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 prevent 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. It
is owing to the presence of hydrochloric acid that gastric juice
can be kept for prolonged periods without undergoing putrefaction.
The quantity of juice secreted varies with the nature of the food consumed. Thus in one experiment, after the use of a test meal consisting of 25 grammes bread and 250 c.c. tea, there was a flow of 106 c.c., whereas in another case with an ordinary meal there was an output of practically 600 c.c. gastric juice.
Pawlow has shown that not only does the amount of juice secreted vary with the nature of the food ingested but that the digestive activity of the secretion also varies in the same way. He gives the following table:—
| Hour. | Quantities of Juice in c.c. | Digestive Power in mm. | ||||
| Flesh. | Bread. | Milk. | Flesh. | Bread. | Milk. | |
| 1st | 11·2 | 10·6 | 4·0 | 4·94 | 6·10 | 4·21 |
| 2nd | 11·3 | 5·4 | 3·6 | 3·03 | 7·97 | 2·35 |
| 3rd | 7·6 | 4·0 | 9·2 | 3·01 | 7·51 | 2·35 |
| 4th | 5·1 | 3·4 | 7·7 | 2·87 | 6·19 | 2·55 |
| 5th | 2·3 | 3·3 | 4·0 | 3·20 | 5·29 | 4·63 |
| 6th | 2·2 | 2·2 | 0·5 | 3·58 | 5·72 | 6·12 |
| 7th | 1·2 | 2·6 | . . | 2·25 | 5·48 | . . |
| 8th | 0·6 | 2·6 | . . | 3·87 | 5·50 | . . |
| 9th | . . | 0·9 | . . | . . | 5·75 | . . |
| 10th | . . | 0·4 | . . | . . | . . | . . |
Thus each separate food gives rise to a definite hourly secretion of the juice and to a characteristic alteration in its properties. The meat diet brings about a very rapid flow, the maximum output taking place within the first two hours; with bread the maximum output is even earlier. With milk somewhat later. When the juice is examined as regards its digestive activity, it is found that with meat the most active juice is secreted within the first hour, with bread in the second and third hours, and with milk in the sixth hour.
According to the nature of the food, the stomach seems to be stimulated to form a secretion which will best serve its purpose and give the minimum of waste. It thus works economically.
The principal ferment found in the gastric juice is pepsin, a ferment which acts only in the presence of a mineral acid. The action proceeds 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 and rendered capable of 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 (syntonin), being formed. This body may be regarded mainly as the product of the action of the hydrochloric acid independently of the pepsin.
The following steps of decomposition are the result of the action of pepsin. From the meta protein primary and secondary proteoses, the so-called proto-, hetero- and deutero-albumoscs are formed, and nom these peptones are finally produced. The result of this process of digestion or hydrolysis induced by the pepsin is that complex protein substances of high molecular weight are converted into simpler bodies of comparatively low molecular weight. Formerly it was believed that the action of the pepsin on protein could not carry the decomposition further than the peptones, but recently it has been shown that still further splitting can be brought about, and that the simple amino acids of which the protein molecule is built up can be produced. This latter process, however, takes a very long time even under favourable circumstances, and it probably never occurs under normal conditions. The contents of the stomach-products of protein digestion-are passed on into the duodenum, chiefly as proteoses and peptones.
In addition to the principal ferment of the gastric juice some workers hold that another enzyme is present. This is the ferment rennet, rennin, or chymosin, the sole action of which, so far as is known at present, is to bring about the curdling of milk, the curd formed being dealt with in the ordinary way by the pepsin. Clotting of milk under the action of rennin occurs at a suitable temperature with great rapidity. This process is said to take place in two stages: (1) the rennin converts the caseinogen of the milkinto para casein, and (2) this paracasein 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 clotting has been shown by the fact that, if they are removed by precipitation as by oxalates, no clotting will take place even after the addition of a large amount of active rennin. Immediate clotting takes place, however, when the necessary lime salts are restored. Many observers now hold that this rennet action is not the property of a specific ferment but simply another phase of the action of pepsin. For this view, which has been put forward by well known workers, there is much to be said and certainly the power of curdling milk is not confined to the stomach, but has been found in various tissue extracts, and, indeed, wherever proteolytic enzymes are found.
