Salivary glands, gastric glands, pancreas, and liver secrete enzymes and other substances that break down food into absorbable molecules. The small intestine absorbs monosaccharides, amino acids, and fatty acids through epithelial cells into blood. The large intestine absorbs water and electrolytes. Liver regulates nutrient distribution to tissues.
From your study of gastrointestinal tract anatomy and motility, you know that food moves through a coordinated series of hollow organs — mouth, esophagus, stomach, small intestine, large intestine — driven by peristalsis and segmentation. But mechanical churning alone cannot reduce a meal to molecules small enough to cross a cell membrane. That job belongs to the digestive glands: accessory organs and secretory cells that add precisely timed chemical reagents at each stage of the digestive pipeline.
Salivary glands launch the process in the mouth, contributing salivary amylase to begin starch hydrolysis and mucus to lubricate the bolus. The stomach's gastric glands — embedded in the stomach wall — secrete hydrochloric acid (dropping luminal pH to 1.5–2) and pepsinogen, which the acid converts to the active protease pepsin. This acidic environment is both a digestive tool and a sterilization step: most ingested pathogens are destroyed before reaching the small intestine. The stomach also produces intrinsic factor, essential for vitamin B₁₂ absorption downstream.
The critical chemical transformation occurs in the small intestine, where two accessory organs deliver the bulk of digestive enzymes. The pancreas secretes a bicarbonate-rich juice that neutralizes acid chyme arriving from the stomach, along with a full suite of enzymes: pancreatic amylase (carbohydrates), lipase (fats), and proteases including trypsin and chymotrypsin (proteins). The liver secretes bile, concentrated and stored in the gallbladder, which acts as a biological detergent — emulsifying fats into tiny droplets to vastly increase the surface area accessible to lipase. Fat digestion is uniquely dependent on liver and gallbladder function; without bile, dietary fat passes largely unabsorbed.
Absorption occurs across the specialized epithelium of the small intestine. Monosaccharides and amino acids enter intestinal cells via active transport driven by sodium gradients, then exit into capillaries of the portal circulation, traveling directly to the liver. Fatty acids and monoglycerides reassemble into triglycerides inside epithelial cells, are packaged into chylomicrons, and enter the lymphatic lacteals — bypassing the portal vein entirely and draining into systemic circulation via the thoracic duct. The large intestine absorbs water and electrolytes from remaining undigested material. The liver, receiving portal blood rich with absorbed nutrients, acts as a gatekeeper: storing glucose as glycogen, processing amino acids and lipids, and regulating what reaches peripheral tissues.