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Human digestive system interactive

The lips and cheeks, the roof of the mouth, the floor of the mouth.

• Salivary glands
• Blood and nerve supply
• Stomach contractions
• Gastric mucosa
• Gastric secretion
• Absorption and emptying
• Contractions and motility
• Rectum and anus
• Gross anatomy
• Microscopic anatomy
• Acinar cells
• Islet cells
• General features of digestion and absorption
• Carbohydrates
• Fat-soluble vitamins
• Vitamin B 12
• Intestinal gas
• Production and secretion of peptides
• Somatostatin
• Cholecystokinin
• Gastric inhibitory peptide
• Intestinal glucagon
• Neurotensin
• Pancreatic polypeptide
• Vasoactive intestinal peptide
• Substance P
• Endorphins and enkephalins
• Prostaglandins
• The gastrointestinal tract as an organ of immunity
• Embryonic development
• Evolutionary development

human digestive system

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• Mayo Clinic - See How Your Digestive System Works
• Healthline - Digestive Health Basics
• LiveScience - Digestive System: Facts, Function and Diseases
• Patient - The digestive system
• Cleveland Cliinic - Digestive System
• National Institute of Diabetes and Digestive and Kidney Diseases - Your Digestive System & How it Works
• WebMD - The Digestive System: How It Works
• University of Minnesota Libraries - Human Biology - The Digestive System
• digestive system - Children's Encyclopedia (Ages 8-11)
• digestive system - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

• What are the main organs involved in the human digestive system?
• How does the process of digestion begin in the mouth?
• What role does the stomach play in digestion?
• How do the small and large intestines contribute to the digestive process?
• How does the digestive system interact with other body systems?
• What is the function of the liver in the digestive process?
• How does the pancreas contribute to digestion?
• What are common digestive disorders and their causes?
• How does the nervous system regulate digestion?

human digestive system , system used in the human body for the process of digestion . The human digestive system consists primarily of the digestive tract , or the series of structures and organs through which food and liquids pass during their processing into forms that can be absorbed into the bloodstream. The system also consists of the structures through which wastes pass in the process of elimination and of organs that contribute juices necessary for the digestive process.

In order to function properly, the human body requires nutrients. Some such nutrients serve as raw materials for the synthesis of cellular materials, while others help regulate chemical reactions or, upon oxidation, yield energy. Many nutrients, however, are in a form that is unsuitable for immediate use by the body; to be useful, they must undergo physical and chemical changes, which are facilitated by digestion.

Structures and functions of the human digestive system

The digestive tract begins at the lips and ends at the anus . It consists of the mouth , or oral cavity, with its teeth , for grinding the food, and its tongue , which serves to knead food and mix it with saliva ; the throat, or pharynx ; the esophagus ; the stomach ; the small intestine , consisting of the duodenum , the jejunum, and the ileum ; and the large intestine , consisting of the cecum , a closed-end sac connecting with the ileum, the ascending colon, the transverse colon, the descending colon, and the sigmoid colon , which terminates in the rectum . Glands contributing digestive juices include the salivary glands , the gastric glands in the stomach lining, the pancreas , and the liver and its adjuncts—the gallbladder and bile ducts. All of these organs and glands contribute to the physical and chemical breaking down of ingested food and to the eventual elimination of nondigestible wastes. Their structures and functions are described step by step in this section.

Mouth and oral structures

Little digestion of food actually takes place in the mouth. However, through the process of mastication , or chewing, food is prepared in the mouth for transport through the upper digestive tract into the stomach and small intestine, where the principal digestive processes take place. Chewing is the first mechanical process to which food is subjected. Movements of the lower jaw in chewing are brought about by the muscles of mastication (the masseter, the temporal , the medial and lateral pterygoids, and the buccinator). The sensitivity of the periodontal membrane that surrounds and supports the teeth, rather than the power of the muscles of mastication, determines the force of the bite.

Mastication is not essential for adequate digestion. Chewing does aid digestion, however, by reducing food to small particles and mixing it with the saliva secreted by the salivary glands. The saliva lubricates and moistens dry food, while chewing distributes the saliva throughout the food mass. The movement of the tongue against the hard palate and the cheeks helps to form a rounded mass, or bolus , of food.

The lips, two fleshy folds that surround the mouth, are composed externally of skin and internally of mucous membrane , or mucosa. The mucosa is rich in mucus-secreting glands, which together with saliva ensure adequate lubrication for the purposes of speech and mastication.

The cheeks, the sides of the mouth, are continuous with the lips and have a similar structure. A distinct fat pad is found in the subcutaneous tissue (the tissue beneath the skin) of the cheek; this pad is especially large in infants and is known as the sucking pad. On the inner surface of each cheek, opposite the second upper molar tooth, is a slight elevation that marks the opening of the parotid duct, leading from the parotid salivary gland, which is located in front of the ear. Just behind this gland are four to five mucus-secreting glands, the ducts of which open opposite the last molar tooth.

The roof of the mouth is concave and is formed by the hard and soft palate . The hard palate is formed by the horizontal portions of the two palatine bones and the palatine portions of the maxillae, or upper jaws. The hard palate is covered by a thick, somewhat pale mucous membrane that is continuous with that of the gums and is bound to the upper jaw and palate bones by firm fibrous tissue. The soft palate is continuous with the hard palate in front. Posteriorly it is continuous with the mucous membrane covering the floor of the nasal cavity. The soft palate is composed of a strong, thin, fibrous sheet, the palatine aponeurosis, and the glossopalatine and pharyngopalatine muscles. A small projection called the uvula hangs free from the posterior of the soft palate.

The floor of the mouth can be seen only when the tongue is raised. In the midline is a prominent, elevated fold of mucous membrane ( frenulum linguae ) that binds each lip to the gums, and on each side of this is a slight fold called a sublingual papilla , from which the ducts of the submandibular salivary glands open. Running outward and backward from each sublingual papilla is a ridge (the plica sublingualis) that marks the upper edge of the sublingual (under the tongue) salivary gland and onto which most of the ducts of that gland open.

The gums consist of mucous membranes connected by thick fibrous tissue to the membrane surrounding the bones of the jaw. The gum membrane rises to form a collar around the base of the crown (exposed portion) of each tooth. Rich in blood vessels, the gum tissues receive branches from the alveolar arteries; these vessels, called alveolar because of their relationship to the alveoli dentales, or tooth sockets, also supply the teeth and the spongy bone of the upper and lower jaws, in which the teeth are lodged.

Essay on Human Digestive System

Introduction

The human digestive system is a complex and efficient network responsible for breaking down food, absorbing nutrients, and expelling waste. It comprises organs such as the mouth, esophagus, stomach, intestines, liver, and pancreas. This complex system ensures that our body obtains essential nutrients for energy, growth, and cellular repair. This system is vital to preserving general health and well-being, underscoring its importance.

Anatomy of the Digestive System

It involves a coordinated effort of different body parts to break down food, absorb nutrients, and expel waste. This system divides into the digestive tract and the accessory organs that aid in digestion:

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The Digestive Tract

The tube that connects the mouth to the anus is the digestive tract, and it is continuous. It includes several critical structures:

1. Mouth: The digestive process starts from the mouth, where mechanical digestion occurs through chewing (mastication), and chemical digestion begins with saliva. Amylase, found in saliva, initiates the breakdown of carbohydrates.

2. Pharynx and Esophagus: After chewing and mixing with saliva, the individual swallows the informed bolus. The pharynx (throat) is a pathway for the food to move into the esophagus, a muscular tube that transports the bolus to the stomach through peristalsis (wave-like muscle contractions).

3. Stomach: The stomach, a muscular, J-shaped organ, mixes food with gastric juices, which include hydrochloric acid and digestive enzymes like pepsin. These substances further break down proteins and other components of food into a semi-liquid form called chyme. Additionally, the stomach serves as a temporary storage space.

4. Small Intestine: The small intestine is a long, coiled tube where most digestion and nutrient absorption occur. It has three parts:

• Duodenum: In the first section, chyme is mixed with pancreatic juice (from the pancreas) and bile (from the gallbladder and liver). These fluids contain enzymes that aid in the subsequent digestion of lipids, proteins, and carbohydrates.
• Jejunum: This is the middle section where most nutrient absorption takes place. Its highly folded lining, covered with villi and microvilli, increases the surface area for absorption.
• Ileum: The final section that continues the absorption process and connects to the large intestine.

5. Large Intestine (Colon): The large intestine absorbs water and salts from undigested material and compacts the waste into feces. It includes:

• Cecum: A pouch connects to the ileum, and an appendix is attached.
• Colon: Divided into ascending, transverse, descending, and sigmoid parts, it processes waste to prepare it for elimination.
• Rectum: The final section stores feces before excreting them through the anus.

6. Anus: The digestive tract terminates at the anus, where the body expels feces. Two sphincters (internal and external) control the release of waste.

Process of Digestion

The human digestive system intricately breaks down food into essential nutrients through stages of ingestion, mechanical and chemical digestion, absorption, and egestion, sustaining life:

• Ingestion: The journey begins with ingestion, taking food into the mouth. The complex coordination of muscles opens the mouth, and you introduce food. This simple act marks the commencement of a chain reaction of physiological events crucial for nutrient extraction.
• Mechanical Digestion: Once food enters the mouth, mechanical digestion ensues. This process involves the physical breakdown of food into smaller particles, facilitating subsequent chemical digestion. The primary instrument of mechanical digestion is chewing, where teeth grind and crush food into smaller, more manageable fragments. The tongue helps with swallowing by combining food and saliva to form a cohesive mass called a bolus.
• Digestion: Digestion intensifies as the bolus traverses the esophagus into the stomach. Digestive enzymes and gastric juices secreted by the stomach lining initiate the breakdown of proteins into simpler amino acids. Peristaltic contractions churn and mix the bolus with gastric secretions, forming a semi-liquid substance called chyme. This mechanical action and enzymatic activity set the stage for efficient nutrient extraction.
• Chemical Digestion: Chemical digestion, primarily in the stomach and small intestine, involves the enzymatic breakdown of complex molecules into absorbable nutrients. In the stomach, pepsinogen is activated to pepsin by hydrochloric acid, initiating protein digestion. Simultaneously, gastric lipase initiates the digestion of fats. Upon entering the small intestine, bile from the liver and pancreatic enzymes further break down carbohydrates, fats, and proteins into their constituent molecules—sugars, fatty acids, and amino acids—readied for absorption.
• Absorption: The small intestine is the main organ for absorbing nutrients. Its unique structure, which consists of microvilli and villi, dramatically expands the surface area that absorbs nutrients. Nutrients, now in their simplest forms, are absorbed through the intestinal lining into the bloodstream. The organism absorbs carbohydrates as glucose, proteins as amino acids, and fats as glycerol and fatty acids. The intestinal tract also absorbs water, vitamins, and minerals, nourishing the body and sustaining vital physiological functions.
• Egestion (Excretion): As absorption concludes, undigested and unabsorbed remnants, along with waste products, transit into the large intestine. Here, water reabsorption occurs, forming feces—a semisolid waste material composed of indigestible substances, bacteria, and sloughed-off cells. Peristaltic contractions propel fecal matter towards the rectum, where it is stored until eliminated through defecation—the final stage of the digestive process.

