually quite dilute.
• The main function of the collecting ducts is to reabsorb as much water as the body needs.
• Nutrients such as glucose, amino acids, and vitamins are reabsorbed by active transport. Positive charged ions ions are also reabsorbed by active transport while negative charged ions are reabsorbed most often by passive transport. Water is reabsorbed by osmosis, and small proteins are reabsorbed by pinocytosis.

Tubular secretion

• Tubular secretion takes place from the blood in the peritubular capillaries to the filtrate in the renal tubules and can ensure that wastes such as creatinine or excess H+ or excess K+ ions are actively secreted into the filtrate to be excreted.
• Excess K+ ion is secreted in the tubules and in exchange Na+ ion is reabsorbed otherwise it causes a clinical condition called Hyperkalemia.
• Tubular secretion of hydrogen ions (H+) is very important in maintaining normal blood pH.
• Substances such as , e.g. drugs including penicillin and aspirin, may not be entirely filtered out of the blood because of the short time it remains in the glomerulus. Such substances are cleared by secretion from the peritubular capillaries into the filtrate within the convoluted tubules.
• The tubular filtrate is finally known as urine. Human urine is usually hypertonic.

Composition of human urine

Water – 96%

Urea – 2%

Uric acids, creatinine, pigments- 0.3%

Inorganic salts – 2%

Bad smell is due to Urinoid

Pale yellow color due to urochrome or urobillin (which is a breakdown product of haemoglobin)

Micturation:

• The process of time to time collection and removal of urine from urinary bladder is known as micturition. Collection of more than 300ml of urine in urinary bladder creates pressure on the wall. The pressure stimulates the desire for urination.
• https://opentextbc.ca/anatomyandphysiology/chapter/25-5-physiology-of-urine-formation/
• https://en.wikibooks.org/wiki/Human_Physiology/The_Urinary_System
• https://legacy.owensboro.kctcs.edu/gcaplan/anat2/note/APIINotes3%20urinary%20system.htm
• https://www.visiblebody.com/learn/urinary/urine-creation
• http://study.com/academy/lesson/the-three-processes-of-urine-formation.html
• http://www.columbia.edu/itc/hs/medical/humandev/2005/HD13-4s.pdf
• https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/urinary-system-25/physiology-of-the-kidneys-240/overview-of-urine-formation-1171-2197/
• composition of human urine
• glomerular filtration
• glomerular hydrostatic pressure
• mechanism of urine production
• micturation
• physiology of urine formation
• selective reabsorption
• tubular secretion

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• Biology Article
• Urine Formation Osmoregulation

Urine Formation And Osmoregulation

Table of Contents

What is Excretion?

Urine formation, mechanism of urine formation, osmoregulation, key points on urine formation and osmoregulation.

Every one of us, including plants and animals, depends on the excretion process for the removal of certain waste products from our bodies. During the process of excretion, both the kidneys play an important role in filtering the blood cells.

Excretion is a biological process, which plays a vital role in eliminating toxins and other waste products from the body. In plants and animals, including humans, as part of metabolism, a lot of waste products are produced. Plants usually excrete through the process of transpiration and animals excrete the wastes in different forms such as urine, sweat, faeces and tears. Among all these, the usual and the main form of excretion is urine.

Waste is excreted from the human body, mainly in the form of urine. Our kidneys play a major role in the process of excretion. Constituents of normal human urine include 95 per cent water and 5 per cent solid wastes. It is produced in the nephron, which is the structural and functional unit of the kidney. Urine formation in our body is mainly carried out in three phases namely

• Glomerular filtration

Reabsorption

The mechanism of urine formation involves the following steps:

Glomerular Filteration

Glomerular filtration occurs in the glomerulus where blood is filtered. This process occurs across the three layers- the epithelium of Bowman’s capsule, the endothelium of glomerular blood vessels, and a membrane between these two layers.

Blood is filtered in such a way that all the constituents of the plasma reach the Bowman’s capsule, except proteins. Therefore, this process is known as ultrafiltration.

Around 99 per cent of the filtrate obtained is reabsorbed by the renal tubules. This is known as reabsorption. This is achieved by active and passive transport.

The next step in urine formation is tubular secretion. Here, tubular cells secrete substances like hydrogen ions, potassium ions, etc into the filtrate. Through this process, the ionic, acid-base and the balance of other body fluids are maintained. The secreted ions combine with the filtrate and form urine. The urine passes out of the nephron tubule into a collecting duct.

The urine produced is 95% water and 5% nitrogenous wastes. Wastes such as urea, ammonia, and creatinine are excreted in the urine. Apart from these, the potassium, sodium and calcium ions are also excreted.

Also Read:  Counter-current Mechansim

Osmoregulation is the process of regulating body fluids and their compositions. It maintains the osmotic pressure of the blood and helps in  homeostasis . This is why it is recommended to consume more water about 2-3 litres, which helps in the proper functioning of our kidneys. For example, we consume lots of water during summers, but still, we urinate fewer times in summers than in winters and the concentration of the urine is also more. The reason is that we lose lots of water from our body in summer through sweating. Thus, to maintain the fluid balance in the body our kidneys reabsorb more water.

• Urine is formed in three main steps- glomerular filtration, reabsorption and secretion.
• It comprises 95 % water and 5% wastes such as ions of sodium, potassium and calcium, and nitrogenous wastes such as creatinine, urea and ammonia.
• Osmoregulation is the process of maintaining homeostasis of the body.
• It facilitates the diffusion of solutes and water across the semi-permeable membrane thereby maintaining osmotic balance.
• The kidney regulates the osmotic pressure of blood through filtration and purification by a process known as osmoregulation.

Also Read:  Osmoregulation

Frequently Asked Questions

Define excretion., what is the difference between egestion and excretion.

Excretion is the removal of toxic materials, waste products of metabolism and excess substances from organisms.

Egestion is the passing out of undigested food as faeces, through the anus.

What are the 3 major processes involved in Urine Formation?

The 3 processes are:

a) Glomerular Filteration b) Tubular Reabsorption c) Tubular Secretion

Define Osmoregulation.

Define ultrafiltration..

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Chapter 14: The Urinary System and The Reproductive System

Physiology of urine formation, learning objectives.

By the end of this section, you will be able to:

• Describe the hydrostatic and colloid osmotic forces that favor and oppose filtration
• Describe glomerular filtration rate (GFR), state the average value of GFR, and explain how clearance rate can be used to measure GFR
• Predict specific factors that will increase or decrease GFR
• State the percent of the filtrate that is normally reabsorbed and explain why the process of reabsorption is so important
• Calculate daily urine production
• List common symptoms of kidney failure

Having reviewed the anatomy and microanatomy of the urinary system, now is the time to focus on the physiology. You will discover that different parts of the nephron utilize specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.

Failure of the renal anatomy and/or physiology can lead suddenly or gradually to renal failure. In this event, a number of symptoms, signs, or laboratory findings point to the diagnosis.

Urine Formation

Urine Formation – by filtering the blood the nephrons perform the following functions

(1) regulate concentration of solutes in blood plasma; this also regulates pH

(2) regulate water concentrations; this helps regulate blood pressure

(3) removes metabolic wastes and excess substances

• Urine Formation:
• Glomerular Filtration – water and solutes are forced through the capillary walls of the glomerulus into the Bowman’s capsule (glomerular capsule)
• Filtrate – the fluid that is filtered out into bowman’s capsule

Glomerular Filtration Rate is regulated by mechanisms:

• Autoregulation – the smooth muscle in the afferent arteriole responds to blood pressure changes by constricting and dilating to regulate filtration rate.
• Sympathetic control – causes afferent arterioles to constrict or dilate when activated by a nerve impulse (fight or flight response to keep blood pressure up)

Renin-angiotensin mechanism – triggered by the juxtaglomerular apparatus; when filtration rate decreases, the enzyme renin is released. Renin converts a plasma protein called angiotensinogen into angiotensin I. Angiotensin I is quickly converted into angiotensin II by another enzyme. Angiotensin II causes 3 changes:

• (1) Constriction of the arterioles – decreases urine formation and water loss
• (2) Stimulates the adrenal cortex to release aldosterone – promotes water reabsorption by causing the absorption of salt
• (3) Stimulates the posterior pituitary to release ADH – antidiuretic hormone – promotes water reabsorption
• (4) Stimulates the thirst and water intake (hypothalamus says we’re thirsty so we get a drink)

Tubular Reabsorption – occurs both passive and actively; glucose, amino acids, and other needed ions (Na, K, Cl, Ca, HCO3) are transported out of the filtrate into the peritubular capillaries (they are reabsorbed back into the blood); about 65% of the filtrate is reabsorbed in the proximal convoluted tubule.

• As these substances are reabsorbed, the blood becomes hypertonic so water easily follows by osmosis
• Reabsorption in the distal convoluted tubule is under hormonal control…aldosterone causes more salt to be absorbed, ADH causes more water to be absorbed

Secretion – waste products such as urea and uric acid, drugs and hydrogen and bicarbonate ions are move out of the peritubular capillaries into the filtrate; this removes unwanted wastes and helps regulate pH

• Urine – filtrate after it has passed through the nephron and undergone filtration, reabsorption, and secretion. The urine passes into the collecting duct, which joins with the minor calyx, major calyx, and eventually the renal pelvis. The renal pelvis joins with the ureter.
• Deep yellow to orange – more concentrated, less water
• Light yellow to clear – less concentrated, more water

Glomerular Filtration Rate (GFR)

The volume of filtrate formed by both kidneys per minute is termed the glomerular filtration rate (GFR) . The heart pumps about 5 L blood per min under resting conditions. Approximately 20 percent or one liter enters the kidneys to be filtered. On average, this liter results in the production of about 125 mL/min filtrate produced in men (range of 90 to 140 mL/min) and 105 mL/min filtrate produced in women (range of 80 to 125 mL/min). This amount equates to a volume of about 180 L/day in men and 150 L/day in women. Ninety-nine percent of this filtrate is returned to the circulation by reabsorption so that only about 1–2 liters of urine are produced per day.

GFR is influenced by the hydrostatic pressure and colloid osmotic pressure on either side of the capillary membrane of the glomerulus. Recall that filtration occurs as pressure forces fluid and solutes through a semipermeable barrier with the solute movement constrained by particle size. Hydrostatic pressure is the pressure produced by a fluid against a surface. If you have a fluid on both sides of a barrier, both fluids exert a pressure in opposing directions. Net fluid movement will be in the direction of the lower pressure. Osmosis is the movement of solvent (water) across a membrane that is impermeable to a solute in the solution. This creates a pressure, osmotic pressure, which will exist until the solute concentration is the same on both sides of a semipermeable membrane. As long as the concentration differs, water will move. Glomerular filtration occurs when glomerular hydrostatic pressure exceeds the luminal hydrostatic pressure of Bowman’s capsule. There is also an opposing force, the osmotic pressure, which is typically higher in the glomerular capillary.

Figure 1. The NFP is the sum of osmotic and hydrostatic pressures.

To understand why this is so, look more closely at the microenvironment on either side of the filtration membrane. You will find osmotic pressure exerted by the solutes inside the lumen of the capillary as well as inside of Bowman’s capsule. Since the filtration membrane limits the size of particles crossing the membrane, the osmotic pressure inside the glomerular capillary is higher than the osmotic pressure in Bowman’s capsule. Recall that cells and the medium-to-large proteins cannot pass between the podocyte processes or through the fenestrations of the capillary endothelial cells. This means that red and white blood cells, platelets, albumins, and other proteins too large to pass through the filter remain in the capillary, creating an average colloid osmotic pressure of 30 mm Hg within the capillary. The absence of proteins in Bowman’s space (the lumen within Bowman’s capsule) results in an osmotic pressure near zero. Thus, the only pressure moving fluid across the capillary wall into the lumen of Bowman’s space is hydrostatic pressure. Hydrostatic (fluid) pressure is sufficient to push water through the membrane despite the osmotic pressure working against it. The sum of all of the influences, both osmotic and hydrostatic, results in a net filtration pressure (NFP) of about 10 mm Hg.

A proper concentration of solutes in the blood is important in maintaining osmotic pressure both in the glomerulus and systemically. There are disorders in which too much protein passes through the filtration slits into the kidney filtrate. This excess protein in the filtrate leads to a deficiency of circulating plasma proteins. In turn, the presence of protein in the urine increases its osmolarity; this holds more water in the filtrate and results in an increase in urine volume. Because there is less circulating protein, principally albumin, the osmotic pressure of the blood falls. Less osmotic pressure pulling water into the capillaries tips the balance towards hydrostatic pressure, which tends to push it out of the capillaries. The net effect is that water is lost from the circulation to interstitial tissues and cells. This “plumps up” the tissues and cells, a condition termed systemic edema .

