7 Digestive System Physiology

How food is processed

The digestive system converts food into absorbable molecules, transfers those molecules into blood or lymph, and coordinates their delivery, use, or storage. Digestion takes place in the lumen of the gastrointestinal tract; absorption across the gut lining is what brings nutrients into the body. Most digestion and nutrient absorption occur in the small intestine.

Mechanical processing includes chewing, stomach churning, and intestinal mixing. These actions break food apart or mix it and increase its surface area. Chemical digestion uses enzymes to hydrolyze large food molecules into smaller units.

Digestive secretions and chemical breakdown

Digestive secretions support breakdown of food and help protect or prepare the gut environment.

• Mouth and salivary glands: Chewing breaks food apart, saliva moistens it, and salivary amylase begins starch digestion.

• Stomach: Churning forms chyme. Hydrochloric acid helps denature proteins, and pepsin begins protein digestion. Mucus protects the stomach lining. Intrinsic factor is needed for vitamin B₁₂ absorption later in the small intestine.

• Pancreas: Digestive enzymes act on carbohydrates, proteins, fats, and nucleic acids. Bicarbonate neutralizes acidic chyme, creating conditions suitable for intestinal enzymes.

• Liver and gallbladder: The liver produces bile, and the gallbladder stores and releases it. Bile salts emulsify fats and help their digestion and absorption; bile is not an enzyme.

• Small intestine: Brush-border enzymes complete digestion. Intestinal mucus and fluid support mixing and protect the lining.

• Large intestine: It absorbs remaining water and electrolytes, and mucus helps stool pass. Gut microbes ferment some undigested material and produce certain vitamins.

Most chemical digestion is completed in the small intestine. Pancreatic amylase and intestinal enzymes reduce starch to absorbable sugars. Pancreatic and intestinal proteases reduce proteins to amino acids and small peptides. Bile-assisted pancreatic lipase breaks triglycerides into absorbable lipid products.

Movement and neural coordination

Motility moves, stores, and mixes digestive contents at an appropriate pace. Swallowing initiates transport from the mouth. Coordinated peristaltic waves carry food through the esophagus and propel contents through the intestines.

The stomach temporarily stores food, churns it, and releases chyme gradually into the duodenum. This measured emptying gives the small intestine time to neutralize acid, digest food, and absorb nutrients. In the small intestine, segmentation repeatedly mixes contents with secretions and brings them into contact with the absorptive surface. Colon contractions move and compact residue before defecation.

The enteric nervous system (ENS), located in the gut wall, can coordinate local motility and secretion. The autonomic nervous system modifies its activity: parasympathetic input generally promotes digestive activity, while sympathetic input generally reduces it. Signals from the brain, such as the sight or smell of food, can prepare digestive organs before food arrives.

Absorption and nutrient routes

Folds, villi, and microscopic microvilli in the small intestine create a large absorptive surface. Nutrients cross epithelial cells by transport processes that include diffusion and carrier-mediated or energy-dependent transport. The route into the body depends on the nutrient.

• Carbohydrates and proteins: Carbohydrates are absorbed mainly as monosaccharides, and proteins mainly as amino acids and small peptides. These water-soluble products enter blood capillaries and travel first to the liver through the hepatic portal vein.

• Most dietary fats: Fats enter intestinal cells, are reassembled and packaged into chylomicrons, and then pass into lymphatic lacteals before reaching the bloodstream. Many fat-soluble vitamins travel with dietary fats.

• Water, minerals, and vitamins: These are absorbed along the gut. The large intestine reclaims much of the water and electrolytes remaining in its contents.

The liver receives nutrient-rich portal blood and can process, store, or release nutrients. The lymphatic route used by most long-chain fats is an important exception to the direct portal-blood route used by many sugars and amino acids.

Feedback and hormonal control of digestion

Digestive activity is regulated by local feedback, nerves, and hormones. Stretch, acidity, and the presence of particular nutrients in the gut can trigger enteric reflexes and alter movement or secretion. Hormones coordinate responses across organs.

• Gastrin promotes gastric acid secretion.

• Secretin, released in response to acidic chyme in the duodenum, stimulates pancreatic bicarbonate secretion.

• Cholecystokinin (CCK) promotes pancreatic enzyme secretion and gallbladder contraction, delivering bile to the small intestine.

• Glucose-dependent insulinotropic polypeptide (GIP) helps slow gastric activity and stimulates insulin release after nutrients enter the small intestine.

How the body uses absorbed nutrients

Absorption supplies materials, but hormones and tissues determine how those materials are used. After a meal, rising blood glucose stimulates pancreatic insulin release. Insulin supports glucose uptake and use by tissues and promotes storage of excess energy, including glucose as glycogen and energy as fat.

Between meals, glucagon helps maintain blood glucose by promoting release of stored fuels, especially glucose from liver glycogen. Amino acids support protein synthesis and other cellular functions. Fats can be oxidized for energy or stored. These coordinated responses help maintain blood nutrient levels while meeting changing energy needs.

After a carbohydrate-rich meal, digestion produces absorbable sugars. They enter portal blood and reach the liver; rising blood glucose stimulates insulin, encouraging tissues to use glucose and the body to store some of the surplus. Meanwhile, secretin and CCK help the intestine receive the bicarbonate and enzymes needed to process the meal.