Detailed Functions of Organs

Let’s examine how each organ functions in this system, which is crucial for comprehending how our bodies convert food into the building blocks necessary for survival:

1. Mouth: The digestion process starts from the mouth. Here, food is broken down mechanically by chewing and chemically by saliva. Enzymes like amylase found in saliva produced by salivary glands begin the breakdown of carbohydrates. The tongue manipulates food and forms it into a bolus, which you then swallow.

2. Pharynx and Esophagus: The pharynx (throat) moves food from the mouth to the esophagus, a muscle tube that moves food towards the stomach with wave-like contractions called peristalsis. The lower esophageal sphincter (LES) at the end of the esophagus opens to allow the bolus to enter the stomach and then closes to prevent acid reflux.

3. Stomach: Food is broken down by the muscular organ known as the stomach, further using mechanical and chemical digestion. TGastric glands in the stomach’s lining secrete pepsinogen lining’s gastric glands. The acidic environment generated by aHCl changes the pepsinogen enzyme that breaks down proteins into peptides by acidity. The stomach also churns the food-liquid combination.

4. Small Intestine: Most digestion and nutritional absorption occur in a lengthy, coiled tube in the small intestine. It has three parts:

• Duodenum: Here, pancreatic juice and bile from the liver, gallbladder, and pancreas combine with chyme. Pancreatic enzymes such as lipase, amylase, and proteases break down proteins, lipids, and carbohydrates, while bile emulsifies fats.
• Jejunum: The middle section of the small intestine, where most nutrient absorption occurs. The inner surface has villi and microvilli that increase the surface area for absorption.
• Ileum: The final section, where absorption of vitamin B12, bile salts, and remaining nutrients takes place. The ileocecal valve, which controls the flow into the large intestine, marks the end of the ileum.

5. Liver: The liver has multiple functions, including producing bile, which is essential for emulsifying and digesting fats. It also processes nutrients absorbed from the small intestine, detoxifies harmful substances, and stores glycogen, vitamins, and minerals.

6. Gallbladder: The gallbladder is where the liver’s generated bile is kept and concentrated. When fatty food enters the duodenum, the gallbladder discharges bile down the bile duct towards the small intestine to help digestion.

7. Pancreas: The pancreas’s activities are both exocrine and endocrine. Through the pancreatic duct, it secretes the digesting enzymes lipase, proteases, and amylase into the small intestine. It also releases bicarbonate to neutralize stomach acid in the duodenum. Additionally, the pancreas regulates blood sugar by producing insulin and glucagon.

8. Large Intestine: The large intestine forms solid waste (feces) because it collects water and salts from the material one cannot digest. It consists of:

• Cecum: The beginning of the large intestine, which receives material from the ileum.
• Colon: The central part of the large intestine, divided into ascending, transverse, descending, and sigmoid colon. It absorbs water and salts.
• Rectum: The final section of the large intestine stores feces before elimination.
• Anus: The digestive tract’s aperture expels feces.

Regulation of Digestive Functions

The regulation of digestive activities in the human digestive system presented are as follows:

• Nervous System Regulation: The enteric nervous system controls digestive functions autonomously and communicates with the central nervous system to modulate activities in response to external stimuli.
• Hormonal Regulation: Specialized cells release hormones in the digestive organs, coordinating processes like enzyme secretion and nutrient absorption in response to food presence and nutritional content.
• Chemical Regulation: It involves maintaining optimal pH levels within the digestive tract through the secretion of acids, enzymes, and alkaline substances, ensuring effective digestion and absorption of nutrients.
• Mechanical Regulation: It refers to the physical processes such as chewing, swallowing, and peristalsis that facilitate the movement and breakdown of food within the digestive tract for digestion and absorption.
• Adaptive Changes: Our digestive organs are relatively adaptable, undergoing structural and functional adaptations in response to dietary variations. For instance, they can hypertrophy pancreatic cells with high-fat diets or decrease gastric acid secretion during fasting.
• Feedback Mechanisms: These control systems are marvels of efficiency. They monitor and adjust digestive processes based on internal and external cues. They ensure efficient nutrient absorption and energy balance by regulating secretion, motility, and satiety signals.

Common Digestive Disorders

The signs and symptoms of common digestive disorders which are essential steps in promoting digestive health and well-being:

• Gastroesophageal Reflux Disease (GERD): This chronic illness develops when the stomach’s acidic contents reflux back into the esophagus, irritating and inflaming the lining. Symptoms include heartburn, regurgitation, chest pain, and difficulty swallowing.
• Peptic Ulcer Disease: Peptic ulcers are lesions that develop on the stomach’s lining, esophagus, or small intestine. They occur when stomach acid and the bacteria Helicobacter pylori (H. pylori) erode protective mucosal barriers. Abdominal pain, bloating, nausea, vomiting, and bleeding are possible symptoms.
• Irritable Bowel Syndrome (IBS): IBS is a common gastrointestinal disorder characterized by abdominal pain, bloating, cramping, diarrhea, and constipation. The exact cause of IBS is unknown, but factors such as abnormal gut motility, visceral hypersensitivity, and alterations in gut microbiota may contribute to its development.
• Inflammatory Bowel Disease (IBD): Ulcerative colitis and Crohn’s disease are two examples of chronic inflammatory diseases of the digestive system, including IBD. Inflammation and ulceration of the intestinal lining characterize these conditions, which cause symptoms such as abdominal pain, diarrhea, rectal bleeding, weight loss, and fatigue.
• Gallstones: These are solid collections of digestive juices that form in the gallbladder, a small organ located under the liver. Their size and composition can differ, and if they obstruct the bile ducts, they might result in symptoms including jaundice, nausea, vomiting, and stomach pain.
• Constipation: Constipation, a common digestive ailment characterized by infrequent bowel movements or difficulty passing feces, can result from various factors such as inadequate fiber intake, dehydration, lack of exercise, certain medications, or underlying health conditions.
• Diarrhea: Loose, watery stools characterize diarrhea and can be acute or chronic. It can result from infections, food intolerances, medications, inflammatory conditions, or functional bowel disorders. If left untreated, chronic diarrhea can result in nutrient shortages and dehydration.

Diagnostic Techniques

The human digestive system, a complex network of organs, undergoes various maladies necessitating precise diagnostic techniques. These include:

• Endoscopy: A non-invasive diagnostic technique, endoscopy visualizes the digestive tract via a flexible tube with a camera inserted through the mouth or rectum. It detects inflammation, ulcers, tumors, and bleeding and obtains biopsies. This non-invasiveness is particularly beneficial in diagnosing conditions such as GERD, ulcers, IBD, and colorectal cancer, as it minimizes patient discomfort.
• Colonoscopy: Colonoscopy is a crucial procedure for screening colorectal cancer and detecting polyps and inflammatory bowel disease. A colonoscope visually examines the colon and rectum, allowing for polyp removal and biopsy. It’s the gold standard for early detection and prevention of colorectal cancer.
• Imaging Techniques: X-rays identify intestinal issues; barium contrast studies enhance visibility by coating the digestive tract. CT scans detect tumors and abscesses in detail. MRI provides high-resolution images of soft tissues, aiding in liver, pancreatic, and biliary tract assessments without radiation exposure.
• Laboratory Tests: Laboratory tests assess biochemical markers, enzyme levels, and pathogens in digestive disorders. Blood tests reveal liver function, pancreatic enzymes, and inflammation markers. Stool tests detect blood, infections, and enzyme abnormalities, important for colorectal cancer screening and diagnosing gastrointestinal conditions.

Maintaining Digestive Health

Maintaining digestive health is essential for overall health and well-being. Here are some tips to promote optimal digestive function:

• Balanced Healthy Diet and Nutrition: A balanced healthy diet includes fruits, vegetables, whole grains, lean proteins, and healthy fats, providing essential nutrients for optimal digestion. Variety and appropriate proportions ensure the body’s nutritional needs for digestive health.
• Importance of Fiber: Both soluble and insoluble Fiber promote regular bowel movements and diversity of gut microbiota. Soluble Fiber aids nutrient absorption, while insoluble Fiber adds bulk to stool, aiding its passage. A fiber-rich diet reduces digestive disorders and colorectal cancer risk.
• Hydration: Adequate hydration is vital for digestion, as water dissolves nutrients, aids absorption, and lubricates the digestive tract. Insufficient intake can cause constipation. Drinking water, especially with fiber-rich foods, supports optimal digestion.
• Regular Exercise: Regular exercise aids digestion by stimulating intestinal contractions, reducing constipation risk, and managing stress, which impacts digestive function. Activities like walking, swimming, or yoga promote a healthy digestive system.
• Avoid Harmful Substances: Excessive alcohol and tobacco harm digestion. Alcohol irritates the stomach, causing inflammation and ulcers, while chronic use harms the liver and pancreas. Tobacco weakens the lower esophageal sphincter, causing acid reflux. Avoiding them is vital for digestive health.

FAQs on Human Digestive System

1. What is the human digestive system?

Answer: The digestive system in humans is a sophisticated network of organs and functions that converts meals into nutrients so the body can utilize and absorb them for growth, repair, and energy.

2. What are the main organs of our digestive system?

Answer: The mouth, esophagus, stomach, small intestine, large intestine, liver, gallbladder, and pancreas are some of the organs of the digestive system.

3. How can you maintain a digestive system?

Answer: To maintain digestive health, eat a fiber-rich diet, hydrate, exercise, manage stress, and avoid excess alcohol and tobacco. Regular healthcare check-ups detect and prevent issues.

Let us honor the intricate dance of digestion within us. Respect the nourishment it offers and its pivotal role in our well-being. Adopt mindful eating practices, prioritize nutritious choices, and foster gratitude for this remarkable system sustaining us daily. Understanding and honoring our digestive process empowers us to make informed dietary decisions and nurture a harmonious relationship with our bodies, promoting long-term health and vitality.

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Essay on Digestive System

Students are often asked to write an essay on Digestive System in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Digestive System

Introduction to digestive system.

The digestive system is a group of organs that work together to change the food we eat into energy for our bodies. It’s like a food processing factory. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus.

Process of Digestion

Digestion starts in the mouth when we chew food. It then travels down the esophagus to the stomach. In the stomach, food is mixed with stomach acids to break it down into a liquid. This liquid then moves to the small intestine.

Role of Small Intestine

The small intestine plays a major role in digestion. Here, nutrients from the liquid food are absorbed into the bloodstream. The blood then carries these nutrients to all parts of the body. The leftover food, which the body can’t use, moves to the large intestine.

Role of Large Intestine

The large intestine is the last part of the digestive process. It absorbs water from the leftover food and turns it into waste. This waste then leaves the body through the rectum and anus. This whole process is known as digestion.

250 Words Essay on Digestive System

What is the digestive system.

The digestive system is a group of organs that work together to change the food you eat into energy and basic nutrients to power your body. It is like a food processing plant that takes in raw materials (food) and turns them into something the body can use.

Parts of the Digestive System

The digestive system is made up of several parts. It starts with the mouth, where you chew and swallow your food. Then there’s the esophagus, a tube that carries food to your stomach. The stomach is like a mixer, churning and breaking down food into a liquid.

How Food Travels

From the stomach, the liquid food then goes into the small intestine. Here, it is broken down even more so your body can absorb the nutrients. Finally, what’s left goes into the large intestine, and then out of your body as waste.

