Give composition and function of gastric juice.

Explanation

The mucous of stomach has gastric glands. Gastric glands liave three major types of cells namely:
(1) Mucous neck cells which secrete mucus
(2) Peptic of chief cells which secrete the proenzyme pepsinogen
(3) Parietal or Oxyntic cells which secrete HCl and intrinsic factor (factor essential) for absorption of vitamin `B_12`.
Food material comes into stomach cavity at that time Gaskin is secreted into blood by pyloric stomach which stimulates secretion of gastric juice from the stomach glands.
Stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme.
The proenzyme pepsinogen on exposure to hydrochloric acid gets converted into the active enzyme pepsin. Pepsin converts proteins into proteoses and peptones.
Pepsinogen HCl `to` Pepsin
Proteins `to` Proteoses + Peptones
The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of mucosal epithelium from excoriation by the highly concentrated hydrochloric acid.
HCl provides the acidic pH optimal for pepsins. Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins.
Small amounts of lipases are also secreted by gastric glands.