Valuable notes on the accessory organs of digestionThe stomach is a J-shaped dilated part of the alimentary canal, which is continuous with the oesophagus above and the small intestine below. Its shape and position are altered by changes within the abdominal cavity and the stomach but it lies below the diaphragm, slightly to the left of the midline.
Structure: The stomach has two curvatures. The lesser curvature forms the right or posterior border of the stomach. The greater curvature is on the anterior surface of the stomach.
The upper end of the stomach which is the entrance of the oesophagus is called the cardiac orifice. To the left of the cardiac orifice is the dome-shaped upper part called the funds and the main part of the stomach is called as the body. The lower part of the J is the pyloric atrium, which has an opening into the duodenum called as the pyloric orifice that is guarded by strong sphincter muscles which separates the stomach from the duodenum.
The stomach consists of the four layers of tissues as the alimentary canal they are:
1. The peritoneum or serious coat.
2. The muscular layer consisting of longitudinal, circular and oblique muscle fibers.
3. The sub mucous layer.
4. The mucous layer contains gastric glands which secrets gastric juice and is thrown into folds called rogue thereby increasing the surface area of the stomach, from which secretion can take place.
Digestion of food in the Stomach:
When food reaches the stomach, the digestive fluid, gastric juice is secreted from tiny ducts of the gastric glands, found within the mucous membrane of the stomach. Mixing of the gastric juices takes place gradually to make the food sufficiently acidified, thereby stopping the action of ptyalin, which acts in an alkaline medium in the mouth.
The gastric juice is a clear watery fluid and in contrast to the saliva, is acid in reaction. It contains:
1. Water, mineral salts and mucus
2. Hydrochloric acid (Hcl)
3. Enzymes: Pepsinogen, Rennin and gastric lipase
4. Intrinsic factor.
The hydrochloric acid converts the inactive pepsinogen to the active enzyme pepsin. Pepsin begins the chemical digestion of protein by converting them to peptones. Whereas rennin curdles milk by converting the soluble protein of milk, caseinogens into insoluble casein which in turn is converted by Pepsin into Peptones. It is present in a gastric juice of children only. The action of gastric lipase is quiet unimportant here, as it merely splits the fat molecules without the actual digestion taking place.
In addition to the onward movement of the gastric contents, Peristaltic action in the stomach that is the churning movement of the muscle tissues causes further mechanical break down of the food, mixing it with the gastric juice and pushing the food contents onward into the duodenum.
Functions of the Stomach
1. It acts as a temporary reservoir for food.
2. It produces gastric juice, which begins the chemical digestion of food.
3. It further breaks down the food, by churning up the food into chyme.
4. It is a place, where glucose, water, alcohol and some drugs are absorbed.
5. It secretes the intrinsic factor that is essential for the absorption of Vitamin B12.