Digestive System

The Esophagus and the Stomach In human anatomy and physiology, the esophagus is a long tube run¬ning downward through the throat and thorax and connecting to the stomach in the upper portion of the abdominal cavity. Food moves quickly through the esophagus, pushed along by waves of mus¬cular contraction in a process called peristalsis. Circular muscles in the wall of the esophagus, when seen using teaching microscopes, contract just behind the food bolus squeez¬ing the food forward. As the food moves, the muscles it passes also contract, so that a region of contraction follows the bolus and constantly pushes it forward, much as though you were to keep a ball moving through a soft rubber tube by giving the tube a series of squeezes, with your hand always just behind the ball.

At the junction between the esophagus and the stomach is a special ring of muscle called a sphincter, which can be studied further using teaching microscopes, that contracts in order to close the entrance to the stomach. It is normally closed, thus preventing the contents of the stomach from moving back into the esophagus when the stomach moves during digestion. It opens when a wave of peris¬taltic contraction coming down the esophagus reaches it.

In human anatomy, the stomach lies slightly to the left side in the upper portion of the abdomen, just below the lower ribs. It is a large muscular sac, which, as we have already seen using teaching microscopes, functions as a storage organ, making possible discontinuous feeding. It has other functions too. When it contains food, its thick muscular wall is swept by powerful waves of contrac¬tion, which churn the food, mixing it and breaking up the larger pieces. In this manner, the stomach supplements the action of the teeth in mechanical breakup of food. The glands of the stomach lining are of several types, as seen when tissue samples were analyzed using a microscope. Some secrete mucus, which covers the stomach lining; others secrete gastric juice, a mixture of hydrochloric acid and digestive enzymes. Enzymatic digestion, then, is a third important function of the human stomach.

The Small Intestine

The food leaves the stomach as a soupy mix¬ture. It passes through the pyloric sphincter into the small intestine, which is the portion of the digestive tract where most of the digestion and absorption takes place. The first section of the small intestine, attached to the stomach, is called the duodenum. It leads into a very long coiled section lying lower in the abdominal cavity. The entire small intestine of an adult man is about 23 feet long and an inch in diameter.

The length of the small intestine shows interesting variations in different animals. The intestine is usually very long and much coiled in herbivores, much shorter in carnivores, and of medium length in omnivores like humans. These differences, like those of the teeth, are correlated with the difficulty of digesting plant material because of the cellulose cell walls. Even if the cellulose has been well broken up, it remains mixed with the digestible portions of the cells and tends to mask them from the digestive enzymes, when examined under a microscope. This interference makes di¬gestion and absorption of plant material much less efficient than the processing of animal material, with the result that a longer intestine is an adaptive advantage in extracting a maximum amount of nutrients from a herbivorous diet.

Since the small intestine is the place where absorption of the prod¬ucts of digestion occurs, we would expect it to have special structural adaptations that increase its absorptive surface area. Clearly, its great length plays a role here. But examination of the internal surface of the vertebrate small intestine using specialized microscopes reveals other modifications that vastly in¬crease its surface area over that of a smooth-walled tube of equal length and girth. First, the epithelial tissue lining the intestine, when viewed under a microscope, is thrown into numerous folds and ridges. Second, small fingerlike outgrowths, called villi, cover the internal surface. And third, as the electron microscope reveals, the individual epithelial cells covering the folds and villi have what is called, for obvious rea¬sons, a brush border, consisting of countless closely packed cylindrical processes, the microvilli. Thus the total internal surface of the small intestine, including folds, villi, and microvilli, is incredibly large, as revealed by specialized microscopes.

The Large Intestine

In human anatomy, the junction between the small intes¬tine and the large intestine, or colon that follows it is usually in the lower right portion of the abdominal cavity. A blind sac, the caecum, projects from the large intestine near the point of juncture. In humans there is a ’small fingerlike process, the appendix, at the tip of the caecum. As you know, the appendix often becomes infected and must be surgically removed. In humans the caecum is small and functionally unimportant, but in some mammals, particularly herbivorous ones, it is large and con¬tains many microorganisms such as bacteria and Protozoa that are capable of digest¬ing cellulose. Since the mammal cannot itself digest cellulose, it benefits from the action of the microbes which are so minute they can only be seen under a microscope.

From the caecum, the large intestine of humans ascends on the right side to the mid-region of the abdominal cavity, then crosses to the left side, and descends again. One of its chief functions is reabsorption of much of the water used in thee digestive process. If all the water in which enzymes are secreted into the digestive tract were lost with the feces, there would be a severe problem of desiccation. Occasionally the intestine becomes irritated, and peristalsis moves material through it too fast for enough water to be reabsorbed; this condition is known as diarrhea. Conversely, if material moves too slowly, too much water is reabsorbed and constipation results. A proper amount of roughage (indigestible material, primarily cellulose) in the diet provides the bulk needed to stimulate enough peristalsis in the large intestine to prevent constipation.

A second function of the large intestine involves the excretion of certain salts, such as those of calcium and iron, when their concen¬tration in the blood is too high. The salts are excreted from the blood into the large intestine and are eliminated from the body in the feces.

The last, portion of the large intestine, the rectum, functions as storage chamber for the feces until defecation. The feces are elimi¬nated from the rectum through an opening called the anus.