The speed with which the stomach is emptied depends to a great extent on the nature of the food. Plain water leaves the stomach almost at once, salt and sugar solutions at a somewhat slower rate. Milk under the action of rennin curdles. The whey rapidly leaves the stomach, whereas the casein and fat are retained for further treatment. On a mixed diet, emptying of the stomach in man proceeds very slowly, requiring about four hours. Cannon, by feeding with food impregnated with bismuth and using X-rays, showed that carbohydrates leave most rapidly, then mixtures of carbohydrates and proteins, then proteins, then fats, and finally mixtures of fats. and proteins. The diet which remains longest in the stomach is a mixture of fats and proteins-rich food, as it is popularly called. Here two factors enter to prevent rapid emptying: (1) the presence of much fat, and (2) the acid secretion engendered by the abundant protein.
There is no doubt that fats present in fine emulsion can be decomposed in the stomach. The action proceeds in a medium which is slightly acid or neutral, being entirely prevented by the presence of strong acids and alkalis. Many workers believe this gastrolipase to be of pancreatic or intestinal origin, and suppose that it gains entrance to the stomach by a reflux flow through the pylorus. Evidence is accumulating to show that this view is correct.
By means of pepsin and gastrolipase proteins and fats are dealt with. No specific enzyme for carbohydrates has been found in the stomach in man. Certainly a small amount of polysaccharide decomposition takes place, but this is dependent (1) on the ptyalin which comes from the mouth, and (2) on a certain amount of hydrolysis due to the action of the free hydrochloric acid.
4. Digestion in the Intestine.—The passage of food from the stomach to the intestine will be considered later. The food so far digested in the stomach is known as chyme, and it is passed on to undergo intestinal digestion under the influence of (1) the enzymes of the pancreas, and (2) of other enzymes present in the different secretions of the intestine. Digestion in the intestine may accordingly be described under these two heads.
(a) Pancreatic Digestion.—The pancreatic juice is the secretion from the pancreas and is discharged into the duodenum. The secretion obtained from a fistula of the pancreatic duct varies in character according to whether the opening into the duct has been made recently or some time before the examination. It is a clear, usually thin fluid with a specific gravity of about 1008, and with an alkaline reaction. It contains a certain amount of protein and ash. The most important inorganic constituent is sodium carbonate, which gives the alkaline reaction (alkalinity is, as NaOH＝0·47%). This alkaline salt, along with that contained in the intestinal juice, plays an important part in neutralizing the acid chyme. "
In the pancreatic secretion there are at least three important enzymes, each with a definite action: (a) trypsin, the proteolytic enzyme which brings about the further breakdown of the food proteins; (b) a diastase which deals with the carbohydrates, and (c) a lipase which acts on the fats.
(a) Trypsin.—This ferment, in the form in which it is secreted—trypsinogen—is inert. Before it can exert its hydrolytic action it must be activated. This activation is brought about by another enzyme which is found in the intestinal tract—enterokinase. The conversion is brought about as soon as the trypsinogen comes into contact with the enterokinase, the merest trace of which suffices to activate a large amount of trypsinogen.
Trypsin acts on the protein just as pepsin does, by bringing about hydrolytic changes. It differs from the latter in acting best in an alkaline or neutral medium. Its effect is much more energetic than that of pepsin, so that the protein molecule is more completely decomposed. Whilst it generally finishes the decomposition which the pepsin has begun, it can break down the original protein quite as easily if not more easily than does pepsin, and it carries the splitting as far as the comparatively simple crystalline bodies, the amino acids, or groups of these, the polypeptides, bodies intermediate between the complex peptones and the simple amino acids of which the protein is built up.
