Stomach - the stomach has a complex function: in addition to storing, mixing and administering food, it also has disinfectant, protein and fat-digesting properties.
The stomach is a hollow, muscular organ; it is divided into several parts: the gastric orifice (cardia), the fundus (the dome-shaped part of the stomach to the left of the cardia), the body (corpus), the horizontal section (antrum) following it, and the gastric cap (pylorus).
The strong muscles of the stomach use peristaltic movements to mix food with the gastric juice and move the contents of the stomach towards the lower part of the stomach.
The characteristic movement of an empty stomach is the peristalsis of hunger, a rumbling sound that can be heard with the unaided ear. The cells lining the stomach secrete three important substances: mucus, hydrochloric acid and pepsinogen, the latter of which is activated only in an acidic medium to pepsin (an enzyme that breaks down proteins).
Approximately 1-1.5 litres of gastric juice is produced each day, containing water, mineral salts, hydrochloric acid and enzymes.
Mucus coats the cells on the surface of the stomach, protecting them from damage caused by acid and enzymes. In the event of damage to the mucosa by Helicobacter pylori infection or aspirin (NSAIDs), a peptic ulcer may develop.
Protein breakdown requires an acidic environment. Pepsin is the only enzyme that can digest collagen, a protein that is the main component of meat. Hydrochloric acid in the stomach also acts as a defence against infection, killing most bacteria.
Acid secretion is stimulated by nerve impulses, gastrin (a hormone released by the stomach) and histamine (a substance released by the stomach).
Depending on the composition of the food, chyme remains in the stomach for 1-4 hours as a partially digested, semi-liquid food. After digestion is complete, the lower pyloric sphincter opens and the chyme passes into the horseshoe intestine, the initial stage of the small intestine.