Human Digestive System
Overview of the Digestive System
The Human Digestive System is a long coiled tube extending from the Oral Cavity to the Anus, collectively called the Alimentary Canal. Associated with the canal are three major Digestive Glands: the Salivary Glands, Liver, and Pancreas. The system performs five characteristic processes: Ingestion, Digestion, Absorption, Assimilation, and Egestion. There are three main sites of digestion in the human system — the oral cavity, stomach, and Small Intestine.
Ingestion: Taking in of complex food into the oral cavity
Digestion: Breakdown of complex organic compounds into simpler diffusible molecules by the action of enzymes
Absorption: Uptake of diffusible food molecules from the digestive region into the blood or lymph
Assimilation: Utilisation of digested products for energy production or synthesis of cellular material
Egestion: Elimination of undigested matter from the body through the anus
Regions of the Alimentary Canal (in order of food passage)
1
Oral (Buccal) Cavity
2
Pharynx
3
Esophagus
4
Stomach
5
Small Intestine: Duodenum, Jejunum, Ileum
6
Large Intestine: Caecum, Colon (Ascending, Transverse, Descending), Rectum
7
Anus
Oral Cavity and Salivary Glands
The Oral Cavity is bounded by the palate, tongue, teeth, and cheeks. It performs four functions: Selection of Food, Mastication, Lubrication, and Digestion. The tongue, being both sensory and muscular, plays the most important role through its Taste Buds. After selection and grinding, the food is mixed with Saliva produced by three pairs of Salivary Glands to form a Bolus, which is then swallowed.
Selection of Food: The oral cavity tastes, smells, and feels food — rejecting unpleasant or harmful items with the help of taste buds on the tongue
Mastication: Molar teeth grind food into smaller pieces, increasing surface area for enzyme action and allowing the esophagus to pass the food through
Lubrication: Water and mucus in saliva moisten and lubricate food for efficient chewing and smooth passage through the esophagus
Bolus Formation: The softened, partly digested, slimy food mass is rolled into an oval lump by the tongue before swallowing
Saliva is produced by three pairs of Salivary Glands: the Sublingual Glands below the tongue, the Submandibular Glands behind the jaws, and the Parotid Glands in front of the ears. Saliva contains three important components: water and mucus for lubrication, sodium bicarbonate and salts for pH stabilisation, and the carbohydrate-digesting enzyme Amylase (also called Ptyalin).
Sublingual Glands: Located below the tongue; secrete saliva containing mucus
Submandibular Glands: Located behind the jaws; secrete saliva with amylase and mucus
Parotid Glands: Located in front of the ears; secrete saliva with amylase
Sodium Bicarbonate: Fresh saliva is alkaline (pH ≈ 8), which stabilises the pH of food; it quickly loses CO₂ and shifts to pH 6
Amylase (Ptyalin): Digests starch and glycogen into maltose
Components of Saliva and Their Functions
•
Water + Mucus: Moisten and lubricate food for chewing and swallowing
•
Sodium Bicarbonate + Salts: Slightly antiseptic; stabilise pH of food (fresh saliva pH ≈ 8)
•
Amylase (Ptyalin): Digests starch and glycogen into maltose
Swallowing begins as a voluntary action but becomes automatic once food reaches the back of the mouth. The tongue moves upward and backward, pushing the bolus toward the pharynx. The soft palate closes the nasal opening, and the Epiglottis — a flap of cartilage — deflects the food away from the Glottis (windpipe opening) into the Esophagus. The larynx moves upward, and a ring of muscles partly closes the glottis for additional protection.
Tongue Action: Moves upward and backward, forcing the bolus to the back of the oral cavity
Soft Palate: Closes the nasal opening at the back to prevent food from entering the nasal cavity
Epiglottis: A flap of cartilage that covers the glottis and diverts food into the esophagus
Larynx Movement: Moves upward under the back of the tongue; the glottis is partly closed by a ring of muscle
Voluntary to Automatic: The initial action is voluntary, but once food reaches the pharynx the process becomes a reflex
Esophagus and Peristalsis
Food moves from the pharynx to the stomach through the Esophagus via rhythmic contractions called Peristalsis. Peristalsis consists of a wave of contraction of circular and longitudinal muscles preceded by a wave of relaxation, squeezing the bolus along the canal. These contractions are powerful enough to move food even against gravity — a person can swallow while doing a headstand.
