Food digestion is a chemical process where the consumed food is broken down into smaller molecules. The process is very important for absorption of nutrients present in food so that they can be used as energy fuel. Digestion of food takes place when the food is churned and mixed with numerous digestive juices.
Digestive process begins right from mouth and ends in small intestine. One of the main functions of stomach is to break large food particles into smaller ones, especially the protein and fat molecules. This procedure is carried out by various gastric juices secreted by the stomach such as hydrochloric acid, pepsin, water, electrolytes and mucus.
With the help of gastric juices the consumed food is converted into semisolid mass called as chyme which makes intestinal absorption easier. Normally the stomach secretes 1.5 to 3 liters of gastric juices in a day.
Regulation Of Gastric Juice Secretion in 3 Phases
The stomach is a J shaped organ located on the left side of abdomen. It is made up of thick muscular wall which helps to prevent damage from the acidic content of stomach. The pH of gastric juices varies from 1-3 and it helps to kill the bacteria and breakdown of food.
Gastric wall consists of three layers. The innermost layer is called mucosa, the middles layer is submucosa and the outermost layer is known as muscularis serosa. Acid is produced by the parietal cells of the stomach lining. The secretion of gastric juices is divided into three phases namely cephalic, gastric, and intestinal.
- Cephalic phase: In this phase the secretion of gastric juice is stimulated by smell, sight, and thought of food. Cephalic phase is entirely an involuntary process and it is mediated by the vagus nerve. Once the vagus nerve, which innervates its branches in stomach is stimulated either by presence of food or by several different senses, it triggers the cells present in stomach lining to produce enzymes and acidic juice.
- Gastric phase: Almost half of the gastric juice is secreted in this phase. The cells in the walls of stomach lining begin to produce juices after distention of stomach once the food enters into the stomach or by the presence of proteins present in food. Specific foodstuffs that increase the secretion of stomach acid include proteins, coffee, wine, tea, beer, milk, colas, etc. The gastric phase is also controlled by vagus nerve and a hormone called gastrin. The acidic pH remains 3 for at least 2 hours after meal. Thus the combined effect of three factors namely; chemical, hormonal and physical are involved during gastric phase for secreting gastric juice. The gastric phase continues till the stomach content moves into the intestine.
- Intestinal phase: The intestinal phase is complex and it is not completely understood by researchers. Intestinal phase begins when the stomach content moves into the small intestine. In this phase both stimulatory and inhibitory mechanism is involved. Although proteins are stimulants for secretion of gastric acid, the chyme inhibits its secretion. As a result the pH of gastric juices falls below 2.5 and inhibits secretion of gastrin hormone. Only ten percent of acid is therefore secreted during this phase.
Thus the regulation of gastric juices secretion reflects an intricate balance of inputs from nerve cells, hormonal, and autocrine pathways. It involves both stimulatory and inhibitory mechanism.