The Role of the Liver and Pancreas

The liver and the pancreas also play important roles in digestion. The liver makes a juice called bile that helps to break down fats. The pancreas makes juices that help to break down carbohydrates, fats, and proteins.

Importance of the Digestive System

The digestive system is very important. Without it, our bodies wouldn’t get the nutrients they need. It keeps us healthy and gives us energy. So, remember to eat a balanced diet to keep your digestive system happy and healthy.

500 Words Essay on Digestive System

The digestive system: an introduction.

The digestive system is a group of organs that work together to change the food we eat into energy our bodies can use. It’s like a food processing factory inside our body. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. The liver and pancreas also play a big role in digestion.

Starting Point: The Mouth

Digestion begins in the mouth. When we eat, our teeth break down the food into smaller pieces. Our saliva, a liquid made by the salivary glands, mixes with these pieces, making them easier to swallow. Saliva also starts the process of breaking down the food chemically.

The Esophagus: The Food Pipe

The esophagus is a long tube that connects the mouth to the stomach. It uses a process called peristalsis to move food. This process is like a wave of muscle contractions that pushes the food down into the stomach.

The Stomach: The Mixing Pot

The stomach is like a mixing pot. Here, the food is mixed with stomach acid and enzymes, which break it down into a liquid. This liquid is then sent to the small intestine.

The Small Intestine: The Nutrient Absorber

The large intestine: the water saver.

The large intestine, also known as the colon, is the final part of the digestive system. Its job is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body.

The End of the Journey: The Rectum and Anus

The rectum and anus are the last parts of the digestive system. The rectum stores the waste until it’s ready to leave the body. Then, it passes through the anus and out of the body as feces.

Conclusion: The Importance of the Digestive System

Remember, eating a balanced diet and drinking plenty of water can help keep your digestive system healthy and working well. Regular exercise is also important as it helps keep food moving through the digestive system, reducing the risk of constipation.

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The Digestive System and Its Functions Essay

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One of the most significant components of human life is digestion, because namely during this process, the necessary proteins, fats, carbohydrates, vitamins, minerals, and other useful ingredients enter the body. That is why the proper functioning of the human digestive system serves as the basis for full-fledged life support during the main processes in the digestive tract. Moreover, the digestive system is also responsible for the water-electrolytic balance, regulating the rate of fluid intake from food. The functions of the gastrointestinal tract can be summarized as follows (Hoffman 9-14):

• Motor function. Due to the middle (muscle) membrane of the digestive tract, muscle contraction-relaxation, food taking is carried out, following chewing, swallowing, mixing, and moving food along the digestive canal.
• Secretory function is carried out due to the digestive juices, that are produced by the glandular cells located in the mucous membrane (inner) of the canal. These secrets contain enzymes (reaction accelerators) that carry out the chemical processing of food (hydrolysis of food substances).
• Excretory function provides the secretion of metabolic products by the digestive glands in the gastrointestinal tract.
• Absorption function ‑ the process of assimilation of nutrients through the wall of the gastrointestinal tract into the blood and lymph.

The gastrointestinal tract is a convoluted tube that begins with the mouth and ends with the anus. The digestive system includes the following: the oral cavity with organs located in it and the adjacent large salivary glands; pharynx; esophagus; stomach; small and large intestine; liver; pancreas (Rogers 15).

The oral cavity, pharynx, and esophagus located in the area of the human head, neck, and chest cavity have a relatively straight direction. In the oral cavity, food enters the throat, where there is a cross of digestive and respiratory tracts. Then the esophagus comes, through which food mixed with saliva enters the stomach. In the oral cavity, the primary processing of food occurs, which consists of its mechanical grinding with the help of the tongue and teeth and turning into a food lump.

The salivary glands secrete saliva, the enzymes of which start the breakdown of carbohydrates in food (Smith and Morton 29). Then, through the throat and esophagus, food enters the stomach, where it is digested under the influence of gastric juice.

The stomach is a thick-walled muscle sac located under the diaphragm in the left half of the abdominal cavity. By reducing the walls of the stomach, its contents are mixed. Many glands concentrated in the mucous wall of the stomach secrete gastric juice containing enzymes and hydrochloric acid. After this, partially digested food enters the anterior part of the small intestine ‑ the duodenum.

The small intestine consists of the duodenum, jejunum, and ileum. In the duodenum, food is exposed to the action of pancreatic juice, bile, and also the juice of the glands located in its wall. In the jejunum and ileum, the final digestion of food and absorption of nutrients into the blood occurs. Undigested residues enter the colon. Here they are accumulated and are subject to removal from the body in the form of feces. The initial part of the colon is called the blind, and the appendix is following it.

Digestive glands include salivary glands, microscopic glands of the stomach and intestines, pancreas, and liver. The liver is the largest gland in the human body. It is located on the right under the diaphragm (Rogers 42). Bile is produced in the liver, which flows through the ducts into the gall bladder, where it accumulates and enters the intestine as needed. The liver retains toxic substances and protects the body from poisoning. The pancreas also belongs to the digestive glands that secrete juices and turn complex nutrients into simpler and more soluble in water. It is located between the stomach and the duodenum. Pancreatic juice contains enzymes that break down proteins, fats, and carbohydrates; 1–1.5 liters of pancreatic juice is secreted per day (Hoffman 30).

The correct sequential operation of the elements of the digestive system in time and space is ensured by regular processes of various levels. Enzymatic activity is characteristic of each section of the digestive tract and is maximum at a certain pH value of the medium. For example, in the stomach, the digestive process is carried out in an acidic environment.

Acidic content passing into the duodenum is neutralized, and intestinal digestion occurs in a neutral and slightly alkaline environment created by secrets secreted into the intestine ‑ bile, pancreatic juices, and intestinal secretions, which inactivate gastric enzymes (Smith and Morton 24). Intestinal digestion occurs in a neutral and slightly alkaline environment, first in the type of abdominal and then parietal digestion, ending with the absorption of hydrolysis products ‑ nutrients.

The degradation of nutrients by the type of cavity and parietal digestion is carried out by hydrolytic enzymes, each of which has specificity expressed to one degree or another. A set of enzymes in the secretions of the digestive glands has specific and individual characteristics, adapted to the digestion of the food that is characteristic of this region, and those nutrients that prevail in the diet.

Each digestion department has its internal environment, which serves as the basis for the functions assigned to it. The organs of the gastrointestinal tract, together with the auxiliary glands, gradually break down each component of the food, separating what the body needs and sending the rest of the absorbed food to waste. If at any of these stages a malfunction occurs, the organs and systems do not receive enough energy resources and, therefore, cannot fully perform their functions, causing an imbalance of the whole organism. Violations of the normal functioning of the digestive system can lead to the development of several diseases.

Works Cited

Hoffman, Gretchen. Digestive System . Benchmark Books, 2008.

Rogers, Kara. The Digestive System . Rosen Education Service, 2010.

Smith, Margaret E., and Dion G. Morton. The Digestive System: Systems of the Body Series . Churchill Livingstone, 2011.

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• Biology Article

Human Digestive System

Digestive system of humans.

The digestive tract of humans starts with the mouth and ends with the anus. It includes different structures such as the mouth, oesophagus, pancreas, stomach, small intestine, large intestine, liver, gall bladder, and anus.

Table of Contents

Introduction Structure Parts

• Small Intestine

Large Intestine

Accessory organs, digestion process.

• Mixing and Movement

Disorders Functions  Notes

The Human Digestive System

The digestive system of the human body comprises a group of organs working together to convert food into energy for the body. Anatomically, the digestive system is made up of the gastrointestinal tract, along with accessory organs such as the liver, pancreas and gallbladder. The hollow organs that make up the gastrointestinal tract (GI tract) include the mouth, stomach, oesophagus, small intestine and large intestine that contains the rectum and anus.

Human Digestive System and Nutrition involve the intake of food by an organism and its utilization for energy. This is a vital process which helps living beings to obtain their energy from various sources. The food which we eat undergoes much processing before the nutrients present in them are utilized to generate energy. This processing is known as digestion. Humans and other animals have specialized organs and systems for this process.

The digestion process involves the alimentary canal along with various accessory organs and organ systems. In humans, the process is quite simple due to our monogastric nature. This means that we have a one-chambered stomach, unlike other animals such as cows, which have four chambers.

Some parts of nervous and circulatory systems also play a significant role in the digestion process. A combination of nerves, bacteria, hormones, blood and other organs of the digestive system completes the task of digestion.

Let us have a detailed look at the human digestive system, its parts and functions. Also provided at the end of the chapter are digestive system notes.

Also Read:  Alimentary Canal

Diagram Of The Human Digestive System

The diagram given below represents different parts of the human digestive system that convert food into essential nutrients absorbed by the body.

Parts of the Human Digestive System

The digestive system of the human body comprises a group of organs that work together in converting food into energy and other basic nutrients to power the body. The food we take in is digested and utilized by our body, and the unused parts of the food are defecated.

The digestive system of the human body is the sum of the gastrointestinal tract (GIT; also called alimentary canal) and accessory organs (tongue, liver, pancreas, etc.). These two parts together help in the digestion process.

The alimentary canal is the long tube through which the food that we eat is passed. It begins at the mouth (buccal or oral cavity), passes through the pharynx, oesophagus or food pipe, stomach, small intestines, large intestines, rectum and finally ends at the anus. The food particles gradually get digested as they travel through various compartments of the alimentary canal.

Accessory organs are organs which participate in the digestion process but are not actually a part of GIT. They stimulate the digestion by releasing certain enzymes that help in breaking down the food.

Let us have a detailed look at the digestive system of the human body, along with its parts and functions:

Food starts its journey from the mouth or the oral cavity. There are many other organs that contribute to the digestion process, including teeth, salivary glands, and tongue. Teeth are designed for grinding food particles into small pieces and are moistened with saliva before the tongue pushes the food into the pharynx.

A fibromuscular y-shaped tube attached to the terminal end of the mouth. It is mainly involved in the passage of chewed/crushed food from the mouth through the oesophagus. It also has a major part in the respiratory system, as air travels through the pharynx from the nasal cavity on its way to the lungs.

This is a muscular tube that connects the pharynx, which is a part of an upper section of the gastrointestinal tract. It supplies swallowed food along with its length.

Also Read:  Food Pipe

It serves as a muscular bag which is situated towards the left side of the abdominal cavity, beneath the diaphragm. This vital organ acts as a storage for the food and provides enough time to digest meals. The stomach also produces digestive enzymes and hydrochloric acid that maintains the process of digestion.

Mucous : It is an aqueous secretion produced by the mucous membranes. It functions by protecting the stomach lining and gastric pits from the acid, which is produced by the glands to destroy the bacteria that entered along with the food particles.

Digestive enzymes : They are the group of enzymes which functions by breaking down polymeric macromolecules like biopolymers into their smaller and simpler substances.

Hydrochloric acid : It is the digestive fluid formed by the stomach during the process of digestion. It functions by destroying harmful microorganisms present in the food particles.

The small intestine is a thin, long tube of about 10 feet long and a part of the lower gastrointestinal tract. It is present just behind the stomach and acquires a maximum area of the abdominal cavity. The complete small intestine is coiled and the inner surface consists of folds and ridges.

This is a thick, long tube measuring around 5 feet in length. It is present just beneath the stomach and wraps over the superior and lateral edges of the small intestine. It absorbs water and consists of bacteria (symbiotic) that support the breakdown of wastes to fetch small nutrients.