Net Filtration Pressure (NFP)

NFP determines filtration rates through the kidney. It is determined as follows:

NFP = Glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)] = 10 mm Hg

NFP = GBHP – [CHP + BCOP] = 10 mm Hg

NFP = 55 – [15 + 30] = 10 mm Hg

As you can see, there is a low net pressure across the filtration membrane. Intuitively, you should realize that minor changes in osmolarity of the blood or changes in capillary blood pressure result in major changes in the amount of filtrate formed at any given point in time. The kidney is able to cope with a wide range of blood pressures. In large part, this is due to the autoregulatory nature of smooth muscle. When you stretch it, it contracts. Thus, when blood pressure goes up, smooth muscle in the afferent capillaries contracts to limit any increase in blood flow and filtration rate. When blood pressure drops, the same capillaries relax to maintain blood flow and filtration rate. The net result is a relatively steady flow of blood into the glomerulus and a relatively steady filtration rate in spite of significant systemic blood pressure changes. Mean arterial blood pressure is calculated by adding 1/3 of the difference between the systolic and diastolic pressures to the diastolic pressure. Therefore, if the blood pressure is 110/80, the difference between systolic and diastolic pressure is 30. One third of this is 10, and when you add this to the diastolic pressure of 80, you arrive at a calculated mean arterial pressure of 90 mm Hg. Therefore, if you use mean arterial pressure for the GBHP in the formula for calculating NFP, you can determine that as long as mean arterial pressure is above approximately 60 mm Hg, the pressure will be adequate to maintain glomerular filtration. Blood pressures below this level will impair renal function and cause systemic disorders that are severe enough to threaten survival. This condition is called shock.

Determination of the GFR is one of the tools used to assess the kidney’s excretory function. This is more than just an academic exercise. Since many drugs are excreted in the urine, a decline in renal function can lead to toxic accumulations. Additionally, administration of appropriate drug dosages for those drugs primarily excreted by the kidney requires an accurate assessment of GFR. GFR can be estimated closely by intravenous administration of inulin . Inulin is a plant polysaccharide that is neither reabsorbed nor secreted by the kidney. Its appearance in the urine is directly proportional to the rate at which it is filtered by the renal corpuscle. However, since measuring inulin clearance is cumbersome in the clinical setting, most often, the GFR is estimated by measuring naturally occurring creatinine, a protein-derived molecule produced by muscle metabolism that is not reabsorbed and only slightly secreted by the nephron.

Chapter Review

The entire volume of the blood is filtered through the kidneys about 300 times per day, and 99 percent of the water filtered is recovered. The GFR is influenced by hydrostatic pressure and colloid osmotic pressure. Under normal circumstances, hydrostatic pressure is significantly greater and filtration occurs. The hydrostatic pressure of the glomerulus depends on systemic blood pressure, autoregulatory mechanisms, sympathetic nervous activity, and paracrine hormones. The kidney can function normally under a wide range of blood pressures due to the autoregulatory nature of smooth muscle.

Answer the question(s) below to see how well you understand the topics covered in the previous section.

Critical Thinking Questions

• Give the formula for net filtration pressure.
• Name at least five symptoms of kidney failure.
• Net filtration pressure (NFP) = glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)]
• Symptoms of kidney failure are weakness, lethargy, shortness of breath, widespread edema, anemia, metabolic acidosis or alkalosis, heart arrhythmias, uremia, loss of appetite, fatigue, excessive urination, and oliguria.

glomerular filtration rate (GFR): rate of renal filtration

inulin: plant polysaccharide injected to determine GFR; is neither secreted nor absorbed by the kidney, so its appearance in the urine is directly proportional to its filtration rate

net filtration pressure (NFP): pressure of fluid across the glomerulus; calculated by taking the hydrostatic pressure of the capillary and subtracting the colloid osmotic pressure of the blood and the hydrostatic pressure of Bowman’s capsule

systemic edema: increased fluid retention in the interstitial spaces and cells of the body; can be seen as swelling over large areas of the body, particularly the lower extremities

• Anatomy & Physiology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]

• Types of Bones
• Axial Skeleton
• Appendicular Skeleton
• Skeletal Pathologies
• Muscle Tissue Types
• Muscle Attachments & Actions
• Muscle Contractions
• Muscular Pathologies
• Functions of Blood
• Blood Vessels
• Circulation
• Circulatory Pathologies
• Respiratory Functions
• Upper Respiratory System
• Lower Respiratory System
• Respiratory Pathologies
• Oral Cavity
• Alimentary Canal
• Accessory Organs
• Absorption & Elimination
• Digestive Pathologies
• Lymphatic Structures
• Immune System

Urinary System Structures

• Urine Formation
• Urine Storage & Elimination
• Urinary Pathologies
• Female Reproductive System
• Male Reproductive System
• Reproductive Process
• Endocrine Glands

Filtration, Reabsorption, Secretion: The Three Steps of Urine Formation

Watch Video Summary

The kidneys filter unwanted substances from the blood and produce urine to excrete them. There are three main steps of urine formation: glomerular filtration, reabsorption, and secretion. These processes ensure that only waste and excess water are removed from the body.

1. The Glomerulus Filters Water and Other Substances from the Bloodstream

Each kidney contains over 1 million tiny structures called nephrons . Each nephron has a glomerulus , the site of blood filtration. The glomerulus is a network of capillaries surrounded by a cuplike structure, the glomerular capsule (or Bowman’s capsule). As blood flows through the glomerulus, blood pressure pushes water and solutes from the capillaries into the capsule through a filtration membrane. This glomerular filtration begins the urine formation process.

2. The Filtration Membrane Keeps Blood Cells and Large Proteins in the Bloodstream

Inside the glomerulus, blood pressure pushes fluid from capillaries into the glomerular capsule through a specialized layer of cells. This layer, the filtration membrane , allows water and small solutes to pass but blocks blood cells and large proteins. Those components remain in the bloodstream. The filtrate (the fluid that has passed through the membrane) flows from the glomerular capsule further into the nephron.

3. Reabsorption Moves Nutrients and Water Back into the Bloodstream

The glomerulus filters water and small solutes out of the bloodstream. The resulting filtrate contains waste, but also other substances the body needs: essential ions, glucose, amino acids, and smaller proteins. When the filtrate exits the glomerulus, it flows into a duct in the nephron called the renal tubule . As it moves, the needed substances and some water are reabsorbed through the tube wall into adjacent capillaries. This reabsorption of vital nutrients from the filtrate is the second step in urine creation.

4. Waste Ions and Hydrogen Ions Secreted from the Blood Complete the Formation of Urine

The filtrate absorbed in the glomerulus flows through the renal tubule, where nutrients and water are reabsorbed into capillaries. At the same time, waste ions and hydrogen ions pass from the capillaries into the renal tubule. This process is called secretion . The secreted ions combine with the remaining filtrate and become urine. The urine flows out of the nephron tubule into a collecting duct. It passes out of the kidney through the renal pelvis, into the ureter, and down to the bladder.

5. Urine Is 95% Water

The nephrons of the kidneys process blood and create urine through a process of filtration, reabsorption, and secretion. Urine is about 95% water and 5% waste products. Nitrogenous wastes excreted in urine include urea, creatinine, ammonia, and uric acid. Ions such as sodium, potassium, hydrogen, and calcium are also excreted.

Download Nephrons Lab Activity

External Sources

“What’s in Your Pee?” from Popular Science .

An overview of nephron and kidney histology from the Histology Guide (a publication of the University of Leeds).

Visible Body Web Suite provides in-depth coverage of each body system in a guided, visually stunning presentation.

Related Articles

Give It Up for the Kidneys

Urine Storage and Elimination

Common Diseases and Disorders

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25.9 The Urinary System and Homeostasis

Learning objectives.

By the end of this section, you will be able to:

• Describe the role of the kidneys in vitamin D activation
• Describe the role of the kidneys in regulating erythropoiesis
• Provide specific examples to demonstrate how the urinary system responds to maintain homeostasis in the body
• Explain how the urinary system relates to other body systems in maintaining homeostasis
• Predict factors or situations affecting the urinary system that could disrupt homeostasis
• Predict the types of problems that would occur in the body if the urinary system could not maintain homeostasis

All systems of the body are interrelated. A change in one system may affect all other systems in the body, with mild to devastating effects. A failure of urinary continence can be embarrassing and inconvenient, but is not life threatening. The loss of other urinary functions may prove fatal. A failure to synthesize vitamin D is one such example.

Vitamin D Synthesis

In order for vitamin D to become active, it must undergo a hydroxylation reaction in the kidney, that is, an –OH group must be added to calcidiol to make calcitriol (1,25-dihydroxycholecalciferol). Activated vitamin D is important for absorption of Ca ++ in the digestive tract, its reabsorption in the kidney, and the maintenance of normal serum concentrations of Ca ++ and phosphate. Calcium is vitally important in bone health, muscle contraction, hormone secretion, and neurotransmitter release. Inadequate Ca ++ leads to disorders like osteoporosis and osteomalacia in adults and rickets in children. Deficits may also result in problems with cell proliferation, neuromuscular function, blood clotting, and the inflammatory response. Recent research has confirmed that vitamin D receptors are present in most, if not all, cells of the body, reflecting the systemic importance of vitamin D. Many scientists have suggested it be referred to as a hormone rather than a vitamin.

Erythropoiesis

Erythropoetin (EPO) is a hormone produced by the kidney that stimulates the formation of red blood cells in the bone marrow. The kidney produces 85 percent of circulating EPO; the liver, the remainder. If you move to a higher altitude, the partial pressure of oxygen is lower, meaning there is less pressure to push oxygen across the alveolar membrane and into the red blood cell. One way the body compensates is to manufacture more red blood cells by increasing EPO production. If you start an aerobic exercise program, your tissues will need more oxygen to cope, and the kidney will respond with more EPO. If erythrocytes are lost due to severe or prolonged bleeding, or under produced due to disease or severe malnutrition, the kidneys come to the rescue by producing more EPO. Renal failure (loss of EPO production) is associated with anemia, which makes it difficult for the body to cope with increased oxygen demands or to supply oxygen adequately even under normal conditions. Anemia diminishes performance and can be life threatening.

Blood Pressure Regulation

Due to osmosis, water follows where Na + leads. In other words, “water follows salt.” Much of the water the kidneys recover from the filtrate follows the reabsorption of Na + . ADH stimulation of aquaporin channels allows for regulation of water recovery in the collecting ducts. Normally, all of the glucose is recovered, but loss of glucose control (diabetes mellitus) may result in an osmotic diuresis severe enough to produce severe dehydration and death. A loss of renal function means a loss of effective vascular volume control, leading to hypotension (low blood pressure) or hypertension (high blood pressure), which can lead to stroke, heart attack, and aneurysm formation.

The kidneys cooperate with the lungs, liver, and adrenal cortex through the renin–angiotensin–aldosterone system (see Chapter 25 Figure 25.4.2 ). The liver synthesizes and secretes the inactive precursor angiotensinogen. When the blood pressure is low, the kidney synthesizes and releases renin. Renin converts angiotensinogen into angiotensin I, and ACE produced in the lung converts angiotensin I into biologically active angiotensin II ( Figure 25.9.1 ). The immediate and short-term effect of angiotensin II is to raise blood pressure by causing widespread vasoconstriction. angiotensin II also stimulates the adrenal cortex to release the steroid hormone aldosterone, which results in renal reabsorption of Na + and its associated osmotic recovery of water. The reabsorption of Na + helps to raise and maintain blood pressure over a longer term.

Regulation of Osmolarity

Blood pressure and osmolarity are regulated in a similar fashion. Severe hypo-osmolarity can cause problems like lysis (rupture) of blood cells or widespread edema, which is due to a solute imbalance. Inadequate solute concentration (such as protein) in the plasma results in water moving toward an area of greater solute concentration, in this case, the interstitial space and cell cytoplasm. If the kidney glomeruli are damaged by an autoimmune illness, large quantities of protein may be lost in the urine. The resultant drop in serum osmolarity leads to widespread edema that, if severe, may lead to damaging or fatal brain swelling. Severe hypertonic conditions may arise with severe dehydration from lack of water intake, severe vomiting, or uncontrolled diarrhea. When the kidney is unable to recover sufficient water from the forming urine, the consequences may be severe (lethargy, confusion, muscle cramps, and finally, death) .

Recovery of Electrolytes

Sodium, calcium, and potassium must be closely regulated. The role of Na + and Ca ++ homeostasis has been discussed at length. Failure of K + regulation can have serious consequences on nerve conduction, skeletal muscle function, and most significantly, on cardiac muscle contraction and rhythm.

pH Regulation

Recall that enzymes lose their three-dimensional conformation and, therefore, their function if the pH is too acidic or basic. This loss of conformation may be a consequence of the breaking of hydrogen bonds. Move the pH away from the optimum for a specific enzyme and you may severely hamper its function throughout the body, including hormone binding, central nervous system signaling, or myocardial contraction. Proper kidney function is essential for pH homeostasis.

Everyday Connection

Stem Cells and Repair of Kidney Damage Stem cells are unspecialized cells that can reproduce themselves via cell division, sometimes after years of inactivity. Under certain conditions, they may differentiate into tissue-specific or organ-specific cells with special functions. In some cases, stem cells may continually divide to produce a mature cell and to replace themselves. Stem cell therapy has an enormous potential to improve the quality of life or save the lives of people suffering from debilitating or life-threatening diseases. There have been several studies in animals, but since stem cell therapy is still in its infancy, there have been limited experiments in humans.