Peristalsis: Wave-like muscle contractions (contraction behind the bolus, relaxation ahead) that propel food through the entire alimentary canal
Gravity Independent: Peristaltic contractions are sufficiently forceful to push food to the stomach regardless of body position
Anti-peristalsis: Reversed peristaltic movements that push food from intestine back toward the mouth, resulting in vomiting
Hunger Pangs: Peristaltic contractions intensified by low blood glucose levels, usually beginning 12 to 24 hours after the previous meal
Stomach: Structure and Gastric Digestion
The Stomach is situated below the diaphragm on the left side of the abdominal cavity. It is an elastic muscular bag that stores food, making discontinuous feeding possible, and partly digests it. At the junction of the esophagus and stomach is the Cardiac Sphincter, a ring of muscles that prevents stomach contents from flowing back into the esophagus. At the lower end, the Pyloric Sphincter controls the release of Chyme into the Duodenum.
Location: Below the diaphragm, on the left side of the abdominal cavity
Cardiac Sphincter: Ring of muscles between esophagus and stomach; opens when peristaltic waves reach it and closes to prevent reflux
Pyloric Sphincter: Ring of muscles at the stomach-duodenum junction; controls the release of chyme into the duodenum
Chyme: Semi-solid, partly digested food mass produced by the stomach
The stomach wall has three principal layers: an outer connective tissue layer, a middle layer of smooth muscles (outer longitudinal and inner circular), and an inner Mucosa containing numerous tubular Gastric Glands. The gastric glands are composed of three types of cells: Mucous Cells, Parietal Cells (also called Oxyntic Cells), and Zymogen Cells. Their collective secretion is called Gastric Juice.
Mucous Cells: Secrete Mucus, a thick secretion that coats the stomach lining and protects it from being digested by gastric juice
Parietal (Oxyntic) Cells: Secrete Hydrochloric Acid, which adjusts stomach pH to 2–3, softens food, and kills microorganisms
Zymogen (Chief) Cells: Secrete Pepsinogen, the inactive precursor of the protein-digesting enzyme Pepsin
Gastric Juice: The combined secretion of mucous cells, parietal cells, and zymogen cells
Summary of Gastric Gland Cells and Their Secretions
•
Mucous Cells → Mucus: Protects stomach wall from self-digestion
•
Parietal (Oxyntic) Cells → Hydrochloric Acid: Lowers pH to 2–3; kills microbes; activates pepsinogen
•
Zymogen (Chief) Cells → Pepsinogen: Inactive form converted to pepsin by HCl or already-activated pepsin
Pepsin hydrolyses proteins into Peptones and Polypeptides. The muscular walls of the stomach churn and mix the food with Gastric Juice, gradually converting it into Chyme. The secretion of gastric juice is regulated by smell, sight, and quality of food. High protein content stimulates the production of Gastrin — a hormone from the gastric endocrine lining — which is carried by blood to the gastric glands and stimulates them to produce more gastric juice.
Protein Digestion: Pepsin breaks down proteins into peptones and polypeptides (partial digestion)
Churning: Muscular contractions of the stomach wall mix food with gastric juice thoroughly
Gastrin: A hormone produced by endocrine cells in the stomach lining in response to protein-rich food; stimulates gastric gland secretion via the bloodstream
Regulation: Smell, sight, and food quality regulate gastric juice secretion; more protein leads to more gastrin and more gastric juice
Small Intestine: Duodenum and Pancreatic Digestion
The Small Intestine consists of three regions: the Duodenum (20–25 cm), the Jejunum (about 2.4 m, two-fifths of the small intestine), and the Ileum (three-fifths of the small intestine). When acidic Chyme enters the Duodenum, its acidity stimulates the release of Secretin — a hormone produced by the intestinal mucosa. Secretin is carried by blood to the Pancreas, stimulating it to produce Pancreatic Juice. Secretin also inhibits gastric secretion.
Duodenum: First 20–25 cm of the small intestine; receives chyme from stomach and secretions from pancreas and liver
Jejunum: Middle section, about 2.4 m long; continues digestion of food escaping the duodenum
Ileum: Final section (three-fifths of the small intestine); principal site of absorption
Secretin: Hormone produced by duodenal mucosa in response to acidic chyme; stimulates pancreatic secretion and inhibits gastric secretion
The Pancreas is a large gland whose exocrine tissue secretes Pancreatic Juice through the pancreatic duct into the duodenum. This juice contains enzymes that digest all three principal components of food: Pancreatic Amylase (amylopsin) digests starch into maltose, Lipase hydrolyses fats into fatty acids and glycerol, and Trypsin splits proteins into peptones and polypeptides. Trypsin is secreted in its inactive form Trypsinogen, which is activated by Enterokinase — an enzyme secreted by the duodenal lining. Pancreatic juice also contains sodium bicarbonate, which neutralises the acidic chyme from the stomach.
Pancreatic Amylase (Amylopsin): Digests starch into maltose
Lipase: Hydrolyses a small percentage of fats into fatty acids and glycerol
Trypsinogen: Inactive precursor of trypsin; activated by enterokinase from the duodenal lining
Trypsin: Splits proteins into peptones and polypeptides
Sodium Bicarbonate: Neutralises acidic chyme from the stomach so that pancreatic enzymes can function optimally
Enterokinase: Enzyme from the duodenal lining that converts trypsinogen to active trypsin
Pancreatic Juice Components and Their Functions
•
Pancreatic Amylase → Starch → Maltose
•
Lipase → Fats → Fatty Acids + Glycerol
•
Trypsinogen (activated by Enterokinase) → Trypsin → Proteins → Peptones + Polypeptides
•
Sodium Bicarbonate → Neutralises acidic chyme from the stomach
Liver, Gall Bladder, and Bile
The Liver secretes Bile, a green watery fluid that may be temporarily stored in the Gall Bladder before being released into the Duodenum through the bile duct. Bile contains no enzymes. Its green colour comes from Bile Pigments formed from the breakdown of haemoglobin in the liver. Bile also contains Bile Salts, which Emulsify fats — breaking them into small globules so that the water-soluble Lipase can digest them more efficiently.
Bile: Green watery fluid secreted by the liver; contains no digestive enzymes
Bile Pigments: Formed from the breakdown of haemoglobin; give bile its green colour; their accumulation in blood causes jaundice
Bile Salts: Emulsify fats by breaking large fat globules into smaller droplets, increasing surface area for lipase action
Gall Bladder: Stores and concentrates bile by absorbing water and electrolytes; releases bile into the duodenum through the bile duct
Gall Stones: Cholesterol secreted by the liver may precipitate in the gall bladder, forming stones that can block bile release
If Bile Pigments are prevented from leaving the digestive tract, they accumulate in the blood, causing Jaundice — a yellowing of the skin and eyes. Gall Stones form when cholesterol precipitates in the gall bladder and may block the release of bile. The liver is vulnerable to rupture because it is large, fixed in position, and fragile. Liver damage can result from hepatitis, alcohol consumption, heart malfunction, or hepatic cancer.
Jaundice: Yellowing of skin and eyes caused by accumulation of bile pigments in the blood when bile flow is obstructed
Gall Stones: Cholesterol precipitates that form in the gall bladder and can block the bile duct
Liver Vulnerability: The liver is large, fixed, and fragile — it can be ruptured by broken ribs or direct trauma, leading to severe internal bleeding
Digestion in the Jejunum, Ileum, and Absorption
Food that escapes undigested from the duodenum is completely digested in the Jejunum and Ileum by enzymes contained in the Intestinal Juice. The intestinal enzymes complete the breakdown of all major food types into their simplest absorbable forms.
Aminopeptidase: Breaks polypeptides into dipeptides
Erepsin: Breaks dipeptides into individual amino acids
Lipase: Breaks remaining fats into fatty acids and glycerol
Maltase: Breaks maltose into glucose
Lactase: Breaks lactose into glucose and galactose
Complete Enzyme Summary of the Human Digestive System
•
Salivary Amylase (Ptyalin): Starch/Glycogen → Maltose (oral cavity)
•
Pepsin: Proteins → Peptones + Polypeptides (stomach)
•
Pancreatic Amylase: Starch → Maltose (duodenum)
•
Trypsin: Proteins → Peptones + Polypeptides (duodenum)
•
Lipase: Fats → Fatty Acids + Glycerol (duodenum + small intestine)
•
Aminopeptidase: Polypeptides → Dipeptides (jejunum/ileum)
•
Erepsin: Dipeptides → Amino Acids (jejunum/ileum)
•
Maltase: Maltose → Glucose (jejunum/ileum)
•
Lactase: Lactose → Glucose + Galactose (jejunum/ileum)
Nearly all Absorption of digested products takes place in the Ileum. The internal surface of the ileum has circular folds covered with finger-like projections called Villi. Each villus contains blood capillaries and a Lacteal (a vessel of the lymphatic system), all covered by epithelial cells. These epithelial cells have countless closely packed cylindrical processes called Microvilli, forming a brush border. The enfolding of the intestinal wall, along with villi and microvilli, creates an incredibly large surface area for absorption.
Villi: Finger-like outgrowths on the ileum wall that greatly increase the absorptive surface area
Lacteal: A lymphatic capillary inside each villus that absorbs fats
Microvilli: Cylindrical projections on the epithelial cells of villi that further increase surface area (brush border)
Surface Area Amplification: Three structural adaptations — folds, villi, and microvilli — work together to maximise absorption
Simple sugars and amino acids are absorbed by diffusion or active transport into the blood capillaries through the Microvilli. Some fatty acids and glycerol also enter the bloodstream directly. However, a large proportion of fatty acids and glycerol enter the epithelial cells of Villi, where they recombine into fats. These fats then enter the Lacteals as lipoprotein droplets, which pass into the bloodstream via the thoracic lymphatic duct. The epithelial cells of villi are constantly shed and replaced by new cells from rapid cell division in the intestinal crypts.
Sugars and Amino Acids: Absorbed into blood capillaries via diffusion or active transport through microvilli
Fatty Acids and Glycerol: Some enter blood capillaries directly; most recombine into fats inside epithelial cells and enter lacteals
Lipoprotein Droplets: Fats combined with proteins in lymph vessels; carried to bloodstream via the thoracic lymphatic duct
Cell Renewal: Epithelial cells of villi are continuously shed and replaced from crypt cell division
Large Intestine, Rectum, and Egestion
The Large Intestine is composed of the Caecum, Colon, and Rectum. The caecum is a blind sac between the ileum and colon, from which the Appendix — a finger-like process — arises. The colon consists of the ascending, transverse, and descending portions. Material passing from the small intestine to the large intestine contains water, dissolved salts, and undigested material. The large intestine absorbs water and salts into the blood, while undigested material is rejected as Faeces.
Caecum: Blind sac at the junction of the small and large intestine
Appendix: Finger-like process arising from the caecum; can become inflamed (appendicitis) if food debris gets trapped and purifies
Colon: Ascending, transverse, and descending portions; absorbs water and salts from residual material
Faeces: Contains bacteria, plant fibres, sloughed-off mucosal cells, mucus, cholesterol, bile pigments, and water
The large intestine harbours a large population of useful bacteria that synthesise Vitamin K, which is absorbed into the blood. The Rectum is the last part of the large intestine, where faeces are temporarily stored and expelled through the Anus at intervals. The anus is surrounded by two sphincters: an internal sphincter of smooth muscle and an external sphincter of striped muscle. As the rectum fills, the Defecation Reflex is triggered, which can be consciously controlled in adults but not in infants.
Vitamin K Synthesis: Useful bacteria in the large intestine produce vitamin K, which is absorbed into the blood
Rectum: Temporary storage site for faeces before egestion
Anal Sphincters: Internal sphincter (smooth muscle, involuntary) and external sphincter (striped muscle, voluntary)
Defecation Reflex: Triggered when the rectum fills; can be consciously controlled in individuals beyond infancy
Common Disorders of the Large Intestine
•
Diarrhoea: Excessive water loss due to infection, drugs, or emotional disturbance; can lead to fatal dehydration if unchecked
•
Constipation: Excessive absorption of water from faecal matter, making it hard and difficult to expel
•
Appendicitis: Inflammation of the appendix due to trapped food debris; usually requires surgical removal
•
Piles (Haemorrhoids): Masses of dilated, tortuous veins in the anorectal mucosa that may bleed during bowel movements
Disorders of the Digestive System
Dyspepsia (incomplete or imperfect digestion) is characterised by abdominal discomfort, flatulence, heartburn, nausea, and vomiting. It is not a disease itself but symptomatic of other disorders — excessive stomach acidity, faulty stomach or intestinal function, or insufficient bile secretion. Ulcers form when the mucus lining of the stomach or duodenum breaks down, allowing digestive enzymes to erode the wall. Severe ulcers can perforate the wall, spilling contents into the abdominal cavity and causing life-threatening infection.
Dyspepsia: Symptomatic condition — not a disease; caused by excessive acidity, faulty function, or insufficient bile
Ulcer: A sore in the stomach or duodenal wall caused by breakdown of the protective mucus layer
Risk Factors for Ulcers: Smoking, spicy food, alcohol, coffee, tea, stress, and excessive gastric acid secretion
Perforation: A severe complication where the ulcer creates a hole in the digestive tract wall, leading to peritonitis