The mouth and digestive organs

The stomach is an enlarged bag with muscular walls; glands in its wall secrete gastric juice, made of enzymes (pepsin, which splits proteins), hydrochloric acid (kills microbes, activates the enzyme) and mucus; the mucus protects the wall from its own juice. Food is mixed in the stomach for some hours. The small intestine is the main site of digestion and absorption: pancreatic juice and bile from the liver enter its first part, the duodenum. Pancreatic enzymes split proteins, fats and carbohydrates; bile breaks fats into tiny droplets, easing digestion. Countless villi in the intestinal wall increase the absorbing surface: amino acids and glucose pass into blood, fats mostly into lymph. In the large intestine water is absorbed, helpful bacteria make some vitamins (for example K) and undigested residue becomes faeces. The liver makes bile, neutralises harmful substances in blood and stores glucose as glycogen.