Stomach – the stomach has a complex role: besides storing, mixing and portioning out food, it also disinfects and breaks down proteins and fats.
The stomach is a hollow, muscular organ that can be divided into several parts: the cardia, the fundus (the dome-shaped part of the stomach to the left of the cardia), the body (corpus), the horizontal section that follows it (antrum), and the pylorus.
Through peristaltic movement, the strong muscles of the stomach mix the food with gastric juice and move the stomach contents towards its lower part.
The characteristic movement of the empty stomach is hunger peristalsis, which produces a rumbling sound that can be heard with the naked ear. The cells lining the stomach secrete three important substances: mucus, hydrochloric acid and pepsinogen; the latter is activated into pepsin (the enzyme that breaks down proteins) only in an acidic environment.
About 1-1.5 litres of gastric juice are produced every day, containing water, mineral salts, hydrochloric acid and enzymes.
Mucus covers the cells on the surface of the stomach, protecting them from damage caused by acid and enzymes. If the mucosal layer (mucosa) is damaged by Helicobacter pylori infection or aspirin (NSAIDs), a stomach ulcer can develop.
Protein digestion requires an acidic environment. Pepsin is the only enzyme that can digest collagen, a protein that is the main component of meat. The hydrochloric acid in the stomach also protects against infections by 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 its composition, chyme – partly digested, semi-liquid food – stays in the stomach for 1-4 hours. When gastric digestion is complete, the lower ring-shaped sphincter of the stomach (pylorus) opens and the chyme passes into the first section of the small intestine, the duodenum.