Acute kidney injury can be caused by a number of factors, including transplants and other surgeries. It affects 7–10 percent of all hospitalized patients, resulting in the deaths of 35–40 percent of inpatients. In limited studies using mesenchymal stem cells, there have been fewer instances of kidney damage after surgery, the length of hospital stays has been reduced, and there have been fewer readmissions after release.

How do these stem cells work to protect or repair the kidney? Scientists are unsure at this point, but some evidence has shown that these stem cells release several growth factors in endocrine and paracrine ways. As further studies are conducted to assess the safety and effectiveness of stem cell therapy, we will move closer to a day when kidney injury is rare, and curative treatments are routine.

Chapter Review

The effects of failure of parts of the urinary system may range from inconvenient (incontinence) to fatal (loss of filtration and many others). The kidneys catalyze the final reaction in the synthesis of active vitamin D that in turn helps regulate Ca ++ . The kidney hormone EPO stimulates erythrocyte development and promotes adequate O 2 transport. The kidneys help regulate blood pressure through Na + and water retention and loss. The kidneys work with the adrenal cortex, lungs, and liver in the renin–angiotensin–aldosterone system to regulate blood pressure. They regulate osmolarity of the blood by regulating both solutes and water. Three electrolytes are more closely regulated than others: Na + , Ca ++ , and K + . The kidneys share pH regulation with the lungs and plasma buffers, so that proteins can preserve their three-dimensional conformation and thus their function.

Review Questions

1. Which step in vitamin D production does the kidney perform?

• converts cholecalciferol into calcidiol
• converts calcidiol into calcitriol
• stores vitamin D
• none of these

2. Which hormone does the kidney produce that stimulates red blood cell production?

• thrombopoeitin

3. If there were no aquaporin channels in the collecting duct, ________.

• you would develop systemic edema
• you would retain excess Na +
• you would lose vitamins and electrolytes
• you would suffer severe dehydration

Critical Thinking Questions

1. How does lack of protein in the blood cause edema?

2. Which three electrolytes are most closely regulated by the kidney?

Bagul A, Frost JH, Drage M. Stem cells and their role in renal ischaemia reperfusion injury. Am J Nephrol [Internet]. 2013 [cited 2013 Apr 15]; 37(1):16–29. Available from: http://www.karger.com/Article/FullText/345731

Answers for Review Questions

Answers for Critical Thinking Questions

• Protein has osmotic properties. If there is not enough protein in the blood, water will be attracted to the interstitial space and the cell cytoplasm resulting in tissue edema.
• The three electrolytes are most closely regulated by the kidney are calcium, sodium, and potassium.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

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Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

25.5 Physiology of Urine Formation

Learning objectives.

By the end of this section, you will be able to:

• Describe the hydrostatic and colloid osmotic forces that favor and oppose filtration
• Describe glomerular filtration rate (GFR), state the average value of GFR, and explain how clearance rate can be used to measure GFR
• Predict specific factors that will increase or decrease GFR
• State the percent of the filtrate that is normally reabsorbed and explain why the process of reabsorption is so important
• Calculate daily urine production
• List common symptoms of kidney failure

Having reviewed the anatomy and microanatomy of the urinary system, now is the time to focus on the physiology. You will discover that different parts of the nephron utilize specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.

Failure of the renal anatomy and/or physiology can lead suddenly or gradually to renal failure. In this event, a number of symptoms, signs, or laboratory findings point to the diagnosis ( Table 25.3 ).

Glomerular Filtration Rate (GFR)

The volume of filtrate formed by both kidneys per minute is termed the glomerular filtration rate (GFR) . The heart pumps about 5 L blood per min under resting conditions. Approximately 20 percent or one liter enters the kidneys to be filtered. On average, this liter results in the production of about 125 mL/min filtrate produced in men (range of 90 to 140 mL/min) and 105 mL/min filtrate produced in women (range of 80 to 125 mL/min). This amount equates to a volume of about 180 L/day in men and 150 L/day in women. Ninety-nine percent of this filtrate is returned to the circulation by reabsorption so that only about 1–2 liters of urine are produced per day ( Table 25.4 ).

GFR is influenced by the hydrostatic pressure and colloid osmotic pressure on either side of the capillary membrane of the glomerulus. Recall that filtration occurs as pressure forces fluid and solutes through a semipermeable barrier with the solute movement constrained by particle size. Hydrostatic pressure is the pressure produced by a fluid against a surface. If you have a fluid on both sides of a barrier, both fluids exert a pressure in opposing directions. Net fluid movement will be in the direction of the lower pressure. Osmosis is the movement of solvent (water) across a membrane that is impermeable to a solute in the solution. This creates a pressure, osmotic pressure, which will exist until the solute concentration is the same on both sides of a semipermeable membrane. As long as the concentration differs, water will move. Glomerular filtration occurs when glomerular hydrostatic pressure exceeds the luminal hydrostatic pressure of Bowman’s capsule. There is also an opposing force, the osmotic pressure, which is typically higher in the glomerular capillary.

To understand why this is so, look more closely at the microenvironment on either side of the filtration membrane. You will find osmotic pressure exerted by the solutes inside the lumen of the capillary as well as inside of Bowman’s capsule. Since the filtration membrane limits the size of particles crossing the membrane, the osmotic pressure inside the glomerular capillary is higher than the osmotic pressure in Bowman’s capsule. Recall that cells and the medium-to-large proteins cannot pass between the podocyte processes or through the fenestrations of the capillary endothelial cells. This means that red and white blood cells, platelets, albumins, and other proteins too large to pass through the filter remain in the capillary, creating an average colloid osmotic pressure of 30 mm Hg within the capillary. The absence of proteins in Bowman’s space (the lumen within Bowman’s capsule) results in an osmotic pressure near zero. Thus, the only pressure moving fluid across the capillary wall into the lumen of Bowman’s space is hydrostatic pressure. Hydrostatic (fluid) pressure is sufficient to push water through the membrane despite the osmotic pressure working against it. The sum of all of the influences, both osmotic and hydrostatic, results in a net filtration pressure (NFP) of about 10 mm Hg ( Figure 25.16 ).

A proper concentration of solutes in the blood is important in maintaining osmotic pressure both in the glomerulus and systemically. There are disorders in which too much protein passes through the filtration slits into the kidney filtrate. This excess protein in the filtrate leads to a deficiency of circulating plasma proteins. In turn, the presence of protein in the urine increases its osmolarity; this holds more water in the filtrate and results in an increase in urine volume. Because there is less circulating protein, principally albumin, the osmotic pressure of the blood falls. Less osmotic pressure pulling water into the capillaries tips the balance towards hydrostatic pressure, which tends to push it out of the capillaries. The net effect is that water is lost from the circulation to interstitial tissues and cells. This “plumps up” the tissues and cells, a condition termed systemic edema .

Net Filtration Pressure (NFP)

NFP determines filtration rates through the kidney. It is determined as follows:

NFP = Glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)] = 10 mm Hg

NFP = GBHP – [CHP + BCOP] = 10 mm Hg

NFP = 55 – [15 + 30] = 10 mm Hg

As you can see, there is a low net pressure across the filtration membrane. Intuitively, you should realize that minor changes in osmolarity of the blood or changes in capillary blood pressure result in major changes in the amount of filtrate formed at any given point in time. The kidney is able to cope with a wide range of blood pressures. In large part, this is due to the autoregulatory nature of smooth muscle. When you stretch it, it contracts. Thus, when blood pressure goes up, smooth muscle in the afferent capillaries contracts to limit any increase in blood flow and filtration rate. When blood pressure drops, the same capillaries relax to maintain blood flow and filtration rate. The net result is a relatively steady flow of blood into the glomerulus and a relatively steady filtration rate in spite of significant systemic blood pressure changes. Mean arterial blood pressure is calculated by adding 1/3 of the difference between the systolic and diastolic pressures to the diastolic pressure. Therefore, if the blood pressure is 110/80, the difference between systolic and diastolic pressure is 30. One third of this is 10, and when you add this to the diastolic pressure of 80, you arrive at a calculated mean arterial pressure of 90 mm Hg. Therefore, if you use mean arterial pressure for the GBHP in the formula for calculating NFP, you can determine that as long as mean arterial pressure is above approximately 60 mm Hg, the pressure will be adequate to maintain glomerular filtration. Blood pressures below this level will impair renal function and cause systemic disorders that are severe enough to threaten survival. This condition is called shock.

Determination of the GFR is one of the tools used to assess the kidney’s excretory function. This is more than just an academic exercise. Since many drugs are excreted in the urine, a decline in renal function can lead to toxic accumulations. Additionally, administration of appropriate drug dosages for those drugs primarily excreted by the kidney requires an accurate assessment of GFR. GFR can be estimated closely by intravenous administration of inulin . Inulin is a plant polysaccharide that is neither reabsorbed nor secreted by the kidney. Its appearance in the urine is directly proportional to the rate at which it is filtered by the renal corpuscle. However, since measuring inulin clearance is cumbersome in the clinical setting, most often, the GFR is estimated by measuring naturally occurring creatinine, a protein-derived molecule produced by muscle metabolism that is not reabsorbed and only slightly secreted by the nephron.

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The Normal Anatomy and Physiology of the Kidneys: Urine Formation Essay

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The External Anatomy of Kidney

The internal anatomy of kidney, physiological balancing in the nephron.

Kidneys are the major organs of the renal system which perform vital homeostatic processes such as maintenance of water and ionic balance in the body. The kidneys’ primary function is waste removal through ultrafiltration, leading to urine formation. Moreover, they are actively involved in the reabsorption of amino acids, glucose, and water to achieve osmotic balance in the body (Wingerd & Taylor, 2020). In addition, hormones and enzymes are produced from the kidneys, stimulating, and catalysing physiological reactions in the body to achieve homeostasis.

A normal kidney is a brown organ, which has the shape of a bean seed. Each kidney is protected by a cushion called the renal capsule, a fibrous membrane having an irregular network of connecting tissue. The capsule is essential in holding kidneys in their positions within the abdominal cavity (Wingerd & Taylor, 2020). Moreover, it assists in maintaining the shape and responsible for protecting them from mechanical shock. The capsule is also overlayed by the renal fat pad, enhancing its efficacy in reducing the physical impact on the kidneys from an external force. The adipose tissues forming the fat pad are linked to the renal fascia, which in conjunction with the peritoneum, serves as anchorage surfaces for the kidneys to the posterior side of the abdominal cavity (Hickling et al., 2017). The hilum forms the entry point through which renal arteries and veins, and ureters serve the kidneys with fluids flowing in and out.

On top of the kidney is embedded an adrenal gland, which is responsible for modulating the organ’s physiological functions. However, the adrenal glands are part of the endocrine and not the renal system. The anterior interests of kidneys are protected by ribcages that curve into the lumbar region (Hickling et al., 2017). Kidneys are served by blood from the renal artery, which branches from the posterior side of the aorta. The flowing blood from the kidney is channelled into the inferior vena cava through the renal vein (Wingerd & Taylor, 2020). The excretory waste constituting the urine flows from the kidney into the bladder through the ureter. Thus, kidney serves in the purification of blood during the process of urine formation.

The kidney comprises three primary internal layers: the cortex, medulla, and pelvis. The renal cortex is the region immediately after the capsule, and within it are the extended sections of the nephron to form the Bowman’s capsule (Lawrence et al., 2018). It is a granular tissue within the kidney, offering a space for the arterioles and venules emerging from the arteries and veins, respectively. Moreover, the glomerular capillaries, which permit filtration of the blood components, are embedded in the cortex (Wingerd & Taylor, 2020). It is in the renal cortex where erythropoietin hormone is secreted to stimulate erythrocyte formation.

The medulla is the parenchymatous region within the kidney, constituting the immediate layer after the cortex. It is composed of stacked masses of tissue defined as renal pyramids. Each pyramid is comprised of densely interwoven nephrons (Wingerd & Taylor, 2020). The basic unit by which a kidney executes homeostatic function is the nephron. The Bowman capsule located in the cortex connects to the proximal convoluted tubules through which the glomerular filtered plasma flows (Hickling et al., 2017). In the medulla pyramids, the loop of Henle and the distal convoluted tubules form part of the nephron channelling the purified plasma into the collecting ducts.

The renal pelvis is the innermost concaved region of the kidney. Every pyramid of the medulla ends in a renal papilla that supplies concentrated urine into the minor calyces. The pool formed by minor calyces from every renal pyramid constitutes the major calyx. All the significant calyces are consecrated into a single unit called the pelvis (Wingerd & Taylor, 2020). The pelvis connects the kidney to the ureter, thus directing the concentrated urine into the bladder.

The Blood Supply Network

Kidneys are highly vascularized organs, receiving a quarter of the blood circulating within the body at a specific duration of time. The entry of blood into the two kidneys occurs via a pair of renal arteries extending from the aorta. On reaching the hilum of each kidney, the blood is channelled into segmental arteries, which branch into the interlobar arteries (Hickling et al., 2017). The interlobar vessels pass through the columns get into the cortex, in which they branch to form arcuate arteries. Further branching of the blood vessels leads to cortical radiate arteries, which supply their contents into the arterioles. Blood from the arterioles enters the glomerulus via the afferent arteriole. After glomerular filtration, the blood leaves the network of capillaries via the efferent arteriole (McDonald, 2019). Moreover, a portal of blood vessels extends from the afferent and efferent arterioles surrounding the proximal convoluted tubules, the loop of Henle, and the distal convoluted tubule to aid the urine concentration process.

