Digestion is the physical and chemical processing of food into smaller components that can be absorbed and used by an organism. It provides nutrients needed for growth, tissue maintenance, and energy production. In humans, digestion involves coordinated movements and secretions throughout the digestive tract, together with contributions from accessory organs. Digestion is distinct from absorption—the movement of nutrients across the intestinal lining—and from metabolism, the chemical transformations that occur after nutrients become available to tissues. (niddk.nih.gov)
Mechanical and chemical digestion
Mechanical digestion changes the size and distribution of food without changing its chemical composition. Chewing breaks food into particles, stomach contractions churn it, and intestinal mixing brings it into contact with digestive secretions and absorptive surfaces. These actions increase the accessibility of food to enzymes. Propulsion is a related but distinct process: swallowing and peristalsis, coordinated waves of muscular contraction, move material along the digestive tract. (openstax.org)
Chemical digestion breaks large food molecules into smaller units. Much of this breakdown occurs through enzyme-catalyzed hydrolysis, in which water participates in the cleavage of chemical bonds. Different enzymes act on different substrates, including carbohydrates, proteins, fats, and nucleic acids. Mechanical and chemical processes overlap rather than forming entirely separate stages. (openstax.org)
Organization of the human digestive system
The digestive system consists of the gastrointestinal tract and organs that supply digestive secretions. The tract extends from the mouth through the pharynx, esophagus, stomach, small intestine, and large intestine to the anus. Accessory organs include the salivary glands, liver, gallbladder, and pancreas. Food travels through the tract, while accessory organs deliver their products through ducts. (niddk.nih.gov)
Digestion begins in the mouth. Teeth fragment food, while saliva moistens it and contains amylase, which begins the breakdown of starch. The tongue helps form a bolus for swallowing. The esophagus mainly transports this bolus to the stomach rather than supplying digestive enzymes. Sphincters regulate passage between regions and help limit backward movement of their contents. (niddk.nih.gov)
Gastric processing
The stomach temporarily stores food and mixes it with gastric juice, producing a semifluid mixture called chyme. Its muscular wall supports vigorous churning, while controlled emptying delivers chyme to the duodenum, the first part of the small intestine. (openstax.org)
Gastric parietal cells secrete hydrochloric acid. The resulting low pH helps unfold proteins and supports the activation and activity of pepsin. Chief cells release pepsinogen, an inactive precursor that becomes pepsin in the acidic environment; pepsin then cleaves proteins into smaller peptides. Gastric lipase also contributes to fat digestion, although most fat digestion occurs farther along the tract. (openstax.org)
The stomach lining is protected by a mucus–bicarbonate barrier, closely joined epithelial cells, and tissue renewal. Parietal cells also produce intrinsic factor, a protein required for normal vitamin B12 absorption later in the small intestine. The stomach therefore contributes both to food breakdown and to conditions necessary for subsequent nutrient uptake. (openstax.org)
Intestinal digestion and absorption
The small intestine is the principal site of chemical digestion and nutrient absorption. Pancreatic bicarbonate buffers incoming gastric acid, creating conditions suitable for intestinal enzymes. Pancreatic juice supplies enzymes that digest carbohydrates, proteins, fats, and nucleic acids, while enzymes associated with the intestinal brush border complete several final breakdown steps. (openstax.org)
Carbohydrates are reduced to absorbable monosaccharides, including glucose, galactose, and fructose. Protein digestion produces amino acids and small peptides. Fat digestion depends on bile, produced by the liver and stored and concentrated in the gallbladder. Bile salts disperse fat and facilitate access by lipases; they are not digestive enzymes. Lipases hydrolyze triglycerides, yielding fatty acids and monoacylglycerols. (openstax.org)
Circular folds, villi, and microvilli enlarge the intestinal surface. Absorptive epithelial cells take up nutrients through transporters and other mechanisms. Most absorbed sugars and amino acids enter blood capillaries and travel toward the liver through the hepatic portal circulation. Most long-chain dietary fats are repackaged into chylomicrons and enter the lymphatic system before reaching the bloodstream. (openstax.org)
Large intestine and microbial digestion
Material reaching the large intestine contains substances not absorbed upstream, including resistant food components. The colon recovers water and electrolytes, and its movements help form, store, and propel feces toward elimination. Unlike the small intestine, it lacks villi and is not the main site of enzymatic nutrient digestion. (openstax.org)
Resident bacteria metabolize some remaining carbohydrates through fermentation, producing gases and short-chain fatty acids that can be absorbed. Human digestive enzymes cannot hydrolyze cellulose, although microbes can process some components of dietary fiber. Microbial digestion consequently supplements the activities of the host’s own enzymes. (openstax.org)
Neural and hormonal regulation
Digestive activity is regulated by local reflexes, signals from the central nervous system, and hormones. The enteric nervous system, located within the gastrointestinal wall, coordinates muscle activity and secretion in response to changes such as stretching. Signals associated with the sight or smell of food can stimulate salivation before ingestion. (niddk.nih.gov)
Gastrin promotes gastric acid secretion. Secretin stimulates pancreatic bicarbonate secretion, while cholecystokinin promotes pancreatic enzyme secretion and gallbladder contraction. These mechanisms coordinate the delivery of digestive secretions with the arrival of food in different regions of the tract. (openstax.org)
Variation among animals
Digestive arrangements vary among animals. Sponges digest food intracellularly, whereas animals with digestive cavities or tracts can secrete enzymes outside their cells. A complete digestive tract has separate openings for ingestion and elimination. Specialized structures include the muscular gizzard of birds and the compartmentalized stomach of ruminants. In ruminants, microbial fermentation enables extensive processing of cellulose-rich plant material before subsequent intestinal digestion and absorption. (openstax.org)