STOMACH

The stomach is a thick-walled large muscular sac which lies in the left portion of the abdominal cavity below the diaphragm. It is made of three layers of muscles arranged longitudinally, circularly and obliquely. The contractions of these muscles in different directions bring about twisting, squeezing and churning of the stomach. Thus large food particles are further broken down into smaller particles for easy digestion by enzymes. The entrance and exit of the stomach is guided by cardiac and pyloric sphincters respectively.

 

The stomach has a thick and delicate inner folded membrane called gastric mucosa in which gastric glands are embedded. Three types of glands are present in the gastric mucosa. One type of gland secretes the enzymes pepsinogen and renin. Pepsinogen is converted to pepsin on contact with dilute hydrochloric acid. Pepsin converts proteins to peptones while renin coagulates milk protein (caesin) before it is acted upon by pepsin. The digestion of carbohydrates, fats and oils does not start in the stomach. The second type of gland secretes mucus while the third type of gland secretes dilute hydrochloric acid.

Hydrochloric acid also helps to kill germs such as bacteria in the stomach.

The mucus which forms a layer over the stomach prevents it from the attack of the dilute hydrochloric acid. If the mucus is not available, the acid attacks the stomach wall causing gastric ulcer.

These three secretions form what is called gastric juice. When the stomach is empty, muscular contraction still goes on causing hunger sensation.

 

Functions of stomach

1. Food is temporarily stored in the stomach for about one to four hours allowing time for digestion of protein to start.
2. The warmth of the stomach helps to melt fats.
3. The secretion of dilute hydrochloric acid from the stomach wall helps to destroy bacteria and converts pepsinogen to pepsin for digestion of proteins.
4. The contraction of the stomach muscles brings about churning of the food and breaking of large food particles to small particles for easy digestion.

 

Stomach of ruminants

A ruminant is a herbivorous animal which has a four-chambered stomach for effective digestion of plant materials. The four chambers are rumen, reticulum, omasum and abomasum. The rumen is the largest portion and it is here that the vegetables (mainly grasses) taken in are stored temporarily. A huge population of bacteria and protozoans live in the rumen and start the digestion and fermentation of cellulose present in the vegetable. The contents of the rumen are passed into the reticulum where further fermentation takes place. The reticulum also supplies fluid to the rumen and stores sand and stones picked up with vegetables. When the animal is at rest, the partially digested food is brought back (regurgitated) to the mouth for thorough chewing before it is swallowed back. This rechewing is referred to as “chewing the cud”.

 

 

On reswallowing the semi-liquid cud, it is directed to another chamber called omasum. The towel-like omasum further breaks down the food and water is passed out of the cud and absorbed into the body. The solid material is passed into another chamber called abomasum (a true stomach). Glands in the walls of the abomasum secrete protein-digesting enzymes and dilute hydrochloric acid for the digestion of protein. Some bacteria and protozoans entering the abomasum with the cud are killed and digested.