Also Read:  Large Intestine

Waste products are passed into the end of the large intestine called the rectum and eliminated out of the body as a solid matter called stool. It is stored in the rectum as semi-solid faeces which later exits from the body through the anal canal through the process of defecation.

It is a large gland present just behind the stomach. It is short with its anterior connected to the duodenum and posterior pointing towards the left part of the abdominal cavity. The pancreas releases digestive enzymes to complete the process of chemical digestion.

Read More:  Pancreas

The liver is a roughly triangular, reddish-brown accessory organ of the digestive system located to the right of the stomach. It produces bile , which helps in the digestion of fat in the small intestine. The bile is stored and recycled in the gallbladder. It is a small, pear-shaped organ which is located just next to the liver.

Also Read:  Cellulose in Digestion

The process of digestion begins from the mouth and ends in the small intestine – the large intestines’ main function is to absorb the remaining water from the undigested food and enable bacterial fermentation of materials that can no longer be digested.

The alimentary canal or the gastrointestinal tract is a series of hollow organs and tubes that begins from the mouth cavity and continues into the pharynx, through the stomach, small intestines, large intestines, and finally ending at the anus. Food particles gradually get digested as they travel through various compartments of the gastrointestinal tract.

The digestion process takes place in the following steps.

The very first step involves mastication (chewing). The salivary glands, along with the tongue, helps to moisten and lubricate food, before being pushed down into the food pipe.

It involves the process of lubricating and manipulating food and pushing it down the food through the food pipe (using peristalsis ), and into the stomach.

The stomach, small intestine, liver, and pancreas secrete enzymes and acids to aid the process of digestion. It functions by breaking down food particles into simple components and easily absorbable components.

Also Read:  What is Liver

The process of converting complex food particles into simpler substances in the presence of enzymes and acids secreted by different digestive organs.

Read more: What are digestive juices?

This process begins in the small intestine where most of the nutrients and minerals are absorbed. The excess water in the indigestible matter is absorbed by the large intestines.

The process of removing indigestible substances and waste by-products from the body through the process of defecation.

In a nutshell, the digestion process consists of the six following steps:

Ingestion  ⇒ Mixing and Movement  ⇒  Secretion  ⇒  Digestion  ⇒ Absorption  ⇒ Excretion

Also Read:  Gastrointestinal Tract

Disorders of the Human Digestive System

Vomiting : It is the ejection of stomach contents through the mouth.

Diarrhoea : It is the abnormal watery bowel movement. Prolonged diarrhoea eventually leads to dehydration.

Constipation : A condition in which the faeces are clutched within the rectum due to an irregular bowel movement.

Indigestion : A pain or discomfort in the stomach which is caused when food is not digested properly, resulting in the feeling of fullness.  Indigestion is mainly caused due to inadequate enzyme secretion, food poisoning, anxiety, overeating and eating spicy foods.

Also Read:  Difference between trachea and oesophagus

Functions of the Human Digestive System

Digestion and absorption are the two main functions of the digestive system.

Digestion is necessary for breaking down food particles into nutrients that are used by the body as an energy source, cell repair and growth.

Food and drink need to be converted into smaller molecules of nutrients before it is absorbed by the blood and carried to the cells throughout the body. The body breaks the nutrients present in the drinks and food into carbohydrates, vitamins, fats and proteins.

Human Digestive System Notes

• The human digestive system breaks down food to release energy essential for the body to carry out its activities.
• The process of digestion takes place in 6 major steps.
• The food is ingested by the alimentary canal and is propelled through the body for further processing.
• The autonomous nervous system controls the peristalsis, contraction and relaxation of muscles within the alimentary canal wall.
• The food is passed to the small intestine where it is digested, and the nutrients are absorbed.
• Water, electrolytes and vitamins are absorbed by the large intestine and the waste is defecated.

Also Read:  Mouth and Buccal Cavity

To learn more about the human digestive system parts and functions, as well as related topics such as the digestion process and disorders of the digestive system, keep visiting  BYJU’S Biology or download the BYJU’S app, for further reference.

Frequently Asked Questions

Deduce the function of the human digestive system..

The digestive system consists of the alimentary canal and the accessory organs. Their main function is to break down the ingested food into its components and produce vital nutrients and energy required to sustain life.

What are accessory organs?

Accessory organs are organs which are not part of the digestive system; however, they aid in the digestion process by performing many secondary functions. The main accessory organs of the digestive system are the tongue, liver, pancreas and gall bladder.

Outline the process of digestion, step by step.

The process of digestion involves the following steps, namely:

List out the parts of the digestive system.

• Mouth & Buccal Cavity

Explain the role of the tongue as an accessory organ.

The tongue is not a part of the digestive system, but it provides support functions such as moving and manipulating the food within the buccal cavity. Furthermore, moistening food also helps to swallow and pass through the oesophagus without much resistance.

Register at BYJU’S to explore digestive system notes and more.

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Your Digestive System & How it Works

On this page:

What is the digestive system?

Why is digestion important, how does my digestive system work, how does food move through my gi tract, how does my digestive system break food into small parts my body can use, what happens to the digested food, how does my body control the digestive process, clinical trials.

The digestive system is made up of the gastrointestinal tract—also called the GI tract or digestive tract—and the liver , pancreas , and gallbladder. The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus . The hollow organs that make up the GI tract are the mouth, esophagus , stomach, small intestine, large intestine, and anus. The liver, pancreas, and gallbladder are the solid organs of the digestive system.

The small intestine has three parts. The first part is called the duodenum. The jejunum is in the middle and the ileum is at the end. The large intestine includes the appendix , cecum, colon , and rectum. The appendix is a finger-shaped pouch attached to the cecum. The cecum is the first part of the large intestine. The colon is next. The rectum is the end of the large intestine.

Bacteria in your GI tract, also called gut flora or microbiome, help with digestion . Parts of your nervous and circulatory systems also help. Working together, nerves, hormones , bacteria, blood, and the organs of your digestive system digest the foods and liquids you eat or drink each day.

Digestion is important because your body needs nutrients from food and drink to work properly and stay healthy. Proteins , fats , carbohydrates , vitamins , minerals , and water are nutrients. Your digestive system breaks nutrients into parts small enough for your body to absorb and use for energy, growth, and cell repair.

• Proteins break into amino acids
• Fats break into fatty acids and glycerol
• Carbohydrates break into simple sugars

MyPlate offers ideas and tips to help you meet your individual health needs .

Each part of your digestive system helps to move food and liquid through your GI tract, break food and liquid into smaller parts, or both. Once foods are broken into small enough parts, your body can absorb and move the nutrients to where they are needed. Your large intestine absorbs water, and the waste products of digestion become stool . Nerves and hormones help control the digestive process.

The digestive process

Food moves through your GI tract by a process called peristalsis. The large, hollow organs of your GI tract contain a layer of muscle that enables their walls to move. The movement pushes food and liquid through your GI tract and mixes the contents within each organ. The muscle behind the food contracts and squeezes the food forward, while the muscle in front of the food relaxes to allow the food to move.

Mouth. Food starts to move through your GI tract when you eat. When you swallow, your tongue pushes the food into your throat. A small flap of tissue, called the epiglottis, folds over your windpipe to prevent choking and the food passes into your esophagus.

Esophagus. Once you begin swallowing, the process becomes automatic. Your brain signals the muscles of the esophagus and peristalsis begins.

Lower esophageal sphincter. When food reaches the end of your esophagus, a ringlike muscle—called the lower esophageal sphincter —relaxes and lets food pass into your stomach. This sphincter usually stays closed to keep what’s in your stomach from flowing back into your esophagus.

Stomach. After food enters your stomach, the stomach muscles mix the food and liquid with digestive juices . The stomach slowly empties its contents, called chyme , into your small intestine.

Small intestine. The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine, and push the mixture forward for further digestion. The walls of the small intestine absorb water and the digested nutrients into your bloodstream. As peristalsis continues, the waste products of the digestive process move into the large intestine.

Large intestine. Waste products from the digestive process include undigested parts of food, fluid, and older cells from the lining of your GI tract. The large intestine absorbs water and changes the waste from liquid into stool. Peristalsis helps move the stool into your rectum.

Rectum. The lower end of your large intestine, the rectum, stores stool until it pushes stool out of your anus during a bowel movement .

Watch this video to see how food moves through your GI tract .

As food moves through your GI tract, your digestive organs break the food into smaller parts using:

• motion, such as chewing, squeezing, and mixing
• digestive juices, such as stomach acid, bile , and enzymes

Mouth. The digestive process starts in your mouth when you chew. Your salivary glands make saliva , a digestive juice, which moistens food so it moves more easily through your esophagus into your stomach. Saliva also has an enzyme that begins to break down starches in your food.

Esophagus. After you swallow, peristalsis pushes the food down your esophagus into your stomach.

Stomach. Glands in your stomach lining make stomach acid and enzymes that break down food. Muscles of your stomach mix the food with these digestive juices.

Pancreas. Your pancreas makes a digestive juice that has enzymes that break down carbohydrates, fats, and proteins. The pancreas delivers the digestive juice to the small intestine through small tubes called ducts.

Liver. Your liver makes a digestive juice called bile that helps digest fats and some vitamins. Bile ducts carry bile from your liver to your gallbladder for storage, or to the small intestine for use.

Gallbladder. Your gallbladder stores bile between meals. When you eat, your gallbladder squeezes bile through the bile ducts into your small intestine.

Small intestine. Your small intestine makes digestive juice, which mixes with bile and pancreatic juice to complete the breakdown of proteins, carbohydrates, and fats. Bacteria in your small intestine make some of the enzymes you need to digest carbohydrates. Your small intestine moves water from your bloodstream into your GI tract to help break down food. Your small intestine also absorbs water with other nutrients.

Large intestine. In your large intestine, more water moves from your GI tract into your bloodstream. Bacteria in your large intestine help break down remaining nutrients and make vitamin K . Waste products of digestion, including parts of food that are still too large, become stool.

The small intestine absorbs most of the nutrients in your food, and your circulatory system passes them on to other parts of your body to store or use. Special cells help absorbed nutrients cross the intestinal lining into your bloodstream. Your blood carries simple sugars, amino acids, glycerol, and some vitamins and salts to the liver. Your liver stores, processes, and delivers nutrients to the rest of your body when needed.

The lymph system, a network of vessels that carry white blood cells and a fluid called lymph throughout your body to fight infection, absorbs fatty acids and vitamins.

Your body uses sugars, amino acids, fatty acids, and glycerol to build substances you need for energy, growth, and cell repair.

Your hormones and nerves work together to help control the digestive process. Signals flow within your GI tract and back and forth from your GI tract to your brain.

Cells lining your stomach and small intestine make and release hormones that control how your digestive system works. These hormones tell your body when to make digestive juices and send signals to your brain that you are hungry or full. Your pancreas also makes hormones that are important to digestion.

You have nerves that connect your central nervous system—your brain and spinal cord—to your digestive system and control some digestive functions. For example, when you see or smell food, your brain sends a signal that causes your salivary glands to "make your mouth water" to prepare you to eat.

You also have an enteric nervous system (ENS)—nerves within the walls of your GI tract. When food stretches the walls of your GI tract, the nerves of your ENS release many different substances that speed up or delay the movement of food and the production of digestive juices. The nerves send signals to control the actions of your gut muscles to contract and relax to push food through your intestines.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and other components of the National Institutes of Health (NIH) conduct and support research into many diseases and conditions.

