Jan Waggoner, age 14, of Brownsville, Tex., for her question:
HOW IS FOOD DIGESTED?
Digestion is the process by which the food we eat is changed to material that can be used by the body for growth and energy. The many parts of the body devoted to this process are called the digestive system.
The process of digestion starts in the mouth where several glands on each side of the face and beneath the jaw empty their fluid into the mouth. This fluid is called saliva and it helps to break down starches.
After the food is crushed and moistened in the mouth, it slides back through the pharynx and is pushed through the esophagus. The source for the push is a wave of contraction or tightening of the wall of the aLimentary canal. This is called peristalsis.
Next stop is the stomach. It is here that most of the process of digestion takes place. Here juices from the stomach wall are mixed with the food. Hydrochloric acid is one of the juices mixed with the food and pepsin is another.
The secretions of the stomach also destroy bacteria and free iron from combinations allowing it to be absorbed to help form red blood cells.
Food stays in the stomach until it is a liquid. This material is churned to mix digestive juices well throughout the food. The liquid is called chyme as it moves out of the stomach into the small intestine.
The small intestine is a tube from 22 to 25 feet long. Digestion continues in the first part.
The lining of the small intestine has many delicate folds called villi. These folds increase the amount of lining surface. As the chyme or digested material pass over this surface, it is absorbed into the blood and lymph tissues. In this way nourishment reaches all the cells of the body.
One pathway for discarding substances the body does not use is through the intestinal tract.
The blood brings waste material to be discarded to the intestinal lining, where it can pass into the canal. Here it is joined with the remaining unabsorbed food material.
The outer wall of the intestine is formed by layers of muscle. Waves of peristalsis move the material in the intestine. The contents stay liquid until they reach the large intestine.
Near where the small and large intestine join is the appendix. This is a narrow tube which has no useful part in the digestive process. Sometimes it becomes blocked and must be removed surgically.
In the colon or large intestine water is absorbed and the contents become more solid. Bacteria are present. The last part of the large intestine is the rectum, from which the waste material leaves the body.