Human Digestive System

Digestion is the process by which the body breaks down food into smaller, absorbable components. It is crucial for converting food into energy and the raw materials required for growth, repair and maintenance of body functions. It supports the immune system, provides essential nutrients and ensures overall health. Efficient digestion prevents nutrient deficiencies, supports metabolism and maintains energy levels, making it vital for sustaining life.

The human digestive system is composed of the gastrointestinal (GI) tract and accessory digestive organs. The GI tract is a continuous tube that extends from mouth to anus. It includes the oral cavity, pharynx, oesophagus, stomach, small intestine and large intestine. The accessory digestive organs include the salivary glands, liver, gallbladder and pancreas.

📌 Note: Food travels in one direction through this tube — it is chewed and mixed with saliva in the mouth, swallowed through the pharynx and oesophagus, churned and partly digested in the stomach, fully digested and absorbed in the small intestine, and its wastes are collected and expelled through the large intestine.

The oral cavity is a cavity immediately after the opening of the mouth. Lips are made of highly vascularised skeletal muscle tissue with many sensory nerve endings. Lips help to retain food as it is being chewed. They also play a role in phonation (the modification of sound).

Selection of food: The muscular tongue plays a role in the selection of food through its taste buds. When food enters the oral cavity, it is tasted and physically felt. If the taste or smell is unpleasant, or if hard objects like bone or dirt are present in the food, it is rejected. The senses of smell and sight also play a role in the selection of food.

Mechanical digestion of food: The ingested food is physically broken down by the teeth through a process called mastication (chewing). Chewing breaks down food into smaller and more manageable pieces, increasing the surface area for enzymatic action.

Chemical digestion of food: As the chewing of food goes on, the salivary glands pour their secretion, saliva, into the oral cavity. Palate, tongue and cheeks help in the mixing of chewed food with saliva. There are three pairs of salivary glands: sublingual glands situated below the tongue, submaxillary glands located behind the jaws, and parotid glands located in front of the ears.

Saliva contains water and mucus that moisten and lubricate the food. Saliva also contains bicarbonate ions, which buffer chemicals in the oral cavity, and thiocyanate ions, which kill microorganisms. Fresh saliva is alkaline (pH 8) but it quickly loses CO₂ and gets pH 6. Saliva also contains an enzyme, salivary amylase. It partially digests the polysaccharides (starch and glycogen) to disaccharides (maltose). After the mechanical and chemical digestive processes in the oral cavity, the food mass is in the form of a small moist mass called a bolus.

Pharynx: The pharynx is a cavity behind the mouth. It is the common passageway for both the digestive and respiratory tracts. The bolus is pushed to the back of the mouth and is swallowed through the pharynx.

Swallowing of food: During swallowing, the tongue moves upwards and backwards against the roof of the mouth. Due to it, the bolus is forced to the back of the oral cavity. The soft palate is also raised against the back wall of the pharynx. These movements close the passage between the nasal cavity and pharynx. At the same time, the larynx moves upward and it lowers the epiglottis (a flap of cartilage) and closes the opening of the trachea. In this way, the bolus passes over the trachea and enters the oesophagus.

📌 Note: The beginning of the swallowing action is voluntary, but once the food reaches the back of the mouth, swallowing becomes automatic.

Oesophagus: After being swallowed, the food enters the tube called the oesophagus. It connects the pharynx to the stomach. The previous digestive actions of saliva continue in the oesophagus. In an adult human, the oesophagus is about 25 cm long and its lower end opens in the stomach. Food moves down through the oesophagus to the stomach by peristalsis. The exit of food from the oesophagus to the stomach is controlled by the lower oesophageal sphincter or cardiac sphincter, which opens in response to the pressure exerted by food. It also prevents the backflow of stomach contents into the oesophagus.

The following two types of movements occur in the alimentary canal.

Peristalsis: It is the rhythmic sequence of waves of contraction in the smooth muscles of the walls of the alimentary canal. Peristalsis squeezes the food down along the oesophagus and other parts of the alimentary canal.

Segmentation: The small and large intestines also have rings of smooth muscles, which contract and relax repeatedly. These contractions and relaxations create a back-and-forth movement in the same place, called segmentation. This movement mixes the food with digestive secretions and increases the efficiency of absorption.

