Lynn McDaniel, age 12, of National City, Calif., