The Human Digestive System Explained
Digestion is the process by which the body breaks food down into molecules small enough to be absorbed into the bloodstream and transported to cells. It involves mechanical crushing and chemical breakdown across a tube nearly nine metres long, coordinated by a remarkable network of enzymes, hormones, and muscles.
The Alimentary Canal: An Overview
The digestive system is built around the alimentary canal (also called the gastrointestinal tract) — a continuous tube from mouth to anus through which food passes in one direction. Along the way, specialised regions transform a complex meal into simple nutrients: glucose from carbohydrates, amino acids from proteins, fatty acids and glycerol from fats, plus vitamins and minerals. Accessory organs — the salivary glands, liver, gallbladder, and pancreas — contribute secretions but food does not pass through them directly.
Digestion has two components that work together. Mechanical digestion physically breaks food into smaller pieces, increasing the surface area available for chemical attack. Chemical digestion uses enzymes to break the molecular bonds holding large polymers together, producing monomers small enough to cross cell membranes.
The Mouth and Oesophagus
Digestion begins in the mouth before you swallow. The teeth perform mechanical digestion, and the tongue forms food into a rounded mass called a bolus. Meanwhile, three pairs of salivary glands secrete saliva, which contains salivary amylase — an enzyme that begins the chemical breakdown of starch into maltose. Saliva also lubricates the bolus and contains lysozyme, an antibacterial enzyme.
Swallowing propels the bolus into the oesophagus, a muscular tube roughly 25 cm long. The oesophagus does not digest food; its job is transport. A coordinated wave of muscular contractions called peristalsis squeezes the bolus downward. Peristalsis is so powerful that you could swallow food standing on your head. At the base of the oesophagus, the lower oesophageal sphincter opens to allow the bolus into the stomach and closes to prevent stomach acid from refluxing back up.
Peristalsis is the rhythmic, wave-like contraction of smooth muscle in the walls of the alimentary canal. A ring of muscle behind the food contracts while the ring ahead relaxes, pushing the food forward. The same mechanism operates throughout the gastrointestinal tract, from oesophagus to large intestine, and is controlled by the enteric nervous system — sometimes called the “second brain” because it operates largely independently of the central nervous system.
The Stomach
The stomach is a muscular, J-shaped bag capable of expanding from about 75 mL when empty to over 1 litre after a large meal. Its three functions are storage, mechanical churning, and the first major phase of chemical protein digestion.
The stomach lining contains millions of gastric glands that secrete gastric juice, a mixture of hydrochloric acid (HCl) and the enzyme pepsin. HCl lowers the stomach pH to between 1.5 and 3.5 — strongly acidic — which kills most swallowed bacteria and provides the optimal environment for pepsin. Pepsin is secreted in an inactive form called pepsinogen and is activated by the acidic environment; this prevents it from digesting the stomach wall itself. The stomach lining is protected by a thick layer of alkaline mucus.
The churning action of the stomach's three muscle layers — longitudinal, circular, and oblique — mixes food with gastric juice to produce a semi-liquid called chyme. The stomach releases chyme in small pulses into the small intestine through the pyloric sphincter, regulating the rate at which the intestine receives material.
The Small Intestine: The Main Absorption Site
The small intestine is roughly 6–7 metres long and 2.5 cm in diameter. Despite the name, it is the longest section of the alimentary canal, and the site where most chemical digestion and virtually all nutrient absorption occurs.
The first section, the duodenum (about 25 cm), receives chyme from the stomach, bile from the liver via the gallbladder, and pancreatic juice from the pancreas. Bile emulsifies fats — breaking large fat globules into tiny droplets that enzymes can attack more efficiently. Pancreatic juice contains a battery of enzymes: pancreatic amylase (continues starch digestion), pancreatic lipase (digests fats), and proteases including trypsin (continues protein digestion). The pancreatic juice is also alkaline, neutralising the acid in the chyme and creating the slightly alkaline pH that intestinal enzymes require.
