How the Digestive System Works

The Mouth and Esophagus

Digestion begins in the mouth with mechanical breakdown by the teeth and chemical breakdown by salivary enzymes. An adult has 32 teeth specialized for different tasks: incisors cut, canines tear, premolars crush, and molars grind food into a soft mass called a bolus. Three pairs of salivary glands, the parotid, submandibular, and sublingual, produce approximately 1 to 1.5 liters of saliva daily. Saliva contains salivary amylase, an enzyme that begins breaking starch into maltose, and lingual lipase, which initiates fat digestion. Saliva also contains mucus for lubrication, bicarbonate to buffer acids, and lysozyme, an antimicrobial enzyme.

Once chewed and mixed with saliva, the bolus is pushed toward the back of the mouth by the tongue and swallowed. The swallowing reflex is coordinated by the medulla oblongata and involves a precise sequence: the soft palate rises to close off the nasal cavity, the epiglottis folds over the larynx to protect the airway, and the upper esophageal sphincter relaxes to admit the bolus. The esophagus, a muscular tube about 25 centimeters long, transports food to the stomach through peristalsis, rhythmic waves of smooth muscle contraction that push the bolus downward regardless of body position. This is why astronauts can eat normally in zero gravity.

The Stomach

The stomach is a J-shaped muscular organ that stores food, mixes it with gastric secretions, and begins protein digestion. An empty stomach has a volume of about 50 milliliters, but it can expand to hold 1 to 1.5 liters after a large meal. The stomach wall has three layers of smooth muscle (longitudinal, circular, and oblique) that produce powerful churning contractions, mixing food with gastric juice to form a semi-liquid paste called chyme.

Gastric glands in the stomach lining contain several specialized cell types. Parietal cells produce hydrochloric acid (HCl), creating a pH of 1.5 to 3.5, acidic enough to dissolve metal and kill most bacteria. Chief cells produce pepsinogen, the inactive precursor of pepsin, a protease that works optimally at low pH. HCl activates pepsinogen to pepsin, which then cleaves proteins into smaller peptide fragments. Mucous neck cells produce a thick alkaline mucus layer that coats the stomach lining and prevents the acid and pepsin from digesting the stomach itself. G cells produce gastrin, a hormone that stimulates acid and enzyme secretion. The stomach also produces intrinsic factor, a glycoprotein essential for vitamin B12 absorption later in the small intestine.

Food typically remains in the stomach for 2 to 6 hours, depending on composition. Carbohydrate-rich meals empty fastest, protein meals take longer, and fatty meals take the longest because fat inhibits gastric emptying through the hormone cholecystokinin (CCK). The pyloric sphincter at the stomach's exit opens periodically to release small squirts of chyme into the duodenum, the first section of the small intestine. This controlled release ensures the small intestine is not overwhelmed and has time to neutralize the acidic chyme.

The Small Intestine

The small intestine is where most chemical digestion and virtually all nutrient absorption occur. Despite being called "small," it is the longest part of the GI tract, measuring about 6 meters in a living adult. It divides into three segments: the duodenum (about 25 centimeters), the jejunum (about 2.5 meters), and the ileum (about 3.5 meters). The duodenum receives chyme from the stomach, bile from the liver and gallbladder, and pancreatic juice from the pancreas. The jejunum and ileum are the primary absorption sites.

Pancreatic juice contains sodium bicarbonate, which neutralizes the acidic chyme and raises the pH to about 7 to 8, optimal for the pancreatic enzymes that follow. These include trypsin and chymotrypsin (protein digestion), pancreatic lipase (fat digestion), pancreatic amylase (starch digestion), and nucleases (nucleic acid digestion). Bile, produced continuously by the liver and stored in the gallbladder, does not contain enzymes but emulsifies fats, breaking large fat globules into tiny droplets that present more surface area for lipase to act on. Without bile, fat digestion is severely impaired and most dietary fat passes through unabsorbed.

The small intestine's inner surface is elaborately folded to maximize absorption area. Circular folds (plicae circulares) are permanent ridges in the intestinal wall. Villi, finger-like projections about 0.5 to 1.5 millimeters long, cover the surface of these folds. Microvilli, microscopic projections on each villus cell, form the "brush border." Together, these three levels of folding create an absorptive surface area of approximately 250 square meters, roughly the size of a tennis court. Each villus contains a capillary network for absorbing water-soluble nutrients (amino acids, simple sugars, water-soluble vitamins) and a lacteal, a lymphatic capillary, for absorbing fats and fat-soluble vitamins.

Nutrients cross the intestinal epithelium through multiple mechanisms. Simple sugars like glucose and galactose are actively transported across the apical membrane by the SGLT1 transporter, then exit the basolateral membrane via GLUT2 transporters into the capillaries. Amino acids use various specific transporters. Fatty acids and monoglycerides, products of fat digestion, simply diffuse into the epithelial cells due to their lipid solubility, are reassembled into triglycerides inside the cells, packaged into chylomicrons (lipoprotein particles), and exported into the lacteals. From there, they travel through the lymphatic system and enter the bloodstream at the subclavian vein.

