Digestion in the Stomach

Both mechanical and chemical digestion continue in the stomach, where food is stored for 4-5 hours. Mechanically, the thick muscular wall of the stomach churns the food, breaking it into finer particles and mixing it thoroughly with gastric juice.

The mucosa of the stomach contains gastric glands, each built of three principal cell types: mucus cells (secrete mucus, which protects the stomach's own lining from the acid within it), peptic (chief) cells (secrete the inactive proenzyme pepsinogen) and parietal (oxyntic) cells (secrete hydrochloric acid (HCl) and intrinsic factor, which is essential for the later absorption of vitamin B12). In infants, the gastric glands additionally secrete the enzyme rennin.

HCl makes the stomach contents acidic, halts the action of salivary amylase, and kills most germs present in the food. In this acidic medium, pepsinogen is converted into its active form, pepsin:

HCl: Pepsinogen (inactive) -> Pepsin (active), at about pH 1.8

Pepsin then acts on proteins, breaking them down into simpler forms — peptones and proteoses:

Pepsin: Proteins -> Peptones + Proteoses (in the acidic medium) …