What are clinical trials, and are they right for you?

Watch a video of NIDDK Director Dr. Griffin P. Rodgers explaining the importance of participating in clinical trials.

What clinical trials are open?

Clinical trials that are currently open and are recruiting can be viewed at www.ClinicalTrials.gov .

This content is provided as a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health. NIDDK translates and disseminates research findings to increase knowledge and understanding about health and disease among patients, health professionals, and the public. Content produced by NIDDK is carefully reviewed by NIDDK scientists and other experts.

Home — Essay Samples — Nursing & Health — Digestive System — Complementarity of Structure & Function in the Digestive System

Complementarity of Structure & Function in The Digestive System

• Categories: Digestive System

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Words: 423 |

Published: Jan 30, 2024

Words: 423 | Page: 1 | 3 min read

Table of contents

Overview of the digestive system, structure-function relationship in the digestive system, interdependency between structure and function.

• Alberts, Bruce. "Molecular Biology of the Cell." Garland Science, 2002.
• Goodman, Craig. "Medical Physiology: A Cellular and Molecular Approach." Elsevier, 2020.
• Hollins, Martin. "Fundamentals of Anatomy and Physiology." Routledge, 2018.

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Digestive system

Author: Lorenzo Crumbie, MBBS, BSc • Reviewer: Francesca Salvador, MSc Last reviewed: November 02, 2023 Reading time: 30 minutes

Extending from the mouth to the anus , the digestive tract is one of the largest systems in the human body. It contains organs that regulate food intake, its digestion and absorbtion of the useful materia that it contains. In addition to this, the digestive system also eliminates the waste products from food and products from various endogenous metabolic processes.

In a nutshell, the digestive tract has the tumultuous responsibility of converting large chunks of food into their constituent micro-molecules that will subsequently be used to build and repair the body.

This article will discuss the anatomy of the digestive system.

To improve your understanding of digestive system terminology, take a closer look at some commonly used roots, prefixes and suffixes in the digestive system in this video:

Trigger and initiation

Mastication, swallowing and physical digestion, chemical digestion, buccal cavity, bowel mnemonic, salivary glands, gallbladder, arterial supply, innervation, venous drainage, examination of the digestive system.

The digestive system can be broken down into two major components:

• There is the primary digestive tract , which functions mainly as a conduit and storage pathway. This portion is needed in order to move food contents along the tract (peristalsis) so that absorption of nutrients and excretion of undigested substances can occur. The tract also allows for segmentation of food at different stages of digestion. This is important because some enzymes produced in one part of the tract (e.g. pepsin in the stomach) will not function optimally in another part of the tract (e.g. the jejunum).
• The other component is the accessory digestive tract . This group of organs are responsible for the synthesis and secretion enzymes to facilitate chemical digestion.

This article aims to give an overview of the anatomy and function of the digestive tract. Details regarding neurovascular supply, histology , and clinically significant points regarding each segment of the digestive tract can be found in the respective articles. It will also briefly mention a few elements of the abdominal examination.

Give it a shot and test yourself on the main organs of the digestive system!

Functions of the digestive system

The function of the digestive system truly begins within the brain . Whenever the body’s energy stores (i.e. blood glucose, protein, or fat stores) fall below a set point, the hunger centres of the hypothalamus are activated. These centres regulate satiety (fullness) and appetite in order to maintain energy homeostasis. This signals to the brain that there is a need to obtain food. Keep in mind, however, that this is not the only source of hunger stimulation, as there are elements of pleasure and comfort that can be derived from ingesting food.

Monogastric organisms like humans have two kinds of digestive processes occurring in the digestive tract – mechanical and chemical digestion. Once the desired food is obtained, the digestive process begins in the mouth with mechanical digestion . Here the teeth are used to cut, tear, and grind chunks of food into smaller particles. This process of mastication involves the alternating action of the muscles of mastication (namely, the superficial and deep masseter , the pterygoids , and the temporalis muscles ).

Mastication is actually a reflex action that is stimulated once food is present in the mouth. At that point, there is inhibition of the muscles of mastication which results in a fall of the mandible. This causes distention of the muscles of mastication, resulting in reflex contraction of the muscle fibres; thus raising the mandible. That action causes apposition of the upper and lower rows of teeth, crushing the food that is between them. The cycle is repeated until the food particles can be rolled into a bolus .

As the tongue rolls the chewed food into a bolus, the salivary glands secrete saliva to moisten the bolus in order for it to pass smoothly to the stomach. Also note that some amount of chemical digestion also occurs in the mouth, as saliva contains the enzyme amylase , which breaks down some carbohydrates in the mouth.

Once the bolus is prepared, then swallowing will be initiated. This is another intricate reflex arc that involves the action of afferent and efferent tracts of several cranial nerves being relayed to and from the nucleus tractus solitarius and nucleus ambiguus of the brainstem .

The end result is that these nerves send motor signals to the tongue, which moves the bolus against the hard and soft palate, then into the oropharynx (which is also under brainstem regulation). The bolus then continues inferiorly towards the laryngopharynx and the swallowing reflex is initiated at the esophagus . All the actions of swallowing up to this point were under voluntary control ; however, the rest of the action is carried out by involuntary peristaltic contractions that travel in a craniocaudal fashion. At the level of the pharynx:

• Superiorly, the bolus is prevented from entering the nasopharynx by the actions of Passavant’s ridge . This structure is formed by the joint actions of the palatopharyngeal sphincters, the superior constrictor muscles, salpingopharyngeus and the muscles of the soft palate.
• The epiglottis closes off the larynx to prevent food from entering the airway. The vocal cords are also adducted as an additional protective measure.

At the level of the esophagus :

• There is relaxation of the cricopharyngeal sphincter and the bolus enters the proximal esophagus. The presence of the bolus causes distention of the myenteric plexus within the walls of the esophagus, initiating the primary esophageal peristaltic wave.
• The continued presence of the food stimulates the secondary peristaltic waves in a craniocaudal direction.

These waves, along with the action of gravity, move the bolus toward the lower esophageal sphincter at a rate of 4 cm/s. At rest, the sphincter has a high tone. However, the presence of the bolus aids relaxation of the lower esophageal sphincter, and food is able to enter the stomach. Here, the majority of the chemical digestion will take place.

Once the bolus enters the stomach , there is regulated release of a variety of enzymes that facilitate chemical digestion. Some of these enzymes also stimulate the accessory digestive organs to release their enzymes to aid in digestion. In addition to chemical digestion (particularly of proteins), the stomach also functions as:

• A storage point , which gradually releases its contents into the small intestines , to allow adequate time for further digestion and absorption.
• A mixer : the mode of contraction and arrangement of the stomach mucosa results in further mixing of the food contents to form chyme.
• A conduit : essentially passing food from the esophagus to the small intestines.
• Immunological defence : the acidic pH of the stomach aids in dissolving invading pathogens before they are able to cause an infection.
• Other micronutrients : iron (Fe), vitamin B12 ,and folate absorption are heavily regulated by the stomach.

The stomach can be functionally divided into proximal and distal motor pumps, which stores food content and pumps chyme along the conduit, respectively. Portions of chyme are passed into the pylorus and into the small intestines .

Once chyme has entered the first part of the duodenum, it activates the neurohormonal axis which promotes the release of bile (from the liver and gallbladder ) and other enzymes from the pancreas. The peristaltic waves continue to move the chyme along the intestinal tract. The intricate folding of the intestines facilitates absorption of nutrients from the chyme. Majority of the nutrients are absorbed within the small intestines. The remnants are passed through the unidirectional ileocecal valve into the cecum.

As the peristaltic waves continue into the colon, the chyme continues to move along the tract. Further absorption of electrolytes and water from the remaining chyme occurs and the chyme is then converted into stool, which is stored in the rectum. As the rectum becomes distended, the stretch receptors signal to the brain that promotes defecation . While the internal anal sphincters are under autonomic regulation, the external anal sphincters are under voluntary control. Therefore the individual may resist the urge to defecate until an appropriate time and place is identified.

Organs of the digestive system

The digestive tract is also referred to as the alimentary canal. It is a tubular continuum that is segmented into dilated regions. These dilated regions are often separated by thickened regions of the wall that form sphincters . This prevents unintended mixing of the contents in the respective segments.

The buccal cavity marks the opening of the digestive tract. It is made up by the oral vestibule (space between the inner cheeks and the teeth) and the oral cavity proper (behind the teeth). The buccal cavity also contains the teeth and tongue. It is limited anteriorly by the lips and teeth and posterolaterally by the palatopharyngeal arches. Posteriorly the buccal cavity opens into the oropharynx. The roof is formed by the hard palate (at the anterior two-thirds) and the soft palate (at the posterior third), and the floor of the oral cavity also contains the tongue.

The teeth can be subdivided into incisors (designed for cutting), canines (designed for tearing), premolars , and molars  (designed for grinding). The molars and premolars have complementary surfaces that also aid in grinding. The tongue consists of intrinsic and extrinsic muscles. It is also populated  with taste buds that facilitate gustatory sensation.

Posterior to the oral cavity proper is the oropharynx . This is the middle part of the pharynx that communicates superiorly with the nasopharynx and inferiorly with the laryngopharynx. The walls of the oropharynx are formed by the superior and middle pharyngeal constrictor muscles . Anterolaterally, the palatopharyngeal folds form a demarcation between the oral cavity proper and the oropharynx.

The base of the tongue also serves as another landmark in the anteroinferior aspect of the oropharynx. The mucosa of the walls also contains numerous mucosa associated lymphatic tissue (MALT) . It can be separated from the nasopharynx by the muscles of Passavant’s ridge and the supporting structures of the soft palate during deglutition.  

The tubular conduit responsible for transferring food from the oropharynx to the stomach is the esophagus. It can be divided into three parts:

• abdominal components

The total length of the muscular tube is 25 cm, commencing at the cricopharyngeus at the lower border of the cricoid cartilage at the sixth cervical vertebrae (C6) . It journeys posteriorly to the trachea in the neck along its caudal course. It then travels through the superior, then posterior mediastinum alongside the thoracic vertebrae. It pierces the diaphragm at the tenth thoracic vertebra (T10). The remaining 2.5 cm of the esophagus is the abdominal part. It transitions into the stomach at the gastroesophageal junction, where the physiological lower esophageal sphincter exists.

Within the abdominal cavity, the esophagus enters the stomach. This is a dilated area of the alimentary canal that participates in both mechanical and chemical digestion. It is divided into four main parts, namely the:

Laterally, there is a greater curvature , and medially there is a lesser curvature . There are two distinct notches on the stomach. The first is the cardiac incisura formed at the lateral border of the cardioesophageal junction. The other is the less pronounced incisura angularis found distally at the caudal end of the lesser curvature (at the pyloric antrum). The pylorus is the distal aspect of the stomach that is thickened. It acts as a physiological sphincter that regulates the passage of chyme from the stomach to the beginning of the small intestines.

The duodenum marks the beginning of the small intestines. It is roughly 20 – 25 cm long, extending from the pylorus to the ligament of Treitz . Not only is the duodenum the shortest part of the small intestines, but it is also the widest. It can be subdivided into four parts based on its geometrical orientation. The first part is known as pars superioris (the superior part); it is roughly 2 – 3 cm long and travels above the head of the pancreas. The second part is pars descendens which commences behind the neck of the gallbladder. It travels about 8 – 10 cm lateral to the head to the pancreas.