Ultrafiltration in the Glomerulus

The formation of urine begins with the filtration process, which takes place in the glomerulus, a mesh of capillaries connected to the Bowman’s capsule. Ultrafiltration in the glomerulus does not require energy. However, it is accomplished through pressure build-up, which pushes the plasma and solute particles through the capillary walls. The process of filtration is aided through a three-layered membrane system (Lawrence et al., 2018). The fenestrated endothelia of capillaries in the permits plasma to pass through them, and not blood cells. Immediately, the negatively charged basement membrane blocks proteins from passing. Finally, the capsule of the capsule in the glomeruli develops a barrier that allows for the selected particles’ filtration. The efficiency of the filtration process is determined by the pressure create by the cardiac pumps of blood through the aorta, arteries, arterioles, and capillaries (Hickling et al., 2017). The net force for filtration generated in the glomeruli yields the glomerular filtrate channelled into the proximal convoluted tubule via the Bowman’s capsule.

Urine Concentration through Water and Ion Re-absorption

The kidney nephron is characterized by four tubular components in which reabsorption of water, ions, amino acids, and glucose takes place. The proximal convoluted tubule is attributed to the highest capacity of absorbing elements of the glomerular filtrate. From its lumen, sodium ions are taken back to the bloodstream by an active transport mechanism involving basolateral pumping of sodium-potassium ions (Lawrence et al., 2018). The secondary dynamic transport mechanism is involved in the reabsorption of amino acids, glucose, and vitamins. Moreover, water is reabsorbed by osmosis created by the ionic imbalance, which in turn drives the diffusion of lipids across the wall of proximal convoluted tubule into the bloodstream (Gupta & Sharma, 2020). The reabsorption of ions, sugar, amino acids, and lipids is essential in osmoregulation all over the body.

The glomerular filtrate moves into the loop of Henle, having the ascending and descending sections. The reabsorption of water through osmosis into the bloodstream main occurs in the descending spiral. On the other hand, potassium, sodium, and chloride ions are taken back to the bloodstream via the ascending loop of Henle (McDonald, 2019). The ATPase enzyme drives the process by creating an ionic gradient which makes the basolateral membrane of the symporter in the ascending loop to be functional in absorbing ions. Immediately after the loop of Henle, the filtrate enters the distal convoluted tubule where sodium ion absorption occurs. Mainly, active transport is involved in sodium-ion uptake via basolateral membrane (Lawrence et al., 2018). However, its passive absorption into the bloodstream occurs through sodium and chloride ion symporter on the apical plasmalemma. In the distal convoluted tubule, aldosterone hormone regulates sodium ion intake while parathyroid hormone controls the reabsorption of calcium ion (McDonald, 2019). Eventually, concentrated urine remaining in the lumen of the tubules moves into the collecting ducts, where final reabsorption occurs via active transport.

The concentrated urine is channelled into the pelvis; after that, it travels to the urinary bladder via the ureter. The kidney has two major homeostatic roles in the body, that is, maintenance of pH through balancing hydrogen ion concentration and osmoregulation. Moreover, it aids the purification of the blood by removing excess water, salts, and impurities. Its physiological functions encompass energy and the regulated hormonal process leading to the reabsorption of ions into the bloodstream. Thus, the structural and physiological function of the kidney allows urine formation and purification of blood.

Gupta, R., & Sharma, T. (2020). Review of urine formation in Ayurveda. Journal of Ayurveda and Integrated Medical Sciences, 5 (1), 145-148.

Hickling, D. R., Sun, T. T., & Wu, X. R. (2017). Anatomy and physiology of the urinary tract: relation to host defence and microbial infection . In Urinary Tract Infections: Molecular Pathogenesis and Clinical Management (pp. 1-25).

Lawrence, E. A., Doherty, D., & Dhanda, R. (2018). Function of the nephron and the formation of urine . Anaesthesia and Intensive Care Medicine, 19 (5), 249-253.

McDonald, M. D. (2019). The renal contribution to salt and water balance. In Fish Osmoregulation (pp. 309-331). CRC Press.

Wingerd, B., & Taylor, T. B. (2020). The Human Body: Concepts of Anatomy and Physiology . Jones & Bartlett Learning.

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Urine Formation: Definition, Mechanism, Significance

Urine Formation: The liquid waste product of the human body is urine. It is made up of waste products from the body’s many metabolic activities, such as urea, uric acid, salts, water, and other substances. It develops in the kidneys, which are the main excretory organs. Unwanted compounds are removed from the circulation by the kidneys, which also create urine to do so. Urine is produced in three stages: glomerular filtration, reabsorption, and secretion.

These procedures make sure that the body is solely expelled of waste and extra water. The nephrons are the structural and operational unit of the kidneys. The process of producing urine involves millions of nephrons. Each nephron has a glomerulus, which is where blood is filtered. The glomerular capsule (also known as Bowman’s capsule) surrounds the capillary network that makes up the glomerulus. Read through the article to learn about urine formation diagram, process and more!

What is Urine Formation?

Define urine formation: The process of urine formation refers to the build-up of yellow concentrated fluid that consists of wastes and toxic materials which are to be excreted out from the body.

Urine Formation Diagram

Fig: Reabsorption and Secretion of Major Substances at Different Parts of the Nephron (Arrows Indicate the Direction of Movement of Materials)

Definition of Urine Formation

The formation of yellow fluid called urine through different processes in the nephrons of the kidneys is called urine formation. The nephron is the unit of the kidney. Before getting into the mechanism of urine formation, we need to know about the unit of the kidney called the nephron.

Structure of Nephron

The nephron consists of a malpighian body and renal tubules.

• Malpighian Body- It consists of a cup-shaped Bowman’s capsule and glomerulus. The glomerulus is a tuft of capillaries. Afferent arteriole enters the Bowman’s capsule and leaves it through the efferent arteriole.

Fig: Bowman’s Capsule

Bowman’s capsule is a cup-shaped structure that surrounds the glomerulus. Its wall is double-layered and consists of special types of cells called podocytes. These cells have projections, and gaps between these projections form pores. These pores allow the filtration of blood from the glomerulus to the Bowman’s capsule.

Fig: Podocyte

• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule

Types of Nephrons

The types of nephrons are as follows:

• Cortical Nephron – These are short nephrons and almost lie in the cortical region of the kidney. Thus, it is called a cortical nephron.
• Juxtamedullary Nephron – These have a long loop of Henle and digs deep into the medulla and are called Juxtamedullary Nephron.

Fig: Types of Nephrons

Process of Urine Formation

Let us see how urine is formed in our bodies. The process of urine formation includes different processes like glomerular filtration, selective reabsorption, and tubular secretion. These urine formation steps involve different parts of the nephron. Students can check the physiology of urine formation below:

Glomerular Filtration

This is the first step of urine formation which is carried out by the glomerulus. About \(1100 – 1200\, ml\) of blood is filtered from the kidneys per minute. The glomerular capillaries are narrower than the glomerular arterioles. This creates pressure in the glomerular capillaries. Due to high pressure in the capillaries, blood is filtered through pores created by podocytes. The filtered blood is poured into the lumen of Bowman’s capsule.

Through these pores, large cells like RBCs, WBCs, plasma proteins, etc., are unable to pass through these pores. This process of filtration, which occurs through glomerular capillaries in the Bowman’s capsule, is known as Ultrafiltration, and the filtrate formed is called glomerular filtrate.

This glomerular filtrate is the same as that of blood plasma. The only difference is that filtrate does not contain fat and proteins.

After passing through the glomerular capillaries, blood enters the efferent arteriole. The amount of filtrate formed per minute by both the kidneys is called the glomerular filtrate rate. It is about \(125\,ml\) per minute or \(180\,L\) per day.

This glomerular filtration is regulated by-

• Myogenic Mechanism- It refers to the increase in blood pressure which in turn increases the pressure in afferent arterioles. This increase in pressure in afferent arterioles results in increased blood flow to the glomerulus. Increased pressure reduces the diameter of the blood vessels.
• Juxtaglomerular Mechanism- Juxtaglomerular apparatus secretes an enzyme called renin which regulates the blood pressure and thereby glomerular filtration rate (GFR). 
• Neural Control- Nervous system also controls the GFR by signalling the renal artery to constrict

Selective Reabsorption

Under this category, we will describe the reabsorption of useful materials back into the blood. 

• Proximal Convoluted Tubule – From Bowman’s capsule, filtrate enters the proximal convoluted tubule (PCT). Here, reabsorption of almost \(65\% \) of the filtrate is done before reaching the top of the loop of Henle. Reabsorption of glucose, amino acids, vitamins, hormones, chlorides, sodium, potassium, and much water are reabsorbed here. These are reabsorbed by active and passive transport. Water reabsorption occurs by osmosis. Here the filtrate is isotonic to the blood plasma.
• Descending limb of the loop of Henle : Here water is reabsorbed when filtrate moves down the loop of Henle and not the solutes like sodium. This makes the filtrate hypertonic to the blood plasma.
• Ascending limb of the loop of Henle : Here solutes like sodium, potassium, magnesium, calcium, and chloride are reabsorbed. This makes the filtrate hypotonic to the blood plasma.
• Distal Convoluted Tubule (DCT)- In this part of the nephron, under the influence of the aldosterone hormone secreted by the cortex of the adrenal gland, sodium ions are reabsorbed. Under the influence of ADH, water reabsorption occurs here. Chloride ions are reabsorbed too. Again here, filtrate becomes isotonic to the blood plasma.
• Collecting Duct- Now filtrate enters the collecting duct from DCT. Here more water is reabsorbed under the influence of ADH, which makes the filtrate more concentrated and hypertonic in the blood plasma. Sodium ions are also reabsorbed. This concentrated filtrate is now called urine.

Tubular Secretion

In the renal tubule, with reabsorption, secretion also occurs. Secretion of harmful toxic materials like ammonia, creatinine, urea, hippuric acid, drugs, etc., is actively secreted from the DCT. Mostly, secretion occurs in DCT. Some secretions also occur in the loop of Henle and DCT. Urea is secreted in the loop of Henle, while in DCT, potassium, and ammonia is reabsorbed.

Fig: Urine Formation

Mechanism of Concentration of Filtrate

Kidneys not only excrete harmful substances out from the body but also maintain the amount of water and salt in the body. The filtrate from the Bowman’s capsule enters the PCT, which is almost isotonic to the blood plasma. But here in PCT, as \(65\% \) of the filtrate is reabsorbed, it gets hypertonic to the blood plasma in the descending loop of Henle. Then in the ascending loop of Henle, the filtrate gets hypotonic to the blood plasma.

Then again, in DCT, due to the action of aldosterone and ADH, the filtrate becomes isotonic as sodium ions and water are reabsorbed by them, respectively. In the collecting duct, further reabsorption of water takes place which makes filtrate more hypertonic. Now this filtrate is called urine.

Counter Current Mechanism

This unique mechanism of the nephron helps in making the urine concentrated. In the limbs of the loop of Henle, filtrate flows in the opposite direction, which forms the counter current. The flow of blood in capillaries surrounding the loop of Henle is also in the opposite direction and forms a counter current.

Its function is to increase the concentration of sodium ions in the interstitial fluid and concentrate the filtrate in the collecting duct by allowing more reabsorption of water from it. This concentrates urine. Urine formed in the kidneys comes down to the urinary bladder through ureters and then is expelled out of the body. This expulsion is called micturition. 

Significance of Urine Formation

The significance of urine formation are as follows:

• Urine formation helps in the removal of wastes like creatinine and urea.
• It helps in the removal of extra fluid from the body, thereby regulating extracellular fluid volume.
• Urine helps to remove extra acidic components of the blood plasma and thus regulates the pH.
• Osmolarity or electrolyte-water balance is maintained by the kidneys by concentrating or diluting the urine.

Urine Formation Per Day

Glomerular filtration rate (GFR) is termed as the volume of filtrate formed by both kidneys per minute. On average, 125 mL/min filtrate is produced in men and 105 mL/min filtrate is produced in women. However, 99% of produced filtrate is returned to circulation by the process of reabsorption. Therefore, only about 1–2 litres of urine are produced per day in a healthy human body. Let’s see the table below for more information on the quantity of urine formed per day for men and women.

In the table below, we have mentioned the quantity of urine that is formed daily in men and women.

What is Net Filtration Pressure?

Net Filtration Pressure (NFP) is used to determine the filtration rates through the kidney.

NFP = Glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)] = 10 mm Hg

NFP = GBHP – [CHP + BCOP] = 10 mm Hg Or: NFP = 55 – [15 + 30] = 10 mm Hg

Multiple Choice Questions (MCQs) on Urine Formation

Here are some of the commonly asked multiple-choice questions on the “urine formation” topic, which will be asked in various competitive exams like NEET, Pharmacist, GPAT and various nursing exams.

Q: Where does capillary hydrostatic pressure build-up?

a) glomerulus b) PCT c) DCT d) collecting ducts

Q: Which of the following products is reabsorbed very limited?

a) urea b) uric acid c) creatinine d) all of the above

Q: What factor maintains the normal blood pH?

a) tubular secretion of H ions b) tubular secretion of Ca ions c) tubular secretion of K ions d) tubular secretion of Na ions

Q: On average, how much volume of blood is filtered by the kidneys per minute?

a) 500 mL b) 100-150 mL c) 1100-1200 mL d) 5000 mL

Q: What is the full form of GFR?

a) Glomerulus filtering unit b) Glomerulur filtration rate c) Globulin fast rate d) Globulin filtering rate

Summary on Urine Formation

Urine formation involves a few processes like glomerular filtration, selective reabsorption, and tubular secretion. The nephron is the unit of the kidney. Each nephron undergoes all three processes of urine formation and forms concentrated urine which is transported to the urinary bladder through ureters and then is excreted out of the body.

FAQs on Urine Formation

Find some of the frequently asked questions on the process of urine formation below:

Q.1: Where and how is urine produced? Ans: Urine is produced in the nephrons of the kidneys. Urine formation involves the following processes:-  a. Glomerular Filtration b. Selective reabsorption c. Tubular Secretion

Q.2: Why do the kidneys form urine? Ans: Urine is a yellow fluid formed by the nephrons of the kidneys. Urine is formed to excrete excess water, salt and nitrogenous wastes like urea. Other toxic substances are also excreted through urine.

Q.3: What are the \(3\) steps of urine formation? Ans: Following are the steps of urine formation: a. Glomerular Filtration b. Selective reabsorption c. Tubular Secretion

Q.4: What is the composition of urine, and write the steps of urine formation? Ans: Urine is a yellow colour fluid consisting of excess water, salt, and nitrogenous wastes like urea. Urine is produced in the nephrons of the kidneys. Urine formation involves the following processes:-  a. Glomerular Filtration b. Selective reabsorption c. Tubular Secretion

Q.5: What are the \(7\) functions of the kidneys? Ans: The functions are as follows: a. maintains acid-base balance. b. Maintains water balance c. Maintains electrolyte balance d. removes wastes and toxins from the body e. Controls blood pressure f. Produces hormone g. Activates Vitamin D

We hope this detailed article on urine formation helps you in your preparation. Stay tuned to  embibe.com  for any latest news and updates!

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• Urine Formation

Did you know that millions of nephrons are involved in the process of urine formation? That is right, what seems like a straight-forward function is actually a complicated life process of our body. Let us educate ourselves about urine formation and its functions.

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Urine is the liquid waste product of the human body . It contains urea, uric acid, salts , water and other waste products that are the result of various metabolic processes occurring in the body. It is formed in the primary excretory organs – the kidneys. The structural and functional unit of the kidneys is called the nephrons. Millions of nephrons are involved in the process of urine formation.

Browse more Topics under Excretory Products

• Introduction to Excretory System
• Human Excretory System
• Regulation of Excretion
• Micturation
• Role of the Other Organs in Excretion
• Disorders of the Excretory System

The formation process occurs in 3 steps or phases:

Glomerular Filtration

Tubular reabsorption, tubular secretion, anatomy of the nephron.

(Source: sites.google.com)

The anatomy of the nephron is important to understand the urine formation process. Each nephron is made up of two parts:

• Renal Corpuscle
• Renal Tubule

The renal corpuscle is divided into the glomerular capillaries or glomerulus and the Bowman’s capsule. It is in the renal corpuscle that the blood is filtered at high pressure . The arteriole that brings blood into the glomerulus is called the afferent arteriole whereas the artery that takes blood away from the glomerulus is known as the efferent arteriole.

Between these arterioles forms, a network of capillaries called the glomerular capillaries of the glomerulus. The Bowman’s capsule is a cup-shaped structure in which this glomerulus is located. The glomerulus along with the Bowman’s capsule achieve the filtration of blood to form urine. The renal tubule consists of :

• The proximal convoluted Tubule(PCT)
• The U-shaped Loop Of Henle
• The Distal Convoluted Tubule(DCT)

Once the blood is filtered in the renal corpuscle, the resultant fluid is called the glomerular filtrate. This glomerular filtrate now passes into the PCT. In the PCT, substances like NaCl, K+, water, glucose, and bicarbonate are reabsorbed into the filtrate whereas urea, creatinine, uric acid are added to the filtrate.

From the PCT, the filtrate enters the U-shaped Loop of Henle where reabsorption and secretion of water and various metabolites occurs. The filtrate then passes into the DCT. From the DCT, the filtrate passes into the collecting tubules, into the renal pelvis and the ureters as urine to be stored int he urinary bladder.

Process of Urine Formation

This process occurs in the glomerular capillaries . The process of filtration leads to the formation of an ultrafiltrate. The blood gushes into these capillaries with high pressure and gets filtered across the thin capillary walls. Everything except the blood cells and proteins are pushed into the capsular space of the Bowman’s capsule to form the ultrafiltrate. The glomerular filtration rate (GFR) is 125ml/min or 180 Litres/day.

During glomerular filtration, all substances except blood cells and proteins are pushed through the capillaries at high pressure. At the level of the Proximal Convoluted Tubule(PCT), some of the substances from the filtrate are reabsorbed. These include sodium chloride, potassium, glucose, amino acids, bicarbonate, and 75% of water.

Absorption of some substances is passive, some substances are actively transported while others are co-transported. The absorption depends upon the permeability of different parts of the nephron. The distal convoluted tubule shows selective absorption. The substances and water which is reabsorbed are taken up by the peritubular capillaries to be returned to the blood.

The peritubular capillaries that help in transporting the reabsorbed substances into the bloodstream, also help in actively secreting substances like H+ ions, K+ ions. Whenever excess K+ is secreted into the filtrate, Na+ ions are actively reabsorbed to maintain the Na-K balance. Some drugs are not filtered in the glomerulus and so are actively secreted into the filtrate during the tubular secretion phase.

Composition of Urine

Physical characteristics: Urine is the waste product that is eliminated by the kidneys. Urine contains waste products like urea, salts, excess ions, water, and metabolized products of drugs.

Urine is often light or pale yellow in colour and fresh urine has a slight ammoniacal smell. It is often clear in turbidity with a pH of around 4-8. These characteristics vary depending upon the nature of the disease in the body. Often a urine sample analysis helps to detect diseases like diabetes, kidney failures etc.

Chemical composition: Chemically, urine is composed mainly of urea, sodium chloride, potassium ions, creatinine, ammonia products, and some amount of protein, and other metabolites. Certain abnormalities in the urine composition occur in the following:

• Hematuria- When blood is found in the urine, the condition is called as hematuria. This indicates some pathology either injury or infection-related.
• Pyuria- This condition is characterized by the presence of pus cells in the urine. This indicates the presence of infection somewhere in the body.
• Glycosuria- This is a condition characterized by the presence of glucose in the urine. This is indicative of diabetes that is most likely uncontrolled.
• Proteinuria- This is a condition where protein molecules are found in the urine. This indicates some defect in the kidney’s filtration process.

Learn more about the Excretory System here .

Solved Example for You

Q: Which of the following is the Glomerular Filtration Rate of a nephron?

(a)    125ml/day       (b) 180ml/day

(c)     125L/day        (d) 125ml/min

Sol: (d) 125ml/min.  The optimum GFR of a nephron is 125ml/min or 180 Litres/day.

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Excretory Products

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Syllabus Edition

First teaching 2020

Last exams 2024

Formation of Urine ( CIE A Level Biology )

Revision note.

Biology Lead

Formation of Urine in the Nephron

• The nephron is the functional unit of the kidney – the nephrons are responsible for the formation of urine
• The process of urine formation in the kidneys occurs in two stages :

Ultrafiltration

• Selective reabsorption

The Two Stages of Urine Production in the Kidneys Table

• After the necessary reabsorption of amino acids, water, glucose and inorganic ions is complete (even some urea is reabsorbed), the filtrate eventually leaves the nephron and is now referred to as urine
• This urine then flows out of the kidneys, along the ureters and into the bladder , where it is temporarily stored
• Arterioles branch off the  renal artery  and lead to each nephron, where they form a knot of capillaries (the  glomerulus ) sitting inside the cup-shaped  Bowman’s capsule
• The capillaries get  narrower  as they get further into the glomerulus which  increases the pressure  on the blood moving through them (which is already at high pressure because it is coming directly from the renal artery which is connected to the  aorta )
• This eventually causes the smaller molecules being carried in the blood to be  forced out of the capillaries and into the Bowman’s capsule , where they form what is known as the filtrate
• The first cell layer is the endothelium of the capillary – each capillary endothelial cell is perforated by thousands of tiny membrane-lined circular holes
• The next layer is the basement membrane – this is made up of a network of collagen and glycoproteins
• The second cell layer is the epithelium of the Bowman’s capsule – these epithelial cells have many tiny finger-like projections with gaps in between them and are known as podocytes
• The fluid that filters through from the blood into the Bowman’s capsule is known as the glomerular filtrate
• The main substances that pass out of the capillaries and form the glomerular filtrate are: amino acids, water, glucose, urea and inorganic ions (mainly Na + , K + and Cl - )
• Red and white blood cells and platelets remain in the blood as they are too large to pass through the holes in the capillary endothelial cells
• The basement membrane acts as a filter as it stops large protein molecules from getting through

Ultrafiltration occurs when small molecules (such as amino acids, water, glucose, urea and inorganic ions) filter out of the blood and into the Bowman’s capsule to form glomerular filtrate. These molecules must pass through three layers during this process: the capillary endothelium, the basement membrane and the Bowman’s capsule epithelium

How ultrafiltration occurs

• Remember – water moves down a water potential gradient, from a region of higher water potential to a region of lower water potential. Water potential is increased by high pressure and decreased by the presence of solutes

Factors Affecting Water Potential Table

• Overall, the effect of the pressure gradient outweighs the effect of solute gradient
• Therefore, the water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the Bowman’s capsule
• This means that as blood flows through the glomerulus, there is an overall movement of water down the water potential gradient from the blood into the Bowman’s capsule

As blood flows through the glomerulus, there is an overall movement of water down the water potential gradient from the blood plasma (region of higher water potential) into the Bowman’s capsule (region of lower water potential)

Selective Reabsorption

• Many of the substances that end up in the glomerular filtrate actually need to be kept by the body
• These substances are reabsorbed into the blood as the filtrate passes along the nephron
• This process is knowns as selective reabsorption as only certain substances are reabsorbed
• Glucose reabsorption occurs in the proximal convoluted tubule
• Co-transporter proteins
• A high number of mitochondria
• Tightly packed cells
• Water and salts are reabsorbed via the Loop of Henle and collecting duct

Adaptations for Selective Reabsorption Table

How the selective reabsorption of solutes occurs

• As the blood in these capillaries comes straight from the glomerulus, it has very little plasma and has lost much of its water, inorganic ions and other small solutes
• The basal membranes (of the proximal convoluted tubule epithelial cells) are the sections of the cell membrane that are closest to the blood capillaries
• Sodium-potassium pumps in these basal membranes move sodium ions out of the epithelial cells and into the blood, where they are carried away
• This lowers the concentration of sodium ions inside the epithelial cells , causing sodium ions in the filtrate to diffuse down their concentration gradient through the luminal membranes (of the epithelial cells)
• These sodium ions do not diffuse freely through the luminal membranes – they must pass through co-transporter proteins in the membrane
• There are several types of these co-transporter proteins – each type transports a sodium ion and another solute from the filtrate (eg. glucose or a particular amino acid)
• Once inside the epithelial cells these solutes diffuse down their concentration gradients, passing through transport proteins in the basal membranes (of the epithelial cells) into the blood

Molecules reabsorbed from the proximal convoluted tubule during selective reabsorption

• This means no glucose should be present in the urine
• Amino acids, vitamins and inorganic ions are reabsorbed
• This creates a steep water potential gradient and causes water to move into the blood by osmosis
• The concentration of urea in the filtrate is higher than in the capillaries, causing urea to diffuse from the filtrate back into the blood

Selective reabsorption in the proximal convoluted tubule

Reabsorption of water and salts

• As the filtrate drips through the  Loop of Henle  necessary salts are reabsorbed back into the blood by  diffusion and active transport
• As salts are reabsorbed back into the blood,  water  follows by  osmosis
• Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time

Selective reabsorption in the proximal convoluted tubule uses the same method of membrane transport that moves sucrose into companion cells in phloem tissue!As sodium ions move passively down their concentration gradient into the epithelial cells of the proximal convoluted tubule, this provides the energy needed to reabsorb solute molecules (eg. glucose and amino acids) into the epithelial cells, even against their concentration gradients.This is known as indirect or secondary active transport, as the energy (ATP) is used to pump sodium ions, not the solutes themselves.

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Urinary Tract Physiology

The urinary system's primary function is to remove waste products from the blood and remove them from the body in the form of urine. Also known as the urinary tract or excretory system, the typical urinary system includes two kidneys, two ureters, the bladder, and the urethra.

Urine is made in the kidneys in three stages: filtration, reabsorption and secretion. The other parts of the urinary system are responsible for moving the urine out of the body, although in men the urethra (a tube allowing urine to flow from the bladder and out of the genitals) is also involved in ejaculation.

Roles of the Urinary System

In addition to its main function—making and getting rid of urine—the urinary system has other important roles in the body:

• Keeping the blood volume and pressure within normal range
• Controlling blood levels of minerals such as potassium, sodium, chloride, and calcium
• Helping the body stay internally regulated and in balance (a process known as homeostasis)

Composition of Urine

Urine is fluid waste, with color ranging from pale yellow to amber (the darker it is, the more concentrated). It's made up of 95 percent water, with the other 5 percent consisting of:

• Urea: a waste product made when cells break down proteins
• Creatinine: a waste product resulting from the contraction of muscles
• Uric acid: a waste product from the breakdown of nucleic acids (compounds found in molecules like DNA)
• Mineral salts and ions: potassium, sodium, chloride, calcium and others
• Drug byproducts: waste products made when medications are broken down in the body

Anatomy of the Kidney

The two kidneys are each about the size of an adult's fist, and located on either side of the spine near the lower back. Each kidney has an indentation where the following attach:

• Renal artery: carries blood to the kidney
• Renal vein: carries blood away from the kidney
• Ureter: carries urine away from the kidney and to the urinary bladder

The kidney has three layers:

• Outer, light-colored layer (renal cortex)
• >Middle, reddish-brown layer (renal medulla)
• Center layer (renal pelvis), where urine collects before leaving the kidney

Urine is made in microscopic structures in the kidney called nephrons. These extend from the outer to middle layers and have two parts:

• A network of small blood vessels (renal corpuscle)
• A long U-shaped tube (renal tubule) that passes through the renal pyramids in the middle part of the kidney

The renal tubules from different nephrons join together and eventually empty into the renal pelvis in the middle part of the kidney. From here, urine enters the ureters.

Urine Formation

Urine formation occurs in three stages: filtration, reabsorption, and secretion.

Filtration occurs in the renal corpuscles, the small blood vessels in the nephrons. Blood entering the kidney is filtered to form renal filtrate, a combination of blood and waste. From here, most of the fluid and other materials (water, glucose, proteins, vitamins, etc.) will be reabsorbed—a process that happens in the renal tubules, which are the long, thin tubes that pass between the outer and inner layer of the kidneys. But the kidneys also actively secrete certain waste products into the renal filtrate, including ammonia, creatinine, and drug byproducts.

Urine Elimination

Urine collects in the center part of the kidney (the renal pelvis), then the kidney expels the urine through its ureter, the long tube that runs to the bladder. The ureters are lined with smooth muscle that helps propel the urine to the bladder, although gravity does much of the work.

The bladder collects and temporarily stores urine. As it fills up, its wall stretches and when it has amassed around 7 ounces of urine, special receptors in the bladder wall send a signal to the spine. The spine sends another signal back to the bladder, causing the ring of muscle between the bladder and the urethra to relax, allowing urine to leave the bladder and enter the urethra. This causes the urge to urinate.

A ring of muscles at the end of the urethra, the external sphincter, can be controlled consciously. Urination happens only when these muscles are relaxed and the urethra is forced open.

Common Urinary Tract Conditions

One of the most common problems people experience with their urinary tracts is urinary tract infection . Symptoms of UTI include:

• A constant urge to urinate
• A burning sensation while urinating
• Blood in urine
• Cloudy urine
• Unusually strong-smelling urine
• Pelvic pain or rectal pain

Those suspecting they may have a urinary tract infection should consult a physician, as these symptoms can also be indicative of other, more serious urinary tract conditions.

Another common urinary system abnormality is kidney stones . Kidney stones are small mineral deposits that form inside the kidneys. They can be very painful to pass, but typically don't cause any long-term complications.

• Severe pain under the ribs, near the side and back
• Pain while urinating
• Discolored urine
• Persistent, frequent urge to urinate

Kidney stones may be passed simply with the assistance of pain medication and drinking lots of fluids, but in some cases surgery may be required.

UXL Complete Health Resource. The Urinary System. Levchuck CM, McNeil A, Nagel R, et al., eds. 2001.

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Urinary crystals identification and analysis.

Prathap Kumar Simhadri ; Preeti Rout ; Stephen W. Leslie .

Affiliations

Last Update: July 27, 2024 .

• Introduction

Urinary crystal formation is a marker of urinary supersaturation with various substances. Crystals can occur secondary to inherited diseases, metabolic disorders, and medications. The presence of crystals in the urine is not always pathognomic for an abnormal metabolic or renal condition, as uric acid, calcium oxalate, calcium phosphate (CaP), and drug-induced crystals can also be observed under normal physiological conditions. The diagnosis of various monogenetic diseases, such as cystinuria, primary hyperoxaluria, and adenine phosphoribosyl transferase deficiency, can be suggested by examining the urinary sediment and correct identification of the associated urinary crystals.

Urinary crystal formation always precedes renal stone formation but does not always lead to nephrolithiasis. When kidney stones are formed, they have the potential to cause acute kidney injury, obstructive uropathy, hydronephrosis, renal colic, pyonephrosis, and urosepsis. Urinary crystals can also cause renal crystal deposition disease, resulting in acute kidney injury, progressive chronic kidney disease, and end-stage kidney disease.

Urine sediment examination can be considered a liquid biopsy of the kidney because it provides valuable information about the events occurring in an individual's kidney. However, with the increased utilization of automated systems, urine sediment examination is becoming a lost art. Nonetheless, it remains an excellent source of clinical information to help appropriately diagnose and manage patients with various diseases associated with urinary crystal formation. Please see StatPearls' companion resource, " Calcium Deposition and Other Renal Crystal Diseases ," for more information.

• Etiology and Epidemiology

Metabolic factors, inherited diseases, medications, dietary habits, dehydration, and anatomical abnormalities are the common etiologies contributing to urinary crystal formation. Kidney stone formation is a known risk factor for developing diabetes, cardiovascular disease, fractures, and chronic renal disease. Conversely, these conditions also increase the risk of nephrolithiasis.

Kidney stone disease is a polygenetic and multifactorial health problem worldwide that affects between 1% and 13% of the global population and almost 11% of the Western population. [1]  Over the past 30 years, the prevalence of kidney stones has been rising across all ages, genders, racial and ethnic groups globally. Approximately 10% of the United States population experience nephrolithiasis in their lifetime. [2] [3]

Estimating the overall prevalence of urinary crystalluria is challenging, as this condition is underdiagnosed and underreported. Although scant literature is available on the prevalence of urinary crystalluria, several studies showed crystalluria in approximately 8% of urine collections. Calcium oxalate, uric acid, and amorphous urate crystals accounted for most of the crystals encountered. Brushite, ammonium biurate, and cystine crystals were rare. [4] [5] [6]

• Pathophysiology

Urine contains both promoters and inhibitors of crystal formation. Crystallization occurs when the concentration of crystal promoters in the glomerular filtrate exceeds the kidney's ability to keep them as soluble molecules. Stone inhibitors include citrate; uromodulin, also known as Tamm-Horsfall protein; magnesium; glycosaminoglycans; and pyrophosphate. Some chemical mediators decrease the urinary crystal's growth, aggregation, and adhesion to the tubular epithelium and the tubular cell; examples are osteopontin, matrix Gla protein, Tamm-Horsfall protein, and urinary fragment 1 of prothrombin. [7] [8]

Calcium, oxalate, urate, and phosphate ions are the leading promoters of crystal formation. Physiological factors influencing kidney stone formation include high urinary osmolarity or low urine volume, bacteria in the urinary tract, and urine pH. Structural factors influencing nephrolithiasis include urinary reflux, instrumentation, and genetic abnormalities of the urinary tract. [8] Please see StatPearls' companion resource, " Renal Calculi, Nephrolithiasis ," for more information.

• Specimen Requirements and Procedure

A first-morning urine sample (void after awakening) is ideal for crystal analysis as it tends to be the most concentrated for crystal formation. The urine sample should be brought to the lab and processed within 2 hours of collection. Uric acid and phosphorous crystallization can occur if the urine stays stagnant for longer than 2 hours. Refrigeration of the urine specimen is necessary after 2 hours, and examination within 8 hours of collection is recommended, even when refrigerated. [6] [9]

Urine analysis through automated and manual methods can provide significant clues to the type and characteristics of urinary crystals. When examining the sediment, urine pH, specific gravity, and a dipstick analysis should always be recorded. Of note, casts degrade quickly in alkaline urine. Red blood cells (RBCs) and white blood cells (WBCs) can swell in low osmolality and shrink in high osmolality. [10]  

Urine pH plays a key role in crystallization, as CaP, struvite, and ciprofloxacin crystals precipitate in alkaline urine. In contrast, uric acid, cystine, and other drug-induced crystals tend to precipitate in acidic urine. These characteristics assist clinicians in accurately identifying urinary crystals. [6]

For microscopic examination, 10 mL of urine should be centrifuged at 1500 to 3000 revolutions per minute for at least 5 min. The sediment is obtained through a pipette after discarding the top 9.5 cc of supernatant and gentle manual agitation of the test tube. A single drop of urine sediment is placed on the glass slide and covered with a cover slip for direct observation. The sediment is examined using a standard brightfield or phase contrast microscope at low and high magnification. A minimum of 10 fields should be reviewed at each power. The specimen is then examined under polarization to determine birefringence and identify other detailed characteristics. [6]  

Automated urinary analyzers also often miss crystals when compared to manual microscopic evaluation. [11]  Off note, a recent study found significant interoperator variability identifying urinary sediment components, even among experienced nephrologists. Therefore, when possible, a personal examination of urine sediment is preferable. [12]

• Diagnostic Tests

A comprehensive workup is essential for diagnosing the etiology of urinary crystals. This approach includes ob t aining a detailed history and performing a thorough physical examination, and may also include x-ray KUB, renal or abdominal ultrasound, and plain computed tomography (CT) of the abdomen and pelvis to help localize the renal stones. Simple urinalysis and analysis of the urinary crystals can help understand a stone's chemical composition. A chemical analysis of a passed stone can give a definitive answer. [7] [13]

Crystalluria indicates supersaturation of the components in the urine, leading to precipitation of the crystals. However, supersaturation alone is insufficient to cause nephrolithiasis; other inciting factors are also typically present. The process can be related to metabolic disorders, inherited diseases, drug use, or toxins. The initial diagnostic test is generally a 24-hour urine collection evaluating the following parameters—urinary volume, pH, calcium, citrate, creatinine, magnesium, phosphate, uric acid, sodium, serum calcium, oxalate, and uric acid levels. The collection should ideally be performed in nonacute conditions at the patient's home with their normal diet and fluid intake for optimal information. Additional workup is typically guided by the type of crystal identified and patient history. [7] [8] [10] [13] Please see StatPearls' companion resources, " 24-Hour Urine Collection ," and " 24-Hour Urine Testing for Nephrolithiasis: Interpretation and Treatment Guidelines ," for more information.

Further Diagnostic Testing Based on Stone Composition

Calcium oxalate:  Calcium oxalate stones comprise up to 80% of all nephrolithiasis cases. Calcium oxalate and CaP often coexist in stones, but 1 component is typically primary. Patients with calcium oxalate crystals should be considered for primary and secondary causes of hyperoxaluria. Please see StatPearls' companion resource, " Calcium Deposition and Other Renal Crystal Diseases ," for more information. Calcium oxalate dihydrate (COD) crystals are also highly associated with hypercalcemia, which should also be considered. Calcium oxalate crystalluria is not necessarily pathologic, as these crystals can sometimes be found in patients with high oxalate diets. [6]  

Hyperoxaluria of >75 mg/d in a 24-hour urine collection suggests possible hereditary hyperoxaluria (types I, II, or III).

Calcium phosphate: CaP nephrolithiasis is the second most common type, representing about 10% to 20% of kidney stones. The presence of these stones should prompt evaluation for conditions causing hypercalciuria, hyperphosphaturia, and alkaline urine formation. These crystals can also be found under normal physiological conditions.

Struvite:  Struvite comprises ammonium, magnesium, and calcium binding with phosphate (triple phosphate). The presence of struvite stones, even in lower quantities, is almost always associated with infection by urease-producing microorganisms, such as Proteus mirabilis , Klebsiella pneumonia , Staphylococcus aureus , Pseudomonas aeruginosa , Providencia  stuartii , Serratia , and  Morganella morganii . [8]

Uric acid:  Uric acid dihydrate crystals primarily depend on low urine pH, whereas the amorphous ones are mainly related to high urinary urate concentration. Uric acid crystals are only found when the urinary pH is acidic, as uric acid becomes much more soluble as the pH increases. [8] [9] [14]  The presence of uric acid dihydrate crystals should prompt the workup for conditions causing impaired renal ammoniogenesis, such as metabolic syndrome and type 2 diabetes. [10]

Xanthine:  Xanthine crystals are found in xanthinuria, a rare genetic disorder with autosomal recessive pattern inheritance caused by the deficiency of the enzyme xanthine dehydrogenase. Not all laboratories can test for xanthine and hypoxanthine, nor are these part of a routine urine collection. If this rare disorder is suspected, this test must be ordered specifically. Biopsy of the gastrointestinal tract to quantify xanthine dehydrogenase is rarely performed. [15]  An allopurinol loading test can also be used to detect a lack of xanthine oxidase, as allopurinol is abnormally metabolized in patients with hereditary xanthinuria; however, these measurements may not be available in all laboratories. [16] Xanthinuria is also associated with hypouricemia and hypouricosuria. [17]

Patients with 2,8-dihydroxyadenine (DHA) crystalluria, cystinuria, and suspected primary hyperoxaluria can also be genetically tested to confirm the diagnosis. [13] [18]

Please see StatPearls' companion resources, " Calcium Deposition and Other Renal Crystal Diseases ," " Hyperoxaluria ,"  " Uric Acid Nephrolithiasis ," " Struvite and Triple Phosphate Renal Calculi ," and " Cystinuria ," for more information.

• Testing Procedures

Effective testing procedures for urine crystal identification and analysis are crucial for accurate diagnosis and patient care. Techniques commonly used for crystal analysis in clinical practice include the following:

• Standard light microscopy
• Optical polarization microscopy
• Automated microscopy
• Flow cytometry
• Digitalized microscopy
• Scanning electron microscopy

Spectroscopy

Standard Light Microscopy

Manual direct microscopic examination is the gold standard technique used in clinics to identify various urinary crystals based on their morphology. When sufficiently trained and equipped, nephrologists can acquire important diagnostic information through direct microscopy. Brightfield microscopy is more commonly used compared to phase contrast microscopy, but phase contrast microscopy may be more beneficial for analyzing urine sediment. [6] [19]

Optical Polarization Microscopy

Polarized light microscopy is used to observe the color, morphology, and birefringence of crystals for identification. The color, refraction of the light, and double refraction are the parameters used in mineral identification. However, contaminants can sometimes distort or interfere with the interpretations of results. [10]

Automated Microscopy

Automated microscopy is a method used in laboratory practice that does not require urine centrifugation or standardized microscope measuring parameters. These machines are based on the Colter principle and became popular in hematology laboratory procedures in 1954 for the rapid and accurate enumeration of cells in the blood. They have been used for urine microscopy since the 1990s. Automated microscopy has been expanded to laser-based flow cytometry for better cell characterization.  

This technique can utilize a large sample size, reducing the laboratories' workload. Automated microscopy can deliver a complete urine analysis result if urinary biochemical data are combined and integrated with the microscopy analysis. The practical approach is to use automated and manual machines to survey many specimens and expert microscopic examination for selected specimens. Automated systems have significant advantages over manual microscopy, as the specimens do not need centrifugation. They are also highly effective in diagnostic screening for blood, bacteria, and WBCs. However, automated systems significantly underperform in recognizing urinary casts and crystals compared to manual examinations performed by experienced lab technicians, pathologists, and nephrologists. [11] The 2 major types of automated microscopy techniques are as follows:

Flow cytometry:

• These machines use laser-based flow cytometry with fluorescent dyes. Flow cytometry can reduce the need for microscopy or culture and is more useful in screening bacteriuria, pyuria, and hematuria, although sometimes microscopy is still required.
• This machine produces scattergrams, not actual images. Microscopy is required to differentiate similar particles, such as various types of crystals, dysmorphic or isomorphic RBCs, and different types of epithelial cells. The advantage of this machine is that minimum expertise is required to operate it, and approximately 100 samples can be tested in 1 hour. [10] [20]

Digitalized microscopy:

• The digitalized microscopy technique uses computerized software to analyze digitalized monochrome images of urine particles. Two different instruments that supply digitized images are available. One is based on automated intelligent microscopy, which shows particles in the samples by categories such as RBCs and WBCs. The other uses cuvette-based microscopy, which displays particles in the same way they appear in the microscopic field during manual examination.
• A significant advantage of this technique is that it can be used for many samples and selective individual specimens. The images can be stored on a computer and sent to different sites, including other laboratories and clinics, for review by pathologists or nephrologists. In addition, this method is relatively cheaper compared to flow cytometry. [10] [20]

Scanning Electron Microscopy

Scanning electron microscopy produces high-resolution images of the crystal surface, aiding in studying the morphology and texture of urinary calculi. This method can analyze the characteristics of stones ranging from 1 mm to 5 mm without changing the morphology of their components, and it does not destroy the stone sample. This method is also a proposed tool for diagnosing primary hyperoxaluria.

Spectroscopy identifies the molecular structure of a sample by investigating its interaction with electromagnetic radiation. In this technique, infrared light passes through the sample, and the detector analyzes the intensity of the transmitted light. Vibrational spectroscopy identifies a sample's composition and molecular structure through its vibrational characteristics when interacting with a beam source. This technique is analyzed based on the sample's vibrational characteristics when interacting with electromagnetic waves. Other spectroscopy methods, such as Raman and Fourier transform-infrared spectroscopy, are increasingly used in clinical practice. [14]

These rapid, nondestructive, and relatively inexpensive methods have been increasingly used in recent years to identify the chemical nature of urinary crystals and detect organic matter. Infrared spectroscopy is extremely useful in identifying calcium oxalate crystals. [21]

Overall, clinicians should be able to identify most crystals by checking urine pH and urinalysis, then examining the urine sediment under bright field, phase contrast, and polarizing light microscopy. Fourier transform-infrared and Raman spectroscopy and solubility testing can be additional resources for unclear crystal composition. [20] [22]

• Results, Reporting, and Critical Findings

The presence of urinary crystals associated with active urinary sediment, acute kidney injury, or kidney stones suggests a pathogenic process. Identifying and characterizing urinary crystals are essential for diagnosing the type of stone, especially when the actual stone is not available for analysis (see Table 1. Characteristics of Commonly Observed Urinary Crystals). The results of urinary crystal analysis are reported after identifying various characteristics of urinalysis and microscopic examination of the urine sediment.

Commonly Reported Urinary Crystals

Calcium oxalate crystals:  There are 2 significant types of calcium oxalate crystals—monohydrate and dihydrate. The calcium oxalate monohydrate (COM) crystals are typically colorless on light microscopy. Their shape varies from oval, biconvex, and dumbbells to elongated rods (see Image.  Calcium Oxalate Monohydrate Crystals). These crystals are strongly birefringent on polarized microscopy.

COD crystals are colorless on light microscopy and are bipyramidal or envelope-shaped, and their size is variable (see Image. Calcium Oxalate Dihydrate Crystals). These crystals are non-birefringent on polarization. Calcium oxalate crystallization is commonly observed in the urine pH range of 5.0 to 6.5 and is rarely observed when the urine pH increases to >7.0. A rare third form of calcium oxalate crystals, calcium oxalate trihydrate or caoxite, is hexagonal and seldom found in urine. [9] [14]

Calcium phosphate crystals:  There are calcium orthophosphate crystals, mainly amorphous carbonated CaP and carbapatite, whereas brushite is dicalcium phosphate dihydrate. 

CaP crystals are colorless on light microscopy. Calcium orthophosphate crystals are amorphous crystals that appear as small granulations with or without large plates and have no polarization (see  Image.  Amorphous Phosphate and Triple Phosphate Crystals in the Urine). Brushite crystals have a broad spectrum of shapes, such as prisms, rosettes, starts, needle-like or splinter-like, or sticks or rods (see Image. Calcium Phosphate Crystals). These crystals are strongly birefringent on polarized microscopy and precipitate in alkaline urine. [9] [14]

Struvite or triple phosphate crystals:  These are radioopaque magnesium ammonium phosphate hexahydrate or triple phosphate crystals. Struvite crystals are colorless under light microscopy. The most common shape observed is a coffin lid; the other shapes are feather-like structures, elongated prisms, and trapezoids. These crystals are weak to strongly birefringent on polarization and precipitate in alkaline urine (see Images. Triple Phosphate Crystals; and Amorphous Phosphate and Triple Phosphate Crystals in the Urine). [9] [14]

Cystine crystals:  Cystine crystals are colorless on light microscopy and are either found alone or heaped onto each other, resembling perfect hexagonal plates. These crystals show weak birefringence on polarized microscopy and precipitate in urine with an acidic pH (see Image. Cystine Crystals in the Urinary Sediment). [18]  Please see StatPearls' companion resource, " Cystinuria ," for more information.

Uric acid and urate crystals:  There are 4 types of uric acid crystals in urine—amorphous, anhydrous uric acid (uricite), monohydrate, and dihydrate crystals. Among these, uric acid dihydrate and amorphous uric acid crystals are most commonly observed. [9] [14]

Uric acid crystals are often amber-colored on light microscopy and have various shapes, such as rho boards, barrels, rosettes, 4-sided plates, needles, rounds, or parallelograms. These crystals show a very strong polychromatic birefringence on polarization (see  Images. Uric Acid Crystals, Barrel-Shaped; Uric Acid Crystals, Rosette-Shaped; and   Uric Acid Crystals, Birefringent on Polarized Light). These crystals precipitate in urine with an acidic pH. [9] [14]

Xanthine crystals:  Xanthine crystals are nonspecific in appearance and can be observed as polarized granules or sticks without a characteristic aspect. Crystal identification requires infrared spectroscopy, high-pressure liquid chromatography, or chemical analysis of a xanthine stone. These crystals precipitate in urine with an acidic pH. [9] [14]

Dihydroxyadenine crystals: DHA crystals are somewhat brown or reddish-brown with a dark perimeter and central spicules that are round in shape on light microscopy. These crystals turn yellow and may demonstrate a characteristic birefringent Maltese-cross pattern on polarized microscopy. In suspected 2,8-DHA crystalluria, it is mandatory to investigate the crystals using infrared spectroscopy on a centrifuged pellet. These crystals precipitate in urine with an acidic or alkaline pH (4.8 to 9.0) and are relatively insoluble at any urinary pH. [9] [14]

Table 1. Characteristics of Commonly Observed Urinary Crystals.

Drug-Induced Crystals  

Drug-induced urinary crystals are variable in size, shape, and other characteristics depending on the specific medication (see Table 2. Characteristics of Various Drug-Induced Crystals). These rare crystals comprise only 1% to 2% of all nephrolithiasis (see Images.  Sulfamethoxazole Crystals;   Sulfadiazine Crystals; and   Acyclovir Crystals). [7]

Table 2. Characteristics of Various Drug-Induced Crystals.

Rare Crystals

Tyrosine crystals are colorless, thin, and needle-shaped, whereas leucine crystals are oval or round with circular striations. Both these crystals are found in acidic urine and are associated with aminoaciduria secondary to liver disease. Bilirubin crystals, which are clumped needles or yellow granules, are highly birefringent on polarization, and they are observed in conditions associated with hyperbilirubinemia. Cholesterol crystals are thin, transparent plates with cut edges. They are highly birefringent on polarization, typically observed in association with nephrotic syndrome. [9] [14]

Solubility Testing

The solubility test involves exposing the crystals to 10% KOH, 30% acetic acid, 30% HCL, and chloroform. The solubility of crystals in these fluids provides additional clues that aid in crystal identification (see Table 3. Solubility Characteristics of Crystals). This test is beneficial in identifying crystals with similar shapes. [9] [14]  

Table 3. Solubility Characteristics of Crystals.

Interfering Factors 

Identifying urinary crystals can be challenging due to similarities in their shapes. Calcium sulfate, CaP, and sodium urate crystals can be needle-shaped or rectangle-shaped with blunt ends. COM, hippuric acid, uric acid, and ammonium biurate crystals can be rectangles with pointed ends. Amorphous urate and phosphate granules can have similar minor granular appearances. Solubility tests can help differentiate urinary crystals with similar shapes. [14] Key considerations include:

• Free hemosiderin granules are yellow-brown coarse granules that can be confused with amorphous phosphate and urate granules.
• 2,8-DHA crystals can be confused with ammonium hydrogen urate as both have a small spherical shape.
• Starch granules are oval or round and highly refractive. Their irregular central indentation can resemble molar tooth grooves.
• Under polarized light, starch granules can appear similar to a Maltese cross and be confused with 2,8-DHA crystals.
• Cholesterol and leucine crystals can also exhibit a typical Maltese-cross appearance under polarized light.
• Clinical Significance

Renal stone formation is always preceded by crystalluria, although the presence of urinary crystals does not always result in nephrolithiasis. Investigating crystalluria is crucial for diagnosing and managing acute and chronic renal failure due to crystal precipitations associated with various inherited and acquired diseases. Identifying the type of urinary crystals is also helpful in offering specific targeted treatment for nephrolithiasis.

Calcium Oxalate Crystals  

Calcium oxalate stones comprise up to 80% of all nephrolithiasis. Patients with calcium oxalate crystals should be considered for primary and secondary causes of hyperoxaluria. Please see StatPearls' companion resource, " Calcium Deposition and Other Renal Crystal Diseases ," for more information. Calcium oxalate crystalluria is not necessarily pathologic, as these crystals can sometimes be found in those with high oxalate diets. [6]  As an overview, calcium monohydrate crystals are generally associated with high urinary oxalate levels, calcium dihydrate crystals are generally associated with high urinary calcium, and the rare dodecahedral calcium dihydrate crystals are associated with very high urinary calcium.

Secondary hyperoxaluria is much more common compared to primary hyperoxaluria and is often related to 1 of 3 mechanisms as follows:

• Increased dietary intake of oxalate-containing foods, such as peanuts, cashews, almonds, spinach, rhubarb, star fruit, chaga mushroom powder, and black iced tea.
• Increased intestinal absorption of oxalate in a setting of fat malabsorption (enteric hyperoxaluria secondary to intestinal bypass surgery, short gut syndrome, Crohn disease, celiac disease, chronic pancreatitis, and orlistat).
• Increased intake of oxalate precursors, such as ethylene glycol and vitamin C, can develop increased gut oxalate absorption, leading to excess urinary oxalate excretion. These patients are also at high risk for developing calcium oxalate crystalluria, crystalline nephropathy, chronic kidney disease, and end-stage kidney disease. Ingestion of the toxic substance, ethylene glycol, results in severe hyperoxaluria, and a very peculiar kind of monohydrate crystals with elongated and narrow hexagonal shapes similar to diamonds are found in the urine. [9]

COD crystals are typically associated with hypercalciuria, such as hyperparathyroidism; hyperthyroidism; multiple myeloma; paraneoplastic syndrome; bone metastasis; vitamin D toxicity; particular drug use, including loop diuretics and vitamin-D supplements; and milk-alkali syndrome.

According to Daudon et al, over 94% of urine specimens containing calcium oxalate crystals with a calcium-to-oxalate ratio of <5 show either monohydrate crystals alone or along with dihydrate crystals. In contrast, over 90% of urine specimens containing calcium oxalate crystals with a calcium-to-oxalate ratio >14 contain dihydrate crystals alone. Dodecahedral COD crystals typically indicate severe hypercalciuria, >7 mmol/L. All types of calcium-containing crystals are radioopaque. [8] [9]

Calcium Phosphate Crystals

Carbapatite crystals are typically observed in patients with hypercalciuria on a background urine pH of 5.8 to 6.5. In contrast, amorphous CaP crystals are typically observed in patients with near-normal calcium and phosphate concentrations with a pH >6.6. [14]

Amorphous CaP crystals suggest a possible environment created by urinary tract infections or metabolic disorders keeping the urine alkaline. When associated with the calcium monohydrate crystals, they indicate the presence of medullary sponge kidneys and other causes related to urinary stasis. When carbapatite crystals are associated with struvite crystals, they indicate the presence of urea-splitting organisms causing urinary tract infections.

Brushite crystals are made of dicalcium phosphate dihydrate ions. These crystals are mainly observed in conditions associated with marked hypercalciuria and hyperphosphaturia and are generally favored by low concentrations of citrates. The presence of >500/mL brushite crystals should prompt an evaluation for primary hyperparathyroidism. CaP crystals are radioopaque. [8] [9] [14]

Struvite Crystals

Struvite crystals are associated with high ammonium content and precipitate in alkaline urine. These crystals are always associated with urinary tract infections by urea-splitting bacteria, where the bacteria hydrolyze the urea, producing ammonia and leading to alkaline urine. These bacteria are Klebsiella , Pseudomonas , Proteus , Providencia , and some species of Staphylococcus . These are radioopaque, sometimes large enough to be called staghorn stones. [8] [9] [14]

Cystine Crystals  

Cystine crystals are associated with hereditary cystinuria, in which a proximal tubular dysfunction leads to increased excretion of dibasic amino acids such as cystine, ornithine, lysine, and arginine. Cystinuria is an autosomal recessive Mendelian disorder, and there are reports of acquired cystinuria post-renal transplant. Cystine forms radiolucent stones, which are not detected on x-rays but can be detected by ultrasound if the stones are large enough. CT scan is the gold standard for diagnosing renal stones and can detect cystine stones. Cystine becomes more soluble as the pH increases. The therapeutic urinary pH level to maximize solubility is ≥7.5, typically higher compared to uric acid. [23]

Uric Acid Crystals  

Uric acid crystals are associated with medical conditions leading to increased uric acid production or altered uric acid metabolism, such as gout, Lesch-Nyhan syndrome, tumor lysis, chemotherapy, or conditions associated with decreased urine pH, such as chronic diarrhea, metabolic syndrome, and diabetes mellitus. However, over half of patients with uric acid stones do not have hyperuricemia. These crystals form radiolucent renal stones that are not detectable on x-rays but can be visualized on CT and ultrasound. [6]

Xanthine Stones  

Xanthine crystals are found in xanthinuria, a rare genetic disorder with autosomal recessive pattern inheritance caused by the deficiency of the enzyme xanthine dehydrogenase. This disorder leads to xanthine accumulation and highly insoluble xanthine excretion. Xanthinuria is associated with hypouricemia and hypouricosuria. [17]  Rarely, xanthinuria can occur related to allopurinol use for Lesch-Nyan syndrome or tumor lysis syndrome. [24]  More recently, the xanthine oxidase inhibitor, febuxostat, has also been noted to cause xanthinuria when used for tumor lysis syndrome. [25]

2,8-Dihydroxyadenine Crystals  

2,8-DHA crystalluria is secondary to adenine phosphoribosyltransferase (APRT) deficiency, an autosomal recessive condition leading to excess DHA production, 2,8-DHA crystalluria, and kidney stones. The condition is underdiagnosed in the United States, with fewer than 300 cases reported in the medical literature. The worldwide prevalence is 1/5000 to 1/100,000 population. A significant number of patients remain asymptomatic, whereas 90% of patients develop urinary crystals and radiolucent renal stones. In addition, 10% of patients with 2,8-DHA crystalluria progress to end-stage renal failure. The diagnosis is confirmed by kidney stone analysis, histologic findings of crystal nephropathy, genetic testing, or lack of APRT activity in RBC lysate. Treatment is recommended with allopurinol or febuxostat, which blocks the conversion of adenosine to DHA. The dosage should be titrated to eliminate 2,8-DHA crystalluria. [26] [27]

Drug-induced crystals are formed in association with the use of various drugs, as previously described. These crystals precipitate in the urine when used at a high dose, for long durations, or both. They form radiolucent renal stones when they contain no calcium or other radioopaque constituents. Patients experiencing signs and symptoms of nephrolithiasis while using these drugs should prompt the search for renal stones by ultrasound or contrast-enhanced CT imaging for diagnosis. Analyzing these crystals is crucial for identifying at-risk patients and clarifying the drugs or medications involved. [28]

Follow-Up   Monitoring

The complete disappearance of crystalluria is the best indicator of improved lithogenic activity in CaP, uric acid, and other common forms of nephrolithiasis. However, high-risk patients, such as those with cystinuria and PH1, can have active urinary crystallization even when the disease is under reasonable control. 

Global crystal volume (GCV) helps determine lithogenicity in patients with crystalluria. GCV   is the product of the number of crystals/mm 3  multiplied by the average size of crystals (in μm) and by a numerical factor given for each crystal species (the numerical number is provided based on the geometrical form of a crystal). GCV of <500 μm 3 /mm 3 is associated with the lowest risk for tubular obstruction and crystalline nephropathy in patients with PH1. In patients with cystinuria, a GCV of >3000 μm 3 /mm 3  is associated with an increased risk for recurrent nephrolithiasis. [9]

• Enhancing Healthcare Team Outcomes

Clinical outcomes can be significantly improved by emphasizing the importance of urine sediment examination and ensuring that medical students, residents, physician assistants, nurse practitioners, laboratory technicians, and other healthcare professionals receive comprehensive training on its routine application.

The preanalytical phase is the most vulnerable part of the evaluation, accounting for up to 75% of all laboratory errors. Outcomes can be improved by following appropriate protocol-based patient preparation, obtaining mid-stream urine samples, rapid sample transportation, and preparation techniques. Nursing, lab processing, and transporting staff are crucial in these collection steps. Physicians must often collect samples shortly before examination for the optimal specimens. Effective communication among team members is paramount. Conscientious laboratory staff are paramount in the accurate processing and calibration of microscopic and analytic equipment. Clear and open communication facilitates rapid diagnosis and treatment decisions, preventing errors and ensuring a coordinated interprofessional team response.

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Calcium Oxalate Monohydrate Crystals. The calcium oxalate monohydrate crystals are typically colorless on light microscopy, with shapes that vary from oval and biconvex to dumbbell and elongated rod forms. Contributed by R Naik, MD, PhD

Calcium Oxalate Dihydrate Crystals. The calcium oxalate dihydrate crystals are colorless on light microscopy and have a bipyramidal or envelope shape. Contributed by R Naik, MD, PhD

Calcium Phosphate Crystals. Calcium phosphate crystals are colorless on light microscopy. Brushite crystals have a broad spectrum of shapes, such as prisms, rosettes, starts, needle-like or splinter-like, sticks, or rods. Contributed by R Naik, (more...)

Triple Phosphate Crystals. Struvite crystals are colorless under light microscopy and often appear in distinctive coffin-lid shapes. Other shapes, such as feather-like structures, elongated prisms, and trapezoids, can also be observed. Contributed by (more...)

Amorphous Phosphate and Triple Phosphate Crystals in the Urine. Calcium orthophosphate crystals are amorphous crystals that appear as small granulations with or without large plates and have no polarization. Contributed by M Zubair, MBBS, FCPS

Uric Acid Crystals, Barrel-Shaped. The image shows barrel-shaped uric acid crystals on light microscopy. Contributed by R Naik, MD, PhD

Uric Acid Crystals, Rosette-Shaped. The image shows rosette-shaped uric acid crystals on light microscopy. Iqbal Osman, Public Domain, via Wikimedia Commons

Uric Acid Crystals, Birefringent on Polarized Light. Uric acid crystals show a strong polychromatic birefringence on polarization. Contributed by S Bhimji, MD

Sulfamethoxazole Crystals. Urine sediment without staining ×400. Left: bright field light microscopy; right: polarized light microscopy. Seltzer J, Velez JC, Buchkremer F, Poloni JAT. Urine sediment of the month: drugs & crystalluria. (more...)

Sulfadiazine Crystals. Unstained urine sediment ×100. Left: bright field light microscopy; right: polarized light microscopy. Seltzer J, Velez JC, Buchkremer F, Poloni JAT. Urine sediment of the month: drugs & crystalluria . Renal (more...)

Acyclovir Crystals. Left: light microscopy unstained; right: polarized light with birefringence. Seltzer J, Velez JC, Buchkremer F, Poloni JAT. Urine sediment of the month: drugs & crystalluria. Renal Fellow Network Web site. https://www.renalfellow.org/2020/05/28/urine-sediment-of-the-month-drugs-crystalluria/. (more...)

Disclosure: Prathap Kumar Simhadri declares no relevant financial relationships with ineligible companies.

Disclosure: Preeti Rout declares no relevant financial relationships with ineligible companies.

Disclosure: Stephen Leslie declares no relevant financial relationships with ineligible companies.

This book is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits others to distribute the work, provided that the article is not altered or used commercially. You are not required to obtain permission to distribute this article, provided that you credit the author and journal.

• Cite this Page Simhadri PK, Rout P, Leslie SW. Urinary Crystals Identification and Analysis. [Updated 2024 Jul 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.

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Homogeneous solvent-based microextraction method (HSBME) using a task-specific ionic liquid and its application to the spectrophotometric determination of fluoxetine as pharmaceutical pollutant in real water and urine samples

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• Published: 23 August 2024

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• Mehdi Hosseini   ORCID: orcid.org/0000-0001-5999-5178 1 ,
• Aram Rezaei 2 &
• Mousa Soleymani 1  

A homogeneous solvent-based microextraction (HSBME) method based on the use of a task-specific pyridinium-based ionic liquid (TSIL) was described, and its application for preconcentration prior to the determination of fluoxetine in real water, tablet and urine samples was evaluated. To demonstrate this, an ionic liquid of [1-(2-hydroxy-3-(isopropylamino)propyl)pyridinium chloride], abbreviated SIL/Cl, was firstly synthesized, characterized using proton nuclear magnetic resonance ( 1 HNMR) and fourier transform infrared spectroscopy (FTIR) techniques, and subsequently used for the determination of fluoxetine. In this method, SIL/Cl is initially completely miscible in aqueous phase, forming a homogeneous system in the aqueous phase containing fluoxetine, ensuring there are no boundaries between fluoxetine and the extracting agent (SIL/Cl). The parameters affecting microextraction procedure were studied and optimized. Under optimal conditions, the following parameters were obtained: a limits of detection (LOD) of 2.35 µg/L, a limit of quantification (LOQ) of 7.84 µg/L, a linear dynamic range (LDR) of 5.0–250.0 µg/L, intra-day and inter-day relative standard deviations (RSDs) ( n  = 7, 50 µg/L) of 2.26% and 2.94%, preconcentration factor of 100 and enhancement factor of 73, respectively. Finally, to evaluate the method's ability to determine fluoxetine in several real samples, it was validated using the standard addition method, with recovery ranges from 98.0 to 103.0%.

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Research councils of Kermanshah University of Medical Sciences and Biosensor and Energy Research Center, Ayatollah Boroujerdi University are acknowledged for financial support of this work.

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Hosseini, M., Rezaei, A. & Soleymani, M. Homogeneous solvent-based microextraction method (HSBME) using a task-specific ionic liquid and its application to the spectrophotometric determination of fluoxetine as pharmaceutical pollutant in real water and urine samples. Chem. Pap. (2024). https://doi.org/10.1007/s11696-024-03660-7

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DOI : https://doi.org/10.1007/s11696-024-03660-7

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