Role of stomach in mammalian digestion

 

The mammalian stomach is a muscular, distensible sac that contain three
main functions.

(1) It stores and mixes the food bolus that received
from the esophagus.

 (2) Secretes substances such as enzymes, mucus, and hydrochloric acid [HCl] that digest the protein.

(3) Stomach also helps to control the rate at which food moves into the small
intestine through the pyloric sphincter (figure a).
The stomach is made up of an inner mucous membrane containing thousands of gastric glands (figure b).

Types of gastric glands

 Parietal cells that secrete a solution that contain HCl.

 Chief cells that secrete pepsinogen, the make from the enzyme pepsin. Both of the cells are in the pits of the gastric glands.

Mucous cells
The surface of the mucous membrane at the openings of the glands contains numerous mucous cells that secrete mucus that cover the surface of the stomach and protects it from the HCl and digestive enzymes.

Function of some hormone in stomach

 The surfaces of the upper gastrointestinal tract the esophagus and mouth have a much thinner mucous-cell layethan the stomach, which is why vomiting can cause a burning sensation in the esophagus or mouth. Endocrine cells in one part of the stomach mucosa release the hormone gastrin, which travels to target cells in the gastric glands, further stimulating them.

When the bolus of food enters the stomach, it distends the walls of the stomach. This distention, as well as the act of eating, causes the gastric pits to secrete HCl and pepsinogen. The H ions cause pepsinogen to be converted into the active enzyme pepsin. As pepsin, mucus, and HCl mix with and begin to break down proteins, smooth mucosal muscles contract and vigorously churn and mix the food bolus.

 About three to four hours after a meal, the stomach contents have been sufficiently mixed and are a semiliquid mass called chyme .The pyloric sphincter regulates the release of the chyme into the small intestine.

Sensation of hunger pangs
When the stomach is empty, peristaltic waves cease, after about 10 hours of fasting, new waves  comes from the upper region of the stomach. These waves can cause hunger pangs as sensory nerve fibers carry impulses to the brain.