Antiperistalsis: Occasionally, the peristaltic movements may reverse in a process called antiperistalsis, pushing food from the intestines back into the stomach and oral cavity, leading to vomiting.

In contrast, hunger contractions are peristaltic movements triggered by low blood glucose levels, creating the uncomfortable sensation known as hunger pangs.

The stomach is an elastic muscular bag (J-shaped) situated after the oesophagus and before the duodenum. It is located on the left side in the abdominal cavity, right below the diaphragm. It has three portions. The cardiac portion is present immediately after the oesophagus. The fundus portion is present on a side of the cardiac portion. The pyloric portion is located beneath the cardiac portion. The cardiac sphincter opens when a wave of peristaltic contractions coming down the oesophagus reaches it and allows food to enter the stomach.

Mechanical digestion of food: The stomach wall is made of the four layers which are:

• Mucosa: It is the inner layer of the stomach which contains glands. These glands secrete digestive enzymes, HCl and mucus.
• Submucosa: It is a layer of connective tissue which contains blood vessels, lymphatics and nerves.
• Muscularis externa: It is a layer of muscle, i.e., outer longitudinal muscles, middle circular muscles and the inner oblique muscles.
• Serosa: It is an outermost thin layer that forms part of the peritoneum and provides protection.

The muscular walls of the stomach contract vigorously and help in the churning of food (mechanical digestion) and mixing the food with stomach secretions. These contractions also generate enough heat that melts the solid fats.

Chemical digestion of food: The mucosa of the stomach possesses numerous tubular gastric glands. These glands open in the mucosa wall through deep depressions, called gastric pits. The glands contain three kinds of cells:

• (i) Mucous cells which secrete mucus, a thick secretion that covers the inside of the stomach and protects it from HCl and digestive enzymes.
• (ii) Parietal (oxyntic) cells which secrete hydrochloric acid. It adjusts the pH of stomach contents to about 2–3. HCl also softens the food, activates the pepsinogen and kills microorganisms.
• (iii) Chief cells which secrete the enzyme, pepsinogen.

All the secretions of the gastric glands are collectively called gastric juice. When the bolus enters the stomach, the gastric glands secrete gastric juice. The H⁺ ions of the HCl activate pepsinogen into pepsin. Pepsin catalyses the breakdown of proteins to yield polypeptides and peptides. About three to four hours after a meal, the stomach contents have been sufficiently mixed and are a semi-liquid acidic mass called chyme. The pyloric sphincter regulates the release of the chyme into the small intestine.

📌 Note — pyrosis (heartburn): Sometimes there is a back flush of acidic chyme from the stomach into the oesophagus. It causes a painful burning sensation in the chest and this condition is known as pyrosis or heartburn.

📌 Note — protection of mucosa: The mucosa of the stomach is susceptible to damage from acid and pepsin if it had no protection. Protection of the mucosa is provided in two ways: viscous mucus and bicarbonate, which neutralizes acid.

Regulation of secretion of gastric juice: The secretion of gastric juice is regulated by both the nervous system and hormonal mechanisms. In reaction to the smell, sight, or thought of food, the medulla of the brain sends a message to the gastric glands to secrete small amounts of gastric juice. When food arrives in the stomach, the distension of the stomach and decrease in the pH of the gastric contents stimulate more secretion and powerful contractions.

The presence of proteins in food stimulates special endocrine cells present in the mucosa of the stomach to release a hormone called gastrin. Gastrin is carried by blood to the gastric glands where it stimulates them to produce and secrete more gastric juice. When food moves from the stomach to the small intestine, a hormone called somatostatin stops the release of hydrochloric acid.

The Small Intestine: It is the longest part of the alimentary canal. It starts after the stomach and ends at the large intestine. In an adult man it is about 2–3 cm in diameter and 6 m in length. The small intestine is responsible not only for the final digestion of all kinds of food but also for the absorption of digested food into blood and lymph. The small intestine consists of three parts, i.e., duodenum, jejunum and ileum.