The inferior duodenal flexure (where pars descendens begins to turn) marks the transition of the second part of the duodenum to the third part – pars horizontalis . It travels for roughly 10 cm before it begins to curve upwards into the final segment of the duodenum, the pars ascendens (which is only 2.5 cm long).

The transition from the duodenum to jejunum occurs at the ligament of Treitz . The difference in the luminal diameter of the jejunum and duodenum is an important distinguishing feature. The duodenum is significantly wider than the jejunum.

However, it may be more challenging to distinguish between the jejunum and ileum as there are no external anatomical landmarks to guide. The key distinguishing features are as follows:

• The external diameter of the jejunum (4 cm) is greater than that of the ileum (3.5 cm).
• The internal diameter of the ileum (2 cm) is also smaller than that of the jejunum (2.5 cm).
• The walls of the jejunum appear thicker than that of the ileum.
• Additionally, the jejunum appears more hyperaemic than the ileum because it has a more extensive vascular supply.
• The luminal surface of the jejunum is significantly folded into plicae circulares that are more numerous and appear deeper than anywhere else within the digestive tract. Furthermore, the plicae circulares become less abundant distally within the ileum.
• Finally, the luminal mucosa of the ileum has more prominent mucosa-associated lymphoid tissue (MALT) than the jejunum.

The ileum terminates at the ileocecal valve ; which marks the transition from the small intestines to the large intestines . The ileocecal valve is a one way structure that prevents reflux of the bolus from the large intestines to the small intestines.

The proximal end of the large intestines – also known as the colon – is formed by a dilated cul-de-sac known as the cecum . There is also a vermiform appendix attached at variable parts of the cecum. The colon is divided into:

• sigmoid parts

Externally, the colon has a segmented appearance due to the haustrations that are present on the luminal surface of the conduit. The muscular layers of the colon are concentrated into three muscular bands known as taenia coli ; which travel along the length of the colon. The three taenia are:

• Taenia libera is the free taenia that is found at the antimesenteric surface of the colon; which is located on the anterior surface of the colon.
• Taenia omentalis is located posterolaterally and is attached to the omentum of the large intestines.
• Taenia mesocolic is found at the midpoint between the taenia libera and the mesenteric attachment on the colon.

Importantly, the cecum does not have the prominent haustrations seen on the rest of the colon. As the ascending colon travels from the right iliac fossa superiorly, it transitions to the transverse colon at the hepatic (right colic) flexure. The transverse colon travels across from the left to the right hypochondriac regions . It turns caudally at the splenic (left colic) flexure to form the descending colon . As the descending colon travels from the left hypochondrium to the left iliac fossa, it transitions into the sigmoid colon . This distal segment enters the pelvic inlet and terminates at the rectosigmoid junction at the level of the third sacral vertebra (S3).

Unlike the preceding colon, the rectum is circumferentially walled by smooth muscles . It does not have haustrations and is devoid of taenia coli. This distal continuation of the large intestines functions of a reservoir for stool, prior to excretion. It terminates at the level of the sacrococcygeal curvature . It passes over the pelvic diaphragm to form the anorectal junction .

The portion of the alimentary tract between the stomach and the anal canal is known as  bowel  and is formed of the following components:

You can remember this easily if you use the mnemonic  D ow J ones I ndustrial A verage C losing S tock R eport, where the first letter of each word corresponds to the first letter of a bowel component!

The final passageway through which undigested food and exfoliated mucosa will exit the body is called the anal canal. It continues from the anorectal junction and passes through the loop formed by the puborectalis muscle, which swings the anal canal anteriorly. Distally, the mucosa of the anal canal transitions from the columnar epithelium with goblet cells found throughout the colon, to the squamous epithelium of the perianal skin. This point is referred to as the anal verge .  

Organs of the accessory digestive system

The organs of the accessory digestive system have the principal role of synthesizing and secreting digestive enzymes to further break down food into nutrients. The salivary glands are paired structures in the oral cavity that secrete saliva and other enzymes that mix with the masticated food to form the bolus. There are three major salivary glands in the oral cavity:

• Parotid glands
• Submandibular glands
• Sublingual glands

Another important accessory digestive organ is the liver. It is located in the right upper quadrant of the abdomen , beneath the right hemidiaphragm. The liver has two anatomical lobes, but eight functional segments. In addition to producing bile to digest fats, all of the nutrients absorbed from the small intestines enter the liver via the hepatic portal venous system .

From the liver, the nutrients are integrated into various catabolic processes and sent throughout the body. Most of the bile made by the liver is stored in the gallbladder . This muscular, sac-like organ that resides on the posterior surface of the liver drains its contents into the extrahepatic biliary tree following a fatty meal.

Finally, the pancreas is a retroabdominal organ that also provides enzymes for digestion. The head of the pancreas is found within the  C-shaped loop of the duodenum. The body extends superolaterally, behind the gastric antrum. The tail of the pancreas terminates at the hilum of the spleen .

Neurovascular supply and venous drainage

The vast majority of the digestive system is supplied by the abdominal aorta . In the abdominal cavity, the aorta gives off three major branches that, in succession, supply the derivatives of the foregut, midgut and hindgut:

• Celiac trunk , which after a short course further splits three into major branches; left gastric , common hepatic and splenic arteries . Via these branches, the celiac trunk supplies the foregut, including the abdominal part of the esophagus, stomach, upper 1/3 of the duodenum, liver, spleen, and pancreas.
• Superior mesenteric artery , which gives off the inferior pancreaticoduodenal , middle colic , right colic , ileocolic arteries , jejunal and ileal branches, and the marginal artery of Drummond . These branches supply the midgut, which includes the distal 2/3 of the duodenum, jejunum, ileum, cecum, appendix, ascending colon and proximal 1/3 of the transverse colon.
• Inferior mesenteric artery , which gives off the left colic , sigmoid, and superior rectal arteries . These branches provide blood to the hindgut, supplying the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and anus.

The digestive system receives both sympathetic and parasympathetic innervation. Sympathetic innervation stems from the abdominopelvic splanchnic nerves . These nerves arise from the sympathetic trunk and include the thoracic and lumbar splanchnic nerves . The splanchnic nerves synapse with prevertebral ganglia (celiac, aorticorenal, superior mesenteric, and inferior mesenteric ganglia) which project postganglionic fibers to innervate the organs of the digestive system. Sympathetic innervation inhibits peristalsis, constricts blood vessels and redirects blood from the digestive system to the skeletal muscles and contracts the anal sphincters.

The parasympathetic supply to the digestive system stems from the vagus nerve (CN X) and pelvic splanchnic nerves . The vagus nerve supplies the digestive tract from the esophagus to the transverse colon, while the pelvic splanchnic nerves supply the descending colon, sigmoid colon and rectum. Parasympathetic innervation increases peristalsis, promotes secretion of digestive juices, relaxes the internal anal sphincter and contracts the rectum.

There are two venous systems that drain the organs of the digestive tract; portal venous system and the systemic venous system .

In the portal venous system, the nutrient-rich blood from the digestive tract is drained by the hepatic portal vein into the liver to be filtered and detoxified. The hepatic portal vein is formed by the merger of the  superior mesenteric vein and the splenic vein .

After being processed by the liver, the blood is carried into the systemic venous system by the hepatic veins , which drain into the inferior vena cava .

Gastrointestinal pathology ranks high among the chief presenting complaints of patients presenting to both outpatient and emergency settings. Obtaining adequate historical details will aid the clinician in narrowing down the pathology to a particular part of the digestive system. Once the history is complete, the clinical examination is conducted to confirm, rule-in or rule-out any diagnosis. The examination follows the rules of inspection, palpation, percussion, and auscultation.

Adequate exposure of the patient for an abdominal exam should allow visibility from the nipple line to mid thigh. Inspect the abdomen for:

• distention (either symmetrical or asymmetrical)
• distended superficial veins (caput medusae)
• cutaneous discoloration
• visible peristalsis

On inspection, also ask the patient to cover their mouth and cough while the examiner observes the abdominal wall. A visible cough impulse (localized distention of the abdominal wall due to raised intra-abdominal pressure from coughing) suggests that there is a hernia at that site.

Palpation should be performed to assess for any masses as well as to determine the surface texture of the liver. The examiner may also determine if there is an enlargement of the kidneys or spleen at this time as well. The abdominal cavity has the potential to store a large volume of interstitial fluid. Large volumes of fluid can be detected during palpation by eliciting a fluid thrill. Smaller volumes can be picked up during percussion with the shifting dullness technique. Percussion also helps the examiner to confirm the size and location (if enlarged) of the liver and spleen. Be sure to auscultate the abdomen for bowel sounds and bruits. The abdominal exam is concluded with a digital rectal and genital examination.

References:

• Ahima, Rexford S., and Daniel A. Antwi:  "Brain Regulation Of Appetite And Satiety." Endocrinology And Metabolism Clinics Of North America, vol 37, no. 4, 2008, pp. 811-823. Elsevier BV, doi:10.1016/j.ecl.2008.08.005.
• Burnand, Kevin G, and Norman L Browse:  Browse's Introduction To The Symptoms & Signs Of Surgical Disease. 5th ed., Boca Raton, FL, CRC Press, Taylor & Francis Group, 2015,.
• Guyton, Arthur C, and John E Hall:  Textbook Of Medical Physiology. 11th ed., Philadelphia, PA, Elsevier Saunders, 2007,.
• Standring, S., Borley, N. and Gray, H.:  (2008). Gray's Anatomy. 42nd ed. [Edinburgh]: Churchill Livingstone/Elsevier, pp.1191-1195.

Illustrators:

• Digestive system (Small intestine) - Begoña Rodriguez
• Hypothalamus (Ventromedial nucleus - lateral-left view) - Paul Kim
• Masseter muscle - Paul Kim
• Nucleus ambiguus (lateral-left view) - Paul Kim
• Esophagus (lateral-left view) - Yousun Koh
• Circular layer of muscular coat of stomach (ventral view) - Rebecca Betts
• Duodenum (ventral view) - Samantha Zimmerman
• Oral cavity proper (ventral view) - Paul Kim
• Palatopharyngeal muscle (dorsal view) - Yousun Koh
• Greater curvature of the stomach (ventral view) - Irina Münstermann
• Descending part of duodenum (ventral view) - Begoña Rodriguez
• Jejunum (ventral view) - Irina Münstermann
• Taenia mesocolica (ventral view) - Begoña Rodriguez
• Rectum (ventral view) - Samantha Zimmerman
• Sublingual gland - Paul Kim
• Liver (ventral view) - Begoña Rodriguez
• Duodenum and jejunum during a surgical intervention - Prof. Carlos Suárez-Quian
• Cecum, appendix and ascending colon in a cadaver - Prof. Carlos Suárez-Quian
• Pharynx (cadaveric dissection) - Prof. Carlos Suárez-Quian

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Digestive System Anatomy and Physiology

Embark on a fascinating voyage through the alimentary canal with our digestive system anatomy and physiology guide. Nursing students, savor the complexity of the processes that turn morsels into energy, and let curiosity guide your exploration.

