After the mouth and throat, the next parts of the digestive tract are the oesophagus and stomach, both with their own roles to perform.
The oesophagus, or gullet, is a muscular tube 25cm long. It starts in the neck, passes through the chest, behind the heart and lungs, and ends in the abdominal cavity by opening into the stomach. Its main purpose is to transport food and fluid to the stomach but it also allows the occasional regurgitation of swallowed air in the form of belching. The inner lining of the oesophagus is made of flattened squamousand mucus is secreted from glands situated at its top and bottom ends. This mucus aids lubrication of food and protects the lower end of the oesophagus against acid return- ing – refluxing – from the stomach. All swallowed substances are pushed to the stomach by a wave of muscular contractions called peristalsis. This means that food will be propelled toward the stomach even when you are standing on your head. At the top of the oesophagus a muscle (musculus cricopharyngeus) prevents air entering during breathing. At the bottom is the lower sphincter, which together with the acute angle of entry of the oesophagus into the stomach, prevents regurgitation of stomach contents (reflux).
The structure of the stomach
The stomach lies in the left upper part of the abdomen immediately below the diaphragm. It is a J-shaped sac with a capacity of approximately one litre. The upper- most part, located directly beneath the diaphragm, is called the fundus ventriculi and it usually contains a gas bubble. The lower part, or antrum pyloricum, is separated from the duodenum by the muscular pylorus which acts as an extremely efficient sphincter. The largest part of the stomach is the body, which receives and stores the food we swallow. The lining of the stomach, the mucosa, contains many tubular glands within its surface. Among these are the gastric glands which secrete hydrochloric acid and pepsinogen. The glands are made up of three different types of: mucoid cells found at the neck of the glands secrete mucus, chief cells secrete digestive enzymes and parietal cells which secrete hydrochloric acid. The latter also secrete a substance called intrinsic factor, vital for the absorption of vitamin B,2-
The stomach has a number of different functions. It acts mainly as a store for recently digested food known as chyme, until there is room in the small intestine.
The walls of the stomach contain thick muscle layers that allow food to be stirred up and mixed with the digestive secretions. Weak mixing waves and stronger antral contractions known as peristaltic waves act to stir up the chyme and provide an efficient means of mixing. One in five of the peristaltic waves is sufficiently strong to propel a proportion of the stomach contents into the duodenum. Hunger pangs are intense muscular contractions of the stomach wall and begin about 12 to 24 hours after the last meal and serve as one of the body’s defences against starvation.
Pepsinogen, which is secreted by the chief cells of the gastric glands, comes into contact with hydrochloric acid splits to form pepsin, the powerful protein-digesting enzyme. This works effectively in the acid environment of the stomach but becomes inactivated once it reaches the alkaline environment of the small intestine. The production of hydrochloric acid, which also kills bacteria, is by the parietal cells in the stomach’s lining. It is regulated both by autonomic -nervous system – and hormonal control. Under the control of the latter, cells within the gastric antrum react to the presence of food by secreting a hormone called gastrin. Its main effect is on the parietal cells of the gastric glands, as it increases their acid output by as much as eight times. Although the response to gastrin is less than that of nervous stimulation, its effect lasts for several hours. Once the stomach contents have reached the necessary level of acidity there is a feedback mechanism which prevents further gastric secretions, and further acid production.
The rate of stomach emptying is determined by several factors. The most important of these is the degree of fluidity of the chyme, which in turn, depends on the type of food eaten, how well it has been chewed, how long it has been in the stomach, and how well it has been mixed with the digestive juices. The small intestine also regulates gastric emptying by slowing the rate of emptying if the chyme is excessively acidic, if the small intestine is already too full, or if there is a large amount of fat present.