Duodenum: The first 20–25 cm long portion is the duodenum. It is concerned with the digestion of food. It also contains glands, which produce an alkaline secretion containing bicarbonate. Two main secretions are poured into the duodenum.

a- Pancreatic juice: It is the secretion of the pancreas and is poured into the duodenum. It is slightly alkaline (pH 8) due to the presence of bicarbonate. It neutralizes the acidity of chyme. The important enzymes in pancreatic juice are:

• (i) Pancreatic amylase, which digests polysaccharides into maltose and even glucose.
• (ii) Trypsinogen, which is in inactive form. Another enzyme, enterokinase (secreted by the walls of the duodenum), activates trypsinogen into trypsin, which digests proteins into polypeptides.
• (iii) Chymotrypsin and carboxypeptidase, which digest proteins into smaller peptides and then into amino acids.
• (iv) Pancreatic lipase, which digests lipids to glycerol and fatty acids.
• (v) Pancreatic nucleases, which digest DNA and RNA into nucleotides.

📌 Note — bile salts as detergents: Fats are insoluble in water. So, they cannot be attacked readily by lipase enzymes of pancreatic juice. Bile salts act as detergent molecules. They break fats into droplets and keep them separate from one another.

b- Bile: It is the secretion of the liver. Before its release, it is stored in the gallbladder. It contains salts which emulsify fats and break them into small droplets (emulsion). These droplets provide large surface areas for effective action of lipid-digesting enzymes.

📌 Note — jaundice: If bile pigments are prevented from leaving the digestive tract, they may accumulate in blood, causing a condition known as jaundice.

Jejunum and Ileum: Jejunum is a 2.4 metres long part, next to the duodenum. Ileum is the last three-fifth, i.e., about 3.5 metres long part of the small intestine. These parts carry out the rest of digestion and absorption of food. The walls of the jejunum and ileum contain glands which secrete intestinal juice. It contains various enzymes; for example, aminopeptidase digests polypeptides into dipeptides, erepsin digests dipeptides into amino acids, lipase digests fats into fatty acids and glycerol, maltase digests maltose into glucose, sucrase digests sucrose into glucose and fructose, and lactase digests lactose into glucose and galactose. After the action of enzymes of intestinal juice, the chyme is converted into an alkaline emulsion, called chyle.

Absorption of Digested Food and Water: The absorption of digested food, water, and dissolved minerals occurs in the jejunum and ileum. The inner wall of the jejunum and ileum contains large circular folds. These folds have numerous finger-like projections called villi.

Each villus is richly supplied with blood capillaries and a vessel of the lymphatic system, called a lacteal. The blood capillaries and lacteal are covered by a single-cell thick epithelium. The epithelial cells of villi have countless cytoplasmic projections, called microvilli. The total surface area of absorption becomes extraordinarily large due to villi and microvilli.

📌 Note: Due to the presence of folds and numerous villi, the internal surface of the jejunum and ileum appears velvety.

Absorption of simple sugars and amino acids: Simple sugars (e.g., glucose) and amino acids are absorbed by diffusion or active transport into the epithelial cells of villi. From here, these molecules enter the blood capillaries of villi. Blood capillaries of villi join to form the hepatic portal vein which carries sugars and amino acids to the liver. The liver stores extra glucose and amino acids in the form of glycogen and proteins respectively. From the liver, the required amounts of these products pass to the heart via the hepatic vein.

Absorption of fatty acids and glycerol: The products of fat digestion, i.e., fatty acids and glycerol, are absorbed by passive transport into the epithelial cells of villi. Inside villi, they combine to form triglycerides. The triglycerides are coated with proteins. In this way small droplets, called chylomicrons, are formed. The chylomicrons enter the lacteals of villi. From the lacteals, the chylomicrons move into the thoracic lymphatic duct, from where they enter the bloodstream. Blood plasma has enzymes which hydrolyse chylomicrons back into fats and proteins. Fats are ultimately hydrolysed into fatty acids and glycerol and enter body cells.

The Large Intestine: It is the last part of the alimentary canal. It is much shorter than the small intestine, occupying about the last metre of the intestinal tract. It is involved in the absorption of water and salts and vitamin K from the lumen of the intestine into the blood. The large intestine is not convoluted and its inner surface area does not possess villi. It consists of three parts.

Caecum: It is a blind sac that projects from the area of the large intestine between the ileum and colon. From the blind end of the caecum there arises a finger-like process called the vermiform appendix. In the human digestive system, the appendix performs no function so it is vestigial.