Table of Contents

Functions of the digestive system, organs of the alimentary canal, small intestine, large intestine, accessory digestive organs, salivary glands, gallbladder, activities occurring in the mouth, pharynx, and esophagus, activities of the stomach, activities of the small intestine, activities of the large intestine, age-related physiological changes in the gastrointestinal system.

The functions of the digestive system are:

• Ingestion. Food must be placed into the mouth before it can be acted on; this is an active, voluntary process called ingestion.
• Propulsion. If foods are to be processed by more than one digestive organ, they must be propelled from one organ to the next; swallowing is one example of food movement that depends largely on the propulsive process called peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall).
• Food breakdown: mechanical digestion. Mechanical digestion prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces, and examples of mechanical digestion are the mixing of food in the mouth by the tongue, churning of food in the stomach, and segmentation in the small intestine.
• Food breakdown: chemical digestion. The sequence of steps in which large food molecules are broken down into their building blocks by enzymes is called chemical digestion.
• Absorption. Transport of digested end products from the lumen of the GI tract to the blood or lymph is absorption, and for absorption to happen, the digested foods must first enter the mucosal cells by active or passive transport processes.
• Defecation. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces .

Anatomy of the Digestive System

The organs of the digestive system can be separated into two main groups: those forming the alimentary canal and the accessory digestive organs.

The alimentary canal, also called the gastrointestinal tract, is a continuous, hollow muscular tube that winds through the ventral body cavity and is open at both ends. Its organs include the following:

Food enters the digestive tract through the mouth , or oral cavity, a mucous membrane-lined cavity.

• Lips. The lips (labia) protect its anterior opening.
• Cheeks. The cheeks form its lateral walls.
• Palate . The hard palate forms its anterior roof, and the soft palate forms its posterior roof.
• Uvula. The uvula is a fleshy finger-like projection of the soft palate, which extends inferiorly from the posterior edge of the soft palate.
• Vestibule. The space between the lips and the cheeks externally and the teeth and gums internally is the vestibule.
• Oral cavity proper. The area contained by the teeth is the oral cavity proper.
• Tongue. The muscular tongue occupies the floor of the mouth and has several bony attachments- two of these are to the hyoid bone and the styloid processes of the skull .
• Lingual frenulum. The lingual frenulum, a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements.
• Palatine tonsils . At the posterior end of the oral cavity are paired masses of lymphatic tissue, the palatine tonsils .
• Lingual tonsil. The lingual tonsils cover the base of the tongue just beyond.

From the mouth, food passes posteriorly into the oropharynx and laryngopharynx.

• Oropharynx. The oropharynx is posterior to the oral cavity.
• Laryngopharynx. The laryngopharynx is continuous with the esophagus below; both of which are common passageways for food, fluids, and air.

The esophagus or gullet , runs from the pharynx through the diaphragm to the stomach.

• Size and function. About 25 cm (10 inches) long, it is essentially a passageway that conducts food by peristalsis to the stomach.
• Structure. The walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same four basic tissue layers or tunics.
• Mucosa. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen, of the organ; it consists primarily of surface epithelium, plus a small amount of connective tissue ( lamina propria ) and a scanty smooth muscle layer.
• Submucosa. The submucosa is found just beneath the mucosa; it is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic vessels.
• Muscularis externa . The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells.
• Serosa. The serosa is the outermost layer of the wall that consists of a single layer of flat serous fluid-producing cells, the visceral peritoneum .
• Intrinsic nerve plexuses. The alimentary canal wall contains two important intrinsic nerve plexuses- the submucosal nerve plexus and the myenteric nerve plexus , both of which are networks of nerve fibers that are actually part of the autonomic nervous system and help regulate the mobility and secretory activity of the GI tract organs.

Different regions of the stomach have been named, and they include the following:

• Location. The C-shaped stomach is on the left side of the abdominal cavity, nearly hidden by the liver and the diaphragm.
• Function. The stomach acts as a temporary “storage tank” for food as well as a site for food breakdown.
• Cardiac region. The cardiac region surrounds the cardioesophageal sphincter , through which food enters the stomach from the esophagus.
• Fundus. The fundus is the expanded part of the stomach lateral to the cardiac region.
• Body. The body is the midportion, and as it narrows inferiorly, it becomes the pyloric antrum , and then the funnel-shaped pylorus.
• Pylorus. The pylorus is the terminal part of the stomach and it is continuous with the small intestine through the pyloric sphincter or valve .
• Size. The stomach varies from 15 to 25 cm in length , but its diameter and volume depend on how much food it contains; when it is full, it can hold about 4 liters (1 gallon) of food, but when it is empty it collapses inward on itself.
• Rugae. The mucosa of the stomach is thrown into large folds called rugae when it is empty.
• Greater curvature. The convex lateral surface of the stomach is the greater curvature.
• Lesser curvature. The concave medial surface is the lesser curvature.
• Lesser omentum. The lesser omentum, a double layer of peritoneum, extends from the liver to the greater curvature.
• Greater omentum.   The greater omentum, another extension of the peritoneum, drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall, and is riddled with fat, which helps to insulate, cushion, and protect the abdominal organs.
• Stomach mucosa. The mucosa of the stomach is a simple columnar epithelium composed entirely of mucous cells that produce a protective layer of bicarbonate-rich alkaline mucus that clings to the stomach mucosa and protects the stomach wall from being damaged by acid and digested by enzymes.
• Gastric glands. This otherwise smooth lining is dotted with millions of deep gastric pits , which lead into gastric glands that secrete the solution called gastric juice .
• Intrinsic factor. Some stomach cells produce intrinsic factor, a substance needed for the absorption of vitamin b12 from the small intestine.
• Chief cells. The chief cells produce protein-digesting enzymes, mostly pepsinogens .
• Parietal cells . The parietal cells produce corrosive hydrochloric acid , which makes the stomach contents acidic and activates the enzymes.
• Enteroendocrine cells. The enteroendocrine cells produce local hormones such as gastrin , that are important to the digestive activities of the stomach.
• Chyme. After food has been processed, it resembles heavy cream and is called chyme.

The small intestine is the body’s major digestive organ.

• Location. The small intestine is a muscular tube extending from the pyloric sphincter to the large intestine.
• Size. It is the longest section of the alimentary tube, with an average length of 2.5 to 7 m (8 to 20 feet) in a living person.
• Subdivisions. The small intestine has three subdivisions: the duodenum , the jejunum , and the ileum , which contribute 5 percent, nearly 40 percent, and almost 60 percent of the small intestine, respectively.
• Ileocecal valve. The ileum meets the large intestine at the ileocecal valve, which joins the large and small intestine.
• Hepatopancreatic ampulla. The main pancreatic and bile ducts join at the duodenum to form the flasklike hepatopancreatic ampulla, literally, the ” liver-pacreatic-enlargement” .
• Duodenal papilla. From there, the bile and pancreatic juice travel through the duodenal papilla and enter the duodenum together.
• Microvilli. Microvilli are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance, sometimes referred to as the brush border ; the plasma membranes bear enzymes (brush border enzymes) that complete the digestion of proteins and carbohydrates in the small intestine.
• Villi. Villi are fingerlike projections of the mucosa that give it a velvety appearance and feel, much like the soft nap of a towel.
• Lacteal. Within each villus is a rich capillary bed and a modified lymphatic capillary called a lacteal.
• Circular folds. Circular folds, also called plicae circulares , are deep folds of both mucosa and submucosa layers, and they do not disappear when food fills the small intestine.
• Peyer’s patches . In contrast, local collections of lymphatic tissue found in the submucosa increase in number toward the end of the small intestine.

The large intestine is much larger in diameter than the small intestine but shorter in length.

• Size . About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus.
• Functions. Its major functions are to dry out indigestible food residue by absorbing water and to eliminate these residues from the body as feces.
• Subdivisions. It frames the small intestines on three sides and has the following subdivisions: cecum, appendix , colon , rectum, and anal canal .
• Cecum. The saclike cecum is the first part of the large intestine.
• Appendix. Hanging from the cecum is the wormlike appendix, a potential trouble spot because it is an ideal location for bacteria to accumulate and multiply.
• Ascending colon. The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure , to travel across the abdominal cavity.
• Transverse colon. The ascending colon makes a turn and continuous to be the transverse colon as it travels across the abdominal cavity.
• Descending colon. It then turns again at the left colic (or splenic) flexure , and continues down the left side as the descending colon.
• Sigmoid colon. The intestine then enters the pelvis, where it becomes the S-shaped sigmoid colon.
• Anal canal. The anal canal ends at the anus which opens to the exterior.
• External anal sphincter. The anal canal has an external voluntary sphincter, the external anal sphincter, composed of skeletal muscle .
• Internal involuntary sphincter. The internal involuntary sphincter is formed by smooth muscles.

Other than the intestines and the stomach, the following are also part of the digestive system:

The role the teeth play in food processing needs little introduction; we masticate, or chew, by opening and closing our jaws and moving them from side to side while continuously using our tongue to move the food between our teeth.

• Function. The teeth tear and grind the food, breaking it down into smaller fragments.
• Deciduous teeth. The first set of teeth is the deciduous teeth, also called baby teeth or milk teeth , and they begin to erupt around 6 months, and a baby has a full set (20 teeth) by the age of 2 years.
• Permanent teeth. As the second set of teeth, the deeper permanent teeth, enlarge and develop, the roots of the milk teeth are reabsorbed, and between the ages of 6 to 12 years they loosen and fall out.
• Incisors. The chisel-shaped incisors are adapted for cutting.
• Canines. The fanglike canines are for tearing and piercing.
• Premolars and molars. Premolars (bicuspids) and molars have broad crowns with round cusps ( tips) and are best suited for grinding.
• Crown. The enamel-covered crown is the exposed part of the tooth above the gingiva or gum.
• Enamel. Enamel is the hardest substance in the body and is fairly brittle because it is heavily mineralized with calcium salts.
• Root . The outer surface of the root is covered by a substance called cementum, which attaches the tooth to the periodontal membrane (ligament) .
• Dentin. Dentin, a bonelike material, underlies the enamel and forms the bulk of the tooth.
• Pulp cavity. It surrounds a central pulp cavity, which contains a number of structures (connective tissue, blood vessels, and nerve fibers) collectively called the pulp .
• Root canal. Where the pulp cavity extends into the root, it becomes the root canal, which provides a route for blood vessels, nerves , and other pulp structures to enter the pulp cavity of the tooth.

Three pairs of salivary glands empty their secretions into the mouth.

• Parotid glands. The large parotid glands lie anterior to the ears and empty their secretions into the mouth.
• Submandibular and sublingual glands. The submandibular and sublingual glands empty their secretions into the floor of the mouth through tiny ducts.
• Saliva. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids.
• Salivary amylase. The clear serous portion contains an enzyme, salivary amylase, in a bicarbonate-rich juice that begins the process of starch digestion in the mouth.

Only the pancreas produces enzymes that break down all categories of digestible foods.

• Location. The pancreas is a soft, pink triangular gland that extends across the abdomen from the spleen to the duodenum; but most of the pancreas lies posterior to the parietal peritoneum, hence its location is referred to as retroperitoneal .
• Pancreatic enzymes. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid that neutralizes the acidic chyme coming in from the stomach.
• Endocrine function. The pancreas also has an endocrine function; it produces hormones insulin and glucagon .

The liver is the largest gland in the body.

