Digestion In The Stomach

Protein is not chemically altered in the mouth. The process of chewing simply moistens and divides it into small particles so that it is easily swallowed and more readily attacked by the secretion of the stomach. Passing down the oesophagus, through the cardiac sphincter guarding the entrance to the stomach, it enters that portion of the latter organ called the fundus. Here it rests quietly for some time, so that it is not immediately mixed with gastric juice, as formerly taught, but the outer portions of the mass are successively attacked and dissolved. These soft portions are then pressed into the intermediate and pyloric regions of the stomach and subjected to a thorough mixing.

The gastric juice is an acid fluid which may be secreted in large quantities - as much as one or two gallons in 24 hours. Its acidity is due to the presence of about 0.2 per cent. to 0.4 per cent. of hydrochloric acid. This of itself is capable; of causing protein to swell and to some extent dissolve; of hydrolyzing the collagen of connective tissue; of decalcifying bone; and of preventing bacterial action.

The gastric juice contains two enzymes which act on protein, pepsin and rennin. Pepsin can act only in the presence of a definite percentage of hydrochloric acid, while rennin acts in a neutral solution. The relations of rennin and pepsin are not yet very clearly understood. The function of rennin is to curdle milk, which causes it to remain in the stomach long enough to be digested by the pepsin.

Action Of Pepsin

The protein of the diet, more or less swollen, softened, and dissolved by the action of hydrochloric acid, is changed chemically by pepsin, into a series of increasingly simpler and more soluble products, namely, acid proteins, proteoses (often called albumoses) and peptones. The change to peptones is not complete, but these three products of peptic digestion tend to pass together into the intestine, when the pyloric sphincter which guards the entrance to the duodenum opens to let them pass. The opening and closing of this sphincter is controlled by two factors: 1. The consistency of the partly digested food, solid particles tending to keep the pylorus closed. 2. The presence of free hydrochloric acid in the stomach contents. This is the more important factor.

Protein is capable of absorbing and uniting chemically with a certain amount of acid. When it is so saturated, free acid will be present, having now nothing with which to combine, and this free acid is the stimulus which causes the pyloric sphincter to relax and lets a little spurt of food into the duodenum. Once in the duodenum, this same acid acts as the stimulus to close the pylorus, and no more food enters the duodenum until the first portion has been neutralized.

A clear understanding of the way in which the pyloric sphincter is regulated throws considerable light on stomach digestion. We can understand why a certain quantity of indigestible residue, such as is present in most of our staple foods - meats, vegetables, fruits, etc. - is of service in retaining food within the stomach for a time, giving the hydrochloric acid a chance to exercise its bactericidal power, and favoring thorough gastric digestion. It also explains why a meal of " bolted " food (or of food imperfectly masticated through absence of teeth), or a failure of free hydrochloric acid in the stomach contents, as in hypoacidity or achylia gastrica, may set up such a continuous state of pyloric contraction that the stomach remains unemptied for a long period, with a good prospect of fermentation and gas production, of relief by vomiting, or of an attack of acute indigestion, biliousness or diarrhoea. The nurse will understand why a diet of "liquids" for a person in relatively good health does not prove satisfactory. Liquids do not stay long enough in the stomach to give a sense of fullness, give the stomach little to do, and tend to pass into the intestine poorly prepared for the action of the digestive juices there.