📌 Note — appendicitis: Appendicitis is the inflammation of the appendix. It is usually due to bacterial infection. The infected appendix must be removed surgically otherwise it may burst and the inflammation may spread in the entire lining of the abdomen. The surgical removal of the appendix is called appendicectomy.

Colon: Next to the caecum is the colon. It has an ascending, a transverse and a descending limb. Its main function is to absorb water from the alimentary canal. As the water is absorbed, the remaining material becomes more solid. These waste products, called faeces, consist of a large number of bacteria, indigestible plant fibres (e.g., cellulose), other undigested food stuff, sloughed off mucosal cells, bile pigments and water.

Rectum: It is the last part of the large intestine where faeces are temporarily stored. At its distal end, the rectum opens out through the anus. The anus is surrounded by two sphincters; the internal sphincter is made of smooth muscles and the outer is made of striated muscles. Under normal conditions when the rectum is filled up with faeces, it gives rise to a defecation reflex. The defecation reflex is consciously inhibited in adults but in infants it is controlled involuntarily. During growth, the child learns to bring this reflex under voluntary control.

Many bacteria, for example E. coli, live and actively divide within the colon. During their metabolism, they produce amino acids and vitamin K. Vitamin K is necessary for man for the coagulation of blood. It is absorbed from the large intestine into the blood.

Control of Egestion: The involuntary reflex for egestion in infants and the voluntary control in adults represent two stages of neurological and muscular development. In infants, egestion is an involuntary reflex mediated by the spinal cord, where rectal distension triggers automatic relaxation of the internal anal sphincter and expulsion of waste. This occurs because the higher brain centres responsible for voluntary control are not yet fully developed. In adults, egestion becomes voluntary as the cerebral cortex matures, allowing conscious regulation of the external anal sphincter to delay or initiate defecation. This transition reflects the integration of reflex pathways with cognitive control, adapting to social and environmental demands.

Accessory Digestive Organs

1. Liver and Gallbladder: The liver plays a vital role in digestion by producing bile, which is essential for fat digestion. Bile emulsifies fats, making them easier to digest. The liver also processes nutrients absorbed from the small intestine, detoxifies harmful substances, synthesizes proteins and stores glycogen for energy.

The gallbladder stores and concentrates bile produced by the liver. When food enters the small intestine, the gallbladder releases bile through the bile duct.

📌 Note — gallstones: Cholesterol, secreted by the liver, may precipitate in the gallbladder to produce gallstones, which may block release of bile.

2. Pancreas: The pancreas is a large gland situated just ventral to the stomach. It has exocrine and endocrine portions. The exocrine (ducted) portion secretes its secretion, i.e., pancreatic juice, into the pancreatic duct. The pancreatic duct joins with the common bile duct from the liver and enters the duodenum. Pancreatic juice contains enzymes for the digestion of all groups of food. Its major enzymes include trypsin, chymotrypsin, lipases, amylases, nucleases etc. The endocrine (ductless) portion of the pancreas secretes its secretion, i.e., insulin and glucagon hormones, into extracellular fluid from where they diffuse into nearby capillaries.

Hormonal Control of the Secretions of Pancreas and Liver: We have studied the regulation of gastric secretions through the nervous system and hormones. The release of secretions from the pancreas and liver is also controlled by hormones. When chyme enters the duodenum from the stomach, its acidity stimulates the duodenal walls to release a hormone, secretin. Similarly, the partially digested proteins and fats present in chyme stimulate the duodenal walls to secrete another hormone, cholecystokinin (CCK). Both these hormones stimulate the pancreas to release pancreatic juice, and the gallbladder to release bile.

Storage and Metabolic Role of the Liver: The liver performs many important functions, especially in storing nutrients and regulating metabolism. It stores excess nutrients from the food and releases them when the body needs energy or building materials. These nutrients stored in the liver include glucose (stored as glycogen), vitamins (like A, D, B12, and K), minerals (e.g., iron and copper), and fats and fat-soluble substances. It also plays a central role in metabolism. It helps in breaking down, building up, and converting substances in the body. For example, it converts excess glucose into glycogen and back when needed. It also breaks down fats to produce energy and forms cholesterol and lipoproteins. It converts amino acids and removes harmful ammonia by turning it into urea, which is excreted in urine. It breaks down and removes toxins, drugs, and alcohol from the blood. The liver also helps in breaking down and regulating hormones.