The Digestive System

The digestive system processes ingested food into absorbable nutrients and eliminates waste. The gastrointestinal (GI) tract is a continuous tube from mouth to anus with accessory organs including the liver, gallbladder, and pancreas.

GI motility is coordinated by the enteric nervous system (ENS) — often called the "second brain" — and includes two key patterns. Peristalsis is coordinated propulsive contractions that move luminal contents aborally. Segmentation creates mixing contractions that maximise contact with the absorptive epithelium without net propulsion. The migrating motor complex (MMC) sweeps residual contents during fasting, preventing bacterial overgrowth.

Digestion begins in the mouth. Salivary amylase initiates starch hydrolysis. In the stomach, parietal cells secrete HCl via the H+/K+-ATPase (stimulated by histamine, gastrin, and acetylcholine) and intrinsic factor (essential for vitamin B12 absorption in the terminal ileum). Chief cells secrete pepsinogen, activated to pepsin at low pH to begin protein digestion. Gastric secretion occurs in cephalic (30%, triggered by food anticipation via the vagus), gastric (60%, triggered by gastric distension and gastrin from G cells), and intestinal (10%, inhibitory via secretin and GIP) phases.

The pancreas secretes bicarbonate-rich fluid (via ductal cells, stimulated by secretin) to neutralise gastric acid, and digestive enzymes (via acinar cells, stimulated by CCK): lipase/colipase for fat digestion, amylase for starch, and proteases as zymogens activated by enteropeptidase/trypsin in the duodenum.

Bile produced by hepatocytes contains bile salts that emulsify dietary fat into small droplets and then form mixed micelles, solubilising monoglycerides, fatty acids, and fat-soluble vitamins for absorption. CCK triggers gallbladder contraction. Bile salts are recycled via enterohepatic circulation (reabsorbed in the terminal ileum → portal vein → liver).

Absorption occurs mainly in the small intestine, where villi and microvilli create ~600× the surface area of a smooth tube. Glucose and galactose are absorbed by SGLT1 (Na+-coupled, secondary active transport) and exit via GLUT2. Fructose enters via GLUT5. Amino acids and dipeptides enter via specific cotransporters. Long-chain fatty acids and monoglycerides reassemble into triglycerides inside enterocytes, are packaged into chylomicrons, and enter the lymphatic lacteals — bypassing the portal circulation initially.

The liver performs diverse functions: synthesis of plasma proteins (albumin, clotting factors), gluconeogenesis, glycogen storage, lipid metabolism (VLDL synthesis, fatty acid oxidation), drug metabolism via cytochrome P450 enzymes, urea synthesis from ammonia, and bile production. Liver failure impairs all of these functions simultaneously.