The stomach forms the widest part of the digestive tract and contains three basic sections. The upper part is known as the fundus, the main and largest portion is referred to as the stomach body and the lower section as the pyloric antrum.
Three layers of smooth muscle exist within the stomach. The outer layer is made up of longitudinal muscle, the middle layer of circular, and the inner layer of oblique muscle. This structure allows for the churning movement necessary for gastric activity.
The oesophageal sphincter which lies between the stomach and the oesophagus controls the entry of food. The pyloric sphincter, located between the stomach and the duodenum (the upper segment of the small intestine), controls the movement of stomach contents into the duodenum. Contents entering the duodenum are referred to as chyme and it is when digestion reaches this stage that the majority of absorption takes place.
Gastric juice arises from gastric glands within the mucous membrane lining of the stomach. Following a meal, food accumulates in the stomach in layers. The stomach churns the food, breaks it down and mixes it with gastric juice.
Water, mineral salts, hydrochloric acid, intrinsic factor and pepsinogens are the substances that make up gastric juice. Water liquefies food whilst hydrochloric acid kills harmful microbes and provides the necessary acid for pepsin digestion. The hydrochloric acid converts pepsinogens to pepsins which then commence protein digestion.
Intrinsic factor is a protein compound necessary for the absorption of vitamin B12 and this is vital for the maturation of red blood cells.
Mineral salts include calcium, iodine, iron, phosphorus, potassium and sodium. These compounds are vital for all functions within the body.
Mucus protects the stomach wall; it forms a barrier between the wall and acidic juices.
Three phases exist in the secretion of gastric juice; the cephalic phase, gastric phase and the intestinal phase
This is initiated via the vagus nerve and comes about due to the sight, smell, taste and even the thought of food. It is during this phase that gastric juice begins to flow.
When food is present a hormone known as gastrin is released and enters into the blood stream. This then stimulates the gastric glands to produce the gastric juices needed for digestion.
This occurs when stomach contents reach the duodenum within the small intestine. The intestine releases a combination of hormones known as enterogastrone and this acts to slow down the secretion of gastric juice.
Gastric juice reaches maximum levels approximately one hour following the consumption of a meal. Levels then decline to a much smaller amount and after roughly four hours reach the minimum levels known as fasting levels.
Not all substances are absorbed from the small intestine, for example, water, alcohol and certain drugs and medications are absorbed through the stomach wall into the blood stream.
Human Body - A Dorling Kindersley Book - edited by Ann Baggaley