Give the composition and function of gastric juice.

(N/A) The mucosa of the stomach contains gastric glands. Gastric glands consist of three major types of cells:
$(1)$ Mucous neck cells which secrete mucus.
$(2)$ Peptic or chief cells which secrete the proenzyme pepsinogen.
$(3)$ Parietal or oxyntic cells which secrete $HCl$ and intrinsic factor (essential for the absorption of vitamin $B_{12}$).
When food enters the stomach,the hormone gastrin is secreted into the blood by the pyloric region,which stimulates the secretion of gastric juice from the gastric glands.
The stomach stores food for $4-5$ hours. The food mixes thoroughly with the acidic gastric juice through the churning movements of the muscular wall,forming a mass called chyme. The proenzyme pepsinogen,upon exposure to hydrochloric acid,is converted into the active enzyme pepsin. Pepsin converts proteins into proteoses and peptones.
$\text{Pepsinogen} \xrightarrow{HCl} \text{Pepsin} \rightarrow \text{Proteins} \rightarrow \text{Proteoses} + \text{Peptones}$
The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated $HCl$.
$HCl$ provides the acidic $pH$ optimal for pepsin activity. Rennin is a proteolytic enzyme found in the gastric juice of infants,which helps in the digestion of milk proteins. Small amounts of lipases are also secreted by gastric glands.