The stomach is a large muscular sac where food is completely solubilised. The walls of the stomach contain layers of muscle. The functions of which include: churning, mechanical digestion, mixing, and peristalsis.
The stomach wall contains gastric glands that secrete over half a litre of gastric juice into the stomach per day. The gastric juice contains a protein-digesting enzyme (protease). This is the endopeptidase pepsin and is secreted in its inactive form, pepsinogen in order to prevent it from digesting the walls of the stomach, while it is in storage in the gastric glands. The gastric juice also contains hydrochloric acid (HCl). The HCl has a number of roles, it :
- activates the enzyme pepsin
- sterilises gut contents (kills bacteria, which are ingested along with food)
- creates a low pH environment which is the optimum for pepsin
The stomach wall also secretes mucus, which coats the stomach walls, and prevents them from being digested, and also protects the walls from acid.
Pepsin is an endopeptidase, which means it acts on the centre of polypeptide chains within proteins and hydrolyses them to smaller chains. This means that more 'ends' are created for further breakdown (see the section on the
small intestine for more detail).
Food is released from the stomach by periodic relaxation of the pyloric sphincter muscle at the lower end of the stomach.