Digestion

Digestion is the set of processes by which the digestive system breaks down food into nutrients that the body can absorb and use. It includes mechanical breakdown, chemical digestion, nutrient absorption, and elimination of indigestible remnants.

Most digestion occurs within the gastrointestinal tract, beginning in the mouth and ending at the anus. The process relies on coordinated movements (peristalsis) and secretions from digestive glands. Enzymes, acids, and bile act to break carbohydrates, proteins, fats, and other components into absorbable units, while also preparing waste for elimination.

Food passes from the mouth through the esophagus to the stomach, where mechanical churning and gastric acid plus enzymes (notably pepsin) begin protein digestion. The small intestine is the primary site of digestion and absorption, aided by pancreatic enzymes and bile. Pancreatic juice contains amylase, proteases (trypsin and chymotrypsin), and lipase; bile emulsifies fats. The lining of the small intestine features villi and microvilli to maximize nutrient uptake, releasing absorbed monosaccharides, amino acids, fatty acids, glycerol, vitamins, minerals, and water into the bloodstream or lymphatic system.

Most water and electrolytes are absorbed in the large intestine, and feces are formed in the colon and expelled through the rectum. Regulatory mechanisms include hormones such as gastrin, secretin, and cholecystokinin, along with the enteric nervous system, which modulate secretion, motility, and appetite. The digestion process ultimately provides energy and building blocks for the body while disposing of indigestible waste.