The Digestive System and the Liver - The stomach
About ten inches down the esophagus, the food we swallow must pass the cardiac sphincter. Then the food, by now fairly well diced and mashed, passes into the stomach.
Inelegant as it sounds, the stomach is best described as a rough, leather-skinned balloon. When empty, its skin shrivels around itself like a deflated balloon; but when “pumped up” by a hearty meal, the stomach becomes a plump, J-shaped bag about a foot long and six inches wide, holding about two quarts of food and drink.
The Passage of Food through the Stomach
Although its food-processing function tends to get more attention, the stomach's role as a storage reservoir is equally important. A moderate, well-rounded meal with a good blend of carbohydrates, proteins, and fats takes usually a minimum of three hours to pass out of the pyloric sphincter into the small intestine—more if the meal is heavy in fats and rich foods. Thus, a meal that might take us 15 minutes to eat, may take up to 20 times as long to pass from the stomach into the small intestine. This decelerating of food's rate of passage has two very significant results: first, it allows time for the food-processing activities within the stomach; and second, it releases food (in a mushy form called chyme ) in small, well-spaced amounts that can be efficiently handled by the small intestine.
Although the stomach is not an absolutely essential organ—a person can live a full life with part or even all of it removed—it is a tremendous convenience. Without a stomach, frequent, carefully selected, well-chewed small feedings rather than “three square meals a day” are necessary so as not to overburden the small intestine, which can handle only a small quantity of food, well-mashed, at one time. If too much food goes directly to the small intestine, only so much nourishment (carbohydrates, proteins, fats) per meal can be supplied to the body, with the result that we would be weak from hunger after going a few hours without eating.
It will come as no surprise to know that the food processing done in the stomach is both mechanical and chemical. The three layers of crisscrossing muscles in the stomach walls are rarely still. They contract and relax continually, squeezing, pummeling, and mixing the stomach's contents into chyme. So active and relentless is the stomach's muscular activity that it actually “chews up” pieces of food that have been swallowed too hastily.
The various chemicals found in the stomach are produced and secreted into the stomach cavity by some 40 million gland cells that line the interior stomach walls. The constant wiggling and jouncing of the stomach helps to mix these chemicals thoroughly into the food. Each of the chemicals is secreted by a special type of cell and has a specific function. They include the digestive enzymes pepsin, rennin, and lipase; hydrochloric acid; and watery mucus.
A look at the special assignments of rennin, pepsin, hydrochloric acid, and mucus—and how they interact with and depend upon one another—provides a good glimpse into the elegant and complex chemical events that occur when the stomach encounters a swallow of food.
Rennin and Pepsin
Rennin, well known to cheesemakers, has essentially one task: to turn milk into milk curds. But the curds are not ready to pass on to the small intestine until they are further dismantled by pepsin . Pepsin has other duties as well: one of them is to begin the breakdown of proteins. But pepsin can only begin to split up protein foods after they have been worked on by hydrochloric acid .
Hydrochloric acid is a corrosive substance and, except in very dilute strengths, could quite literally eat away the lining of the stomach. (This is apparently what happens in cases of gastric ulcers.) Mucus secretions, with the help of fluids in the food itself, dilute the hydrochloric acid to a point where (in a normal stomach) it is rendered harmless. Even so, the normal, healthy condition inside our stomach is slightly acid. The slight acidity of the stomach serves to inhibit the growth of organisms such as bacteria.