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DIGESTION

The alimentary canal begins above with the cavity of the mouth and terminates below at the anus, traversing in its course the length of the thoracic and abdominal cavities.

After the entrance into the mouth the food undergoes a twofold reduction: a mechanical one and a chemical one. The former process is called mastication. The chemical action taking place in the mouth is accomplished by means of the first digestive secretion, the saliva.

The end product of mastication is the bolus, a mass of food moistened with saliva. In this form the food passes into the stomach where it undergoes a mechanical and chemical reduction. The chemical process is accomplished by means of the gastric juice.

The main purpose of gastric digestion is the formation of the chyme: in this form the food is transferred into the duodenum.

In the duodenum the food is treated by the pancreatic juice and the bile, an important external secretion of the liver. The liver is the largest gland in the body which has the most powerful influence upon all the metabolic functions of the body.

The principal organ of absorption is the small intestine. The function of the small intestine is to separate the useful from the useless constituents of the food. The useful constituents are immediately transferred into the absorbing channels of the body, while the useless are allowed to escape into the large intestine and are included in the faeces.

Ответьте на вопросы:

- What organs take part in the process of digestion?

- How is food treated in the oral cavity?

- How is the end product of mastication called?

- Where is the food treated by the gastric juice?

- What is chyme?

- Where is the food treated by the pancreatic juice and the bile?

- What function does the small intestine perform in the process of digestion?

Текст

Digestion in the stomach

The food which reaches the stomach is mixed with saliva and is semisolid in consistency. It accumulates in the fundus and the mass thus formed is rot penetrated to any extent by the gastric juice, so that salivary digestion can be continued for some time. Gradually, however, due to muscular movements, the food is mixed with the gastric juice and the acidity so increased that the action of the ptyalin ceases. The reaction of the gastric juice is acid, pH 0.9 to 1.5. The more important constituents are hydrochloric acid, mucin, pepsin, rennin and gastric lipase.

The first step in gastric digestion is the action of hydrochloric acid upon protein to form acid metaprotein. The acid causes the protein to swell to a gelatinous mass (acid metaprotein) which goes rapidly into solution in the acid gastric secretions. Pepsin then hydrolyzes acid metaprotein to proteoses and peptones. Protein digestion does not go beyond the peptone stage in the stomach; it is completed in the intestines. There is no carbohydrate enzyme in the gastric juice but some of the cane sugar (sucrose) is hydrolyzed by the hydrochloric acid.

As a result of muscular movements which mix the food with the gastric juice, various chemical and physical changes take place which reduce the gastric contents to a semifluid, more or less homogeneous, creamy mass called chyme. At intervals portions of the chyme are ejected through the pylorus into the duodenum.