Gastric digestion is brought about by the action of the gastric juice, a clear watery, colourless and strongly acid fluid with a specific gravity of about 1003. The amount of hydrochloric acid present in the juice varies with the period of digestion. The acid exists in the stomach in two forms, as free hydrochloric acid and as combined hydrochloric acid. The amount of each depends on various factors: (i) the secretion itself ; (ii.) the nature of the food; and (iii.) the rapidity with which the stomach empties itself, etc. For instance, after a protein-free meal the hydrochloric acid is for the most part free, whereas, when protein is present, it combines with it and, unless secreted in very large amount, most of the acid is in a fixed condition.
The hydrochloric acid is formed by the activities of certain gland cells in the middle region of the stomach, and the fact that it does not exist as such in the blood proves that it is formed within these cells. That the chlorine comes from the sodium chloride in the food has been shown by the fact that, when the tissues are deprived of this salt, and sodium bromide is given, hydrobromic acid may appear in the gastric secretion.
The hydrochloric acid is essential for the action of the gastric enzyme, pepsin, in splitting up the protein of the food. In addition to this, the acid has a slight action in splitting polysaccharides and disaccharides. Lastly, it acts as a bactericidal agent, preventing bacterial decomposition from taking place, and it may thus pre vent certain noxious bacteria, taken in in the food, from gaining access to the intestinal tract, where there is a chance of their flourishing in the rich alkaline medium.
The quantity of juice secreted varies with the nature of the food consumed. Each separate food seems to give rise to a definite hourly secretion of the juice and to a characteristic alter ation in its properties.
The principal ferment found in the gastric juice is pepsin, which acts only in the presence of a mineral acid. The action pro
ceeds best at a temperature of about 37°C in an acid medium of 0.2% to 0.3%. Pepsin is elaborated in the so-called chief cells of the gastric glands as an inert precursor—propepsin. It is only when it comes into contact with the acid of the juice that it is activated into attacking the protein of the food.
As already mentioned, the main function of the gastric juice is to deal with the protein moiety of the food and to prepare it for further digestion in the intestine. The first result of the action of this secretion on protein matter is to render it soluble—a meta protein or acid albumin being formed. This body may be regarded mainly as the product of the action of the hydrochloric acid inde pendently of the pepsin.
The following steps of decomposition are the result of the action of pepsin. From the metaprotein primary and secondary proteoses are formed, and from these peptones are finally pro duced. The result of this process of digestion or hydrolysis in duced by the pepsin is that complex insoluble protein substances of high molecular weight are converted into simpler soluble protein bodies of comparatively low molecular weight. The con , tents of the stomach—products of protein digestion—are passed on into the duodenum, chiefly as proteoses and peptones.
In addition to pepsin some workers hold that another enzyme is present in the gastric juice. This is the ferment rennet, rennin or chymosin, the sole action of which, so far as is known at pres ent, is to bring about the curdling of milk, the curd formed being subsequently dealt with in the ordinary way by the pepsin. Clot ting of milk under the action of rennin occurs at a suitable tem perature with great rapidity. This process is said to take place in two stages : (I ) the rennin converts the caseinogen of the milk into soluble casein, and (2) this soluble casein unites with the lime salts present in the milk and forms the curd or precipitate. That lime salts are absolutely essential for this process of clot ting has been shown by the fact that, if they are removed by pre cipitation as by oxalates, no clotting will take place even after the addition of a large amount of active rennin. Immediate clot ting takes place, however, when the necessary lime salts are restored.