The contents of the gut are outside the body and are potentially dangerous to the body if they cross the gut wall. The contents include hydrochloric acid, proteindigesting enzymes and bacteria. We are protected from the gut contents by its mucosal lining and the cells (macrophages, dendritic cells, B-lymphocytes and T-lymphocytes) in the lymphoid tissue known as Peyer’s patches. The gut also contains ingested food molecules of protein, carbohydrate and triglyceride. Digestion (hydrolysis) is the disassembling of large food molecules and is necessary to reduce the very large polymerised molecules in food to particles small enough to pass into the cells lining the gut. The products of hydrolysis of proteins are amino acids and di- and tripeptides. Carbohydrates are hydrolysed into monosaccharides (glucose, fructose, galactose), while triglycerides are hydrolysed into free fatty acids and monoglycerides. Nucleic acids (DNA and RNA) are hydrolysed by pancreatic nucleases into nucleotides and then hydrolysed by brush border enzymes (nucleosidases and phosphatases) into their free bases, pentose sugars and phosphate ions. Ingested food passes through the oesophagus, stomach, small diameter intestine (duodenum, jejunum, ileum), large diameter intestine (caecum, ascending colon, transverse colon, descending colon, sigmoid colon rectum), before the residue is eliminated. The movement is produced by peristalsis and is unidirectional, being controlled by deglutition, the gastro-oesophageal and pyloric sphincters, the iliocaecal valve and the anal sphincters. Hydrolysis is achieved by enzymes in saliva and from glands in the stomach and duodenal mucosae and from the pancreas. The products of carbohydrate and protein digestion are absorbed by active transport (for glucose and amino acids), enter the blood capillaries and are transported to the liver by the hepatic portal vein. The products of triglyceride digestion diffuse through the plasma membranes of the cells lining the small intestine where they are reconstituted into triglycerides. They then combine with phospholipids and cholesterol, are coated with protein to form water-soluble chylomicrons. Chylomicrons enter the lymphatic vessels (via lacteals) and eventually the bloodstream via the thoracic duct.