The stomach is an enlargement of the alimentary tract located inferior to the diaphragm. It is a J-shaped organ, and we divide it into four regions:
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. The stomach secretes two to three litres of gastric juices (water, salts, mucins, HC1, pepsin, rennin, and perhaps gastric lipase) each day. Secretions arise from gastric glands located in the stomach’s mucosal layer. Their chief cells secrete pepsin and rennin. Meanwhile, parietal cells produce HC1. And, mucous cells 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. The stomach of infants has a much higher concentration of this enzyme, than the stomach of adults. Rennin is present in adults, but not in vast amounts.
Gastric lipase as the name suggests is found in the stomach, but no evidence is present, which indicates that it is produced there. Concentrations of it are very low. So, it may 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. It creates 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 gets into the stomach, the pyloric mucosa released gastrin, a hormone 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 oesophagus.