The Large Intestine

The large intestine, about 1.5 meters long and 6.5 centimeters in diameter, receives the remaining indigestible material from the ileum through the ileocecal valve. Its primary functions are water absorption, electrolyte recovery, and formation of feces. Approximately 1.5 liters of material enters the large intestine daily, and the colon absorbs about 90% of the remaining water, producing 100 to 200 grams of feces per day. The large intestine includes the cecum (with the appendix), the ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.

The large intestine houses the gut microbiome, a community of approximately 38 trillion bacteria, along with smaller numbers of archaea, fungi, and viruses. These microorganisms, collectively weighing about 1.5 to 2 kilograms, perform several important functions. They ferment dietary fiber and other indigestible carbohydrates, producing short-chain fatty acids (acetate, propionate, butyrate) that nourish the colonic epithelial cells and contribute about 5% to 10% of daily caloric intake. Gut bacteria also synthesize vitamins K and B12, metabolize bile acids, train the immune system, and compete with pathogenic bacteria for resources and attachment sites.

The composition of the gut microbiome varies enormously between individuals and is influenced by diet, medications (especially antibiotics), age, geography, and mode of birth. Research published since 2010 has linked alterations in the gut microbiome to conditions ranging from inflammatory bowel disease and obesity to depression, autism spectrum disorder, and cardiovascular disease. While causation has been established in some animal models, human studies are still sorting out which microbial changes are causes versus consequences of disease.

The Liver

The liver, weighing about 1.5 kilograms, is the largest internal organ and performs over 500 known functions. In digestion, its primary role is producing bile, about 500 to 1,000 milliliters daily. But the liver does far more than aid digestion. It processes all nutrients absorbed from the small intestine through the hepatic portal vein before they enter the general circulation, a role called "first-pass metabolism." The liver converts excess glucose to glycogen for storage (and back again when blood sugar drops), synthesizes plasma proteins including albumin and clotting factors, produces cholesterol and lipoproteins for fat transport, detoxifies drugs and alcohol, breaks down hemoglobin from worn-out red blood cells, stores iron and vitamins A, D, and B12, and converts ammonia (a toxic byproduct of amino acid metabolism) to urea for excretion by the kidneys.

The liver's remarkable regenerative capacity is unique among solid organs. If up to 75% of liver tissue is removed or destroyed, the remaining cells can proliferate and restore the organ to its original size within weeks. This regeneration is what makes living-donor liver transplantation possible: a donor gives a portion of their liver, and both the donated portion and the remaining portion regrow.

The Pancreas

The pancreas is a dual-function organ serving both the digestive and endocrine systems. Its exocrine function, producing about 1.5 liters of pancreatic juice daily, accounts for roughly 99% of its mass. Pancreatic acinar cells produce digestive enzymes, while ductular cells produce the bicarbonate-rich fluid that carries those enzymes to the duodenum and neutralizes gastric acid. Pancreatic enzymes are secreted in inactive forms (zymogens) to prevent self-digestion: trypsinogen is activated to trypsin by enterokinase in the duodenal wall, and trypsin then activates the remaining zymogens.

The endocrine function of the pancreas resides in the islets of Langerhans, clusters of hormone-producing cells scattered throughout the organ. Beta cells produce insulin, which lowers blood glucose by stimulating cellular uptake and glycogen synthesis. Alpha cells produce glucagon, which raises blood glucose by stimulating glycogen breakdown and gluconeogenesis in the liver. Delta cells produce somatostatin, which inhibits both insulin and glucagon secretion. Disruption of this endocrine function causes diabetes mellitus: type 1 from autoimmune destruction of beta cells, type 2 from insulin resistance and eventual beta cell exhaustion.

Regulation of Digestion

Digestive function is regulated through a combination of neural and hormonal mechanisms. The enteric nervous system, sometimes called the "second brain," is a network of roughly 100 million neurons embedded in the walls of the GI tract. It can coordinate peristalsis, secretion, and blood flow independently of the central nervous system, though it is modulated by the vagus nerve (parasympathetic, promoting digestion) and sympathetic nerves (inhibiting digestion during stress).

Several hormones coordinate the digestive process. Gastrin, released by G cells in the stomach, stimulates acid secretion when food arrives. Secretin, released by the duodenum when acidic chyme arrives, stimulates bicarbonate secretion from the pancreas. Cholecystokinin (CCK), released by the duodenum in response to fats and proteins, stimulates gallbladder contraction, pancreatic enzyme secretion, and inhibits gastric emptying. Gastric inhibitory peptide (GIP) inhibits gastric activity and stimulates insulin release. Motilin triggers the migrating motor complex, periodic waves of peristalsis that sweep through the intestines between meals to clear residual food and bacteria.