The stomach is an enlargement of the alimentary tract located inferior to the diaphragm. It is a J-shaped organ and can be divided into four regions:
- the cardia is the immediate region surrounding the cardiac sphincter;
- the fundus is the bulbous portion to the left and superior to the cardia;
- the body is the lower central portion of the stomach;
- the pylorus is the region adjacent to the pyloric valve which separates the stomach from the small intestine. The outer border of the J-shape is referred to as the greater curvature and the inner border as the lesser curvature.
The stomach serves as a food reservoir, a site for mechanical and chemical digestion, and as an absorption site for alcohol, water, and some salts. Two to three liters of gastric juices (water, salts, mucins, HC1, pepsin, rennin, and perhaps gastric lipase) are secreted by the stomach each day. Secretions arise from gastric glands located in the stomach's mucosal layer. They are composed of chief cells which secrete pepsin and rennin, parietal cells which produce HC1, and mucous cells which secrete mucin.
Pepsin is the major enzyme in the stomach . It is responsible for reducing proteins to proteoses and peptones. It is secreted in an inactive form,pepsinogen, which is activated in the presence of HC1. Rennin acts on the milk protein casein and has a curdling effect on the milk. This enzyme is more highly concentrated in the stomach of infants, than adults. Rennin is present in adults, but not in great amounts. Gastric lipase as the name suggests is found in the stomach, but no evidence is present which suggests that it is produced there. Concentrations are so low that it is now thought to arrive via the pyloric sphincter from the small intestine. In any event it is only capable of breaking down fats to triglycerides. Mucins are secretions from the pyloric mucosa that combine with water to form mucus. This forms a protective barrier against enzymes on the inner lining of the stomach.
Enzymatic secretions are under nervous and hormonal control. Sight, smell and the taste of food will also initiate gastric secretions via vagal nerve stimulation (= Cephalic Phase). Undigested food in the stomach initiates gastric secretions and muscular contractions of the stomach (= Gastric Phase). When food is present in the stomach, gastrin, a hormone of the pyloric mucosa, is released into the blood. It is this hormone that initiates the muscular contractions and secretions mentioned above. As the final mass of partially digested food (i.e., usually this is undigested fats) passes from the stomach to the intestine, the intestine produces a hormone, enterogastrone, which now inhibits gastric movements and secretions (= Intestinal Phase).
The acidity of the stomach (ph = 1.0) is due to a high concentration of HC1. Hydrochloric acid activates pepsinogen. curds milk, plays a minor role in protein digestion (i.e., swells and softens protein), and kills almost all of the bacteria present in the food bolus obtained from the esophagus.