The jejunum and ileum (the remaining 5–6 metres) complete digestion and carry out absorption. The intestinal wall is enormously folded to maximise surface area. The inner lining is covered with finger-like projections called villi (singular: villus), each of which is in turn covered with microscopic projections called microvilli, forming a brush border. Together, these structures increase the absorptive surface area of the small intestine to approximately 200 square metres — the size of a tennis court.
Glucose and amino acids pass through the villus epithelial cells by active transport and diffusion, then enter capillaries in the villus and are carried to the liver via the hepatic portal vein. Fatty acids and glycerol are reassembled into triglycerides inside epithelial cells, packaged into protein-coated droplets called chylomicrons, and absorbed into the lymphatic vessels (lacteals) in each villus, eventually reaching the bloodstream via the thoracic duct.
The Liver and Pancreas
The liver is the largest internal organ and one of the most metabolically active. In digestion, it receives all blood leaving the small intestine via the hepatic portal vein, allowing it to regulate what reaches general circulation. The liver converts excess glucose into glycogen for storage (glycogenesis) and breaks down amino acids it cannot store through a process called deamination, producing urea as a waste product that is excreted by the kidneys. It also detoxifies alcohol and drugs, produces clotting factors and the plasma protein albumin, and manufactures bile.
The pancreas serves both endocrine functions (secreting insulin and glucagon into the blood to regulate blood sugar) and exocrine functions (secreting digestive enzymes and bicarbonate into the duodenum). Its exocrine output is precisely controlled by hormones: when acid chyme enters the duodenum, cells there release secretin, which stimulates the pancreas to produce bicarbonate, and cholecystokinin (CCK), which stimulates enzyme secretion and triggers the gallbladder to contract and release bile.
The Large Intestine and Absorption of Water
Material that has not been absorbed passes into the large intestine (colon), which is about 1.5 metres long but much wider than the small intestine. The large intestine's primary function is the reabsorption of water and electrolytes from the remaining chyme, concentrating liquid waste into solid faeces. Roughly 1.5 litres of water enter the large intestine daily; nearly all of it is reclaimed.
The large intestine also hosts the gut microbiome — trillions of bacteria that ferment undigested fibre, producing short-chain fatty acids that nourish the colon lining, as well as vitamins K and B12 that the body absorbs. The composition of this microbial community is increasingly recognised as important for immune function, mental health, and the risk of conditions ranging from inflammatory bowel disease to obesity.
Faeces collect in the rectum and are expelled through the anus, which has an internal smooth muscle sphincter under involuntary control and an external skeletal muscle sphincter under conscious control.
Common Digestive Disorders
Acid reflux (GERD) occurs when stomach acid backs up into the oesophagus, causing the burning sensation known as heartburn. The underlying cause is a weakened lower oesophageal sphincter. Long-term untreated reflux can damage the oesophageal lining.
Peptic ulcers are open sores in the stomach or duodenal lining. The majority are caused by infection with the bacterium Helicobacter pylori, which weakens the mucus layer, allowing acid to erode the tissue beneath. NSAIDs (such as ibuprofen) are a secondary cause.
Lactose intolerance results from insufficient production of lactase, the enzyme that cleaves the disaccharide lactose into glucose and galactose. Undigested lactose passes to the large intestine, where bacteria ferment it and produce gas, causing bloating and discomfort. The condition is distinct from a milk allergy, which involves an immune response.
Summary
The digestive system breaks food down through coordinated mechanical and chemical processes. Carbohydrates begin breaking down in the mouth; proteins begin in the stomach; fats are emulsified and digested in the duodenum. The small intestine, with its vast folded surface area of villi and microvilli, absorbs the resulting nutrients into the blood or lymph. The liver processes absorbed material, regulates blood glucose, and produces bile. The large intestine recovers water and hosts the gut microbiome. Each segment of the alimentary canal is specialised for its role, regulated by a precise combination of mechanical signals, hormones, and the enteric nervous system.