• Location. Located under the diaphragm, more to the right side of the body, it overlies and almost completely covers the stomach.
• Falciform ligament. The liver has four lobes and is suspended from the diaphragm and abdominal wall by a delicate mesentery cord, the falciform ligament.
• Function. The liver’s digestive function is to produce bile.
• Bile. Bile is a yellow-to-green, watery solution containing bile salts, bile pigments, cholesterol , phospholipids, and a variety of electrolytes .
• Bile salts. Bile does not contain enzymes but its bile salts emulsify fats by physically breaking large fat globules into smaller ones, thus providing more surface area for the fat-digesting enzymes to work on.

While in the gallbladder , bile is concentrated by the removal of water.

• Location. The gallbladder is a small, thin-walled green sac that snuggles in a shallow fossa in the inferior surface of the liver.
• Cystic duct. When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored.

Physiology of the Digestive System

Specifically, the digestive system takes in food (ingests it), breaks it down physically and chemically into nutrient molecules (digests it), and absorbs the nutrients into the bloodstream, then, it rids the body of indigestible remains (defecates).

The activities that occur in the mouth, pharynx, and esophagus are food ingestion, food breakdown, and food propulsion.

Food Ingestion and Breakdown

Once food is placed in the mouth, both mechanical and chemical digestion begin.

• Physical breakdown. First, the food is physically broken down into smaller particles by chewing.
• Chemical breakdown. Then, as the food is mixed with saliva, salivary amylase begins the chemical digestion of starch, breaking it down into maltose.
• Stimulation of saliva. When food enters the mouth, much larger amounts of saliva pour out; however, the simple pressure of anything put into the mouth and chewed will also stimulate the release of saliva.
• Passageways. The pharynx and the esophagus have no digestive function; they simply provide passageways to carry food to the next processing site, the stomach.

Food Propulsion – Swallowing and Peristalsis

For food to be sent on its way to the mouth, it must first be swallowed.

• Deglutition. Deglutition, or swallowing , is a complex process that involves the coordinated activity of several structures (tongue, soft palate, pharynx, and esophagus).
• Buccal phase of deglutition. The first phase, the voluntary buccal phase, occurs in the mouth; once the food has been chewed and well mixed with saliva, the bolus (food mass) is forced into the pharynx by the tongue.
• Pharyngeal-esophageal phase. The second phase, the involuntary pharyngeal-esophageal phase, transports food through the pharynx and esophagus; the parasympathetic division of the autonomic nervous system controls this phase and promotes the mobility of the digestive organs from this point on.
• Food routes. All routes that the food may take, except the desired route distal into the digestive tract, are blocked off; the tongue blocks off the mouth; the soft palate closes off the nasal passages; the larynx rises so that its opening is covered by the flaplike epiglottis .
• Stomach entrance. Once food reaches the distal end of the esophagus, it presses against the cardioesophageal sphincter, causing it to open, and food enters the stomach.

The activities of the stomach involve food breakdown and food propulsion.

Food Breakdown

The sight, smell , and taste of food stimulate parasympathetic nervous system reflexes, which increase the secretion of gastric juice by the stomach glands

• Gastric juice. Secretion of gastric juice is regulated by both neural and hormonal factors.
• Gastrin. The presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin, which prods the stomach glands to produce still more of the protein-digesting enzymes (pepsinogen), mucus, and hydrochloric acid.
• Pepsinogen. The extremely acidic environment that hydrochloric acid provides is necessary, because it activates pepsinogen to pepsin, the active protein-digesting enzyme.
• Rennin. Rennin, the second protein-digesting enzyme produced by the stomach, works primarily on milk protein and converts it to a substance that looks like sour milk.
• Food entry. As food enters and fills the stomach, its wall begins to stretch (at the same time as the gastric juices are being secreted).
• Stomach wall activation. Then the three muscle layers of the stomach wall become active; they compress and pummel the food, breaking it apart physically, all the while continuously mixing the food with the enzyme-containing gastric juice so that the semifluid chyme is formed.

Food Propulsion

Peristalsis is responsible for the movement of food towards the digestive site until the intestines.

• Peristalsis. Once the food has been well mixed, rippling peristalsis begins in the upper half of the stomach, and the contractions increase in force as the food approaches the pyloric valve.
• Pyloric passage. The pylorus of the stomach, which holds about 30 ml of chyme, acts like a meter that allows only liquids and very small particles to pass through the pyloric sphincter; and because the pyloric sphincter barely opens, each contraction of the stomach muscle squirts 3 ml or less of chyme into the small intestine.
• Enterogastric reflex. When the duodenum is filled with chyme and its wall is stretched, a nervous reflex, the enterogastric reflex, occurs; this reflex “puts the brakes on” gastric activity and slows the emptying of the stomach by inhibiting the vagus nerves and tightening the pyloric sphincter, thus allowing time for intestinal processing to catch up.

The activities of the small intestine are food breakdown and absorption and food propulsion.

Food Breakdown and Absorption

Food reaching the small intestine is only partially digested.

• Digestion. Food reaching the small intestine is only partially digested; carbohydrate and protein digestion has begun, but virtually no fats have been digested up to this point.
• Brush border enzymes. The microvilli of small intestine cells bears a few important enzymes, the so-called brush border enzymes, that break down double sugars into simple sugars and complete protein digestion.
• Pancreatic juice. Foods entering the small intestine are literally deluged with enzyme-rich pancreatic juice ducted in from the pancreas, as well as bile from the liver; pancreatic juice contains enzymes that, along with brush border enzymes, complete the digestion of starch, carry out about half of the protein digestion, and are totally responsible for fat digestion and digestion of nucleic acids.
• Chyme stimulation. When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones; two of these are secretin and cholecystokinin which influence the release of pancreatic juice and bile.
• Absorption. Absorption of water and of the end products of digestion occurs all along the length of the small intestine; most substances are absorbed through the intestinal cell plasma membranes by the process of active transport .
• Diffusion .  Lipids or fats are absorbed passively by the process of diffusion.
• Debris. At the end of the ileum, all that remains are some water, indigestible food materials, and large amounts of bacteria; this debris enters the large intestine through the ileocecal valve.

Peristalsis is the major means of propelling food through the digestive tract.

• Peristalsis. The net effect is that the food is moved through the small intestine in much the same way that toothpaste is squeezed from the tube.
• Constrictions. Rhythmic segmental movements produce local constrictions of the intestine that mix the chyme with the digestive juices, and help to propel food through the intestine.

The activities of the large intestine are food breakdown and absorption and defecation.

What is finally delivered to the large intestine contains few nutrients, but that residue still has 12 to 24 hours more to spend there.

• Metabolism. The “resident” bacteria that live in its lumen metabolize some of the remaining nutrients, releasing gases (methane and hydrogen sulfide) that contribute to the odor of feces.
• Flatus . About 50 ml of gas (flatus) is produced each day, much more when certain carbohydrate-rich foods are eaten.
• Absorption. Absorption by the large intestine is limited to the absorption of vitamin K, some B vitamins, some ions, and most of the remaining water.
• Feces. Feces, the more or less solid product delivered to the rectum, contains undigested food residues, mucus, millions of bacteria, and just enough water to allow their smooth passage.

Propulsion of the Residue and Defecation

When presented with residue, the colon becomes mobile, but its contractions are sluggish or short-lived.

• Haustral contractions. The movements most seen in the colon are haustral contractions, slow segmenting movements lasting about one minute that occur every 30 minutes or so.
• Propulsion . As the haustrum fills with food residue, the distension stimulates its muscle to contract, which propels the luminal contents into the next haustrum.
• Mass movements. Mass movements are long, slow-moving, but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum.
• Rectum. The rectum is generally empty, but when feces are forced into it by mass movements and its wall is stretched, the defecation reflex is initiated.
• Defecation reflex. The defecation reflex is a spinal (sacral region) reflex that causes the walls of the sigmoid colon and the rectum to contract and anal sphincters to relax.
• Impulses. As the feces is forced into the anal canal, messages reach the brain giving us time to make a decision as to whether the external voluntary sphincter should remain open or be constricted to stop passage of feces.
• Relaxation . Within a few seconds, the reflex contractions end and rectal walls relax; with the next mass movement , the defecation reflex is initiated again.

Age-related changes in the gastrointestinal system include reduced saliva, decreased esophageal and stomach motility, decreased stomach emptying time, decreased production of intrinsic factor, and decreased intestinal absorption, motility, and blood flow.  In addition, tooth enamel becomes harder and more brittle, making teeth more susceptible to fractures.

Health promotion teaching for elders include preventive dental care and effective oral hygiene , appropriate diet and sufficient fluid intake, regular bowel maintenance, and importance of colorectal cancer screening.

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10 thoughts on “Digestive System Anatomy and Physiology”

It very helpful for students & teachers

Yes it is very helpfull for the students.

Thank you so much for all this. This website is extremely I’m a third-year nursing student. Did you recently add the study notes because last time I checked they were not there?

“For food to be sent on its way to the MOUTH, it must first be swallowed” it says in the text. Shoudn’t this say “for food to be sent on it’s way to the stomach, it must first be swallowed”?

I love the notes it’s well elaborated

The Note is very helpful Infact I really appreciate

Its really so detailed and helpful for students specially BSc. nursing students

Thank you so much! It’s awesome to hear that the content is hitting the mark, especially for BSc nursing students. If there’s anything else you’d like to see or any specific topics to delve into, don’t hesitate to mention. Keep acing those studies! Thanks for your comment Divya.

Hi Pardeep, Thanks for the feedback! If you have any questions or need further details on the digestive system (or anything else), just give a shout. Happy studying!

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Critical Thinking Questions

Explain how the enteric nervous system supports the digestive system. What might occur that could result in the autonomic nervous system having a negative impact on digestion?

What layer of the alimentary canal tissue is capable of helping to protect the body against disease, and through what mechanism?

Offer a theory to explain why segmentation occurs and peristalsis slows in the small intestine.

It has been several hours since you last ate. Walking past a bakery, you catch a whiff of freshly baked bread. What type of reflex is triggered, and what is the result?

The composition of saliva varies from gland to gland. Discuss how saliva produced by the parotid gland differs in action from saliva produced by the sublingual gland.

During a hockey game, the puck hits a player in the mouth, knocking out all eight of his most anterior teeth. Which teeth did the player lose and how does this loss affect food ingestion?

What prevents swallowed food from entering the airways?

Explain the mechanism responsible for gastroesophageal reflux.

Describe the three processes involved in the esophageal phase of deglutition.

Explain how the stomach is protected from self-digestion and why this is necessary.

Describe unique anatomical features that enable the stomach to perform digestive functions.

Explain how nutrients absorbed in the small intestine pass into the general circulation.

Why is it important that chyme from the stomach is delivered to the small intestine slowly and in small amounts?

Describe three of the differences between the walls of the large and small intestines.

Why does the pancreas secrete some enzymes in their inactive forms, and where are these enzymes activated?

Describe the location of hepatocytes in the liver and how this arrangement enhances their function.

Explain the role of bile salts and lecithin in the emulsification of lipids (fats).

How is vitamin B 12 absorbed?

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

• Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix
• Publisher/website: OpenStax
• Book title: Anatomy and Physiology
• Publication date: Apr 25, 2013
• Location: Houston, Texas
• Book URL: https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
• Section URL: https://openstax.org/books/anatomy-and-physiology/pages/23-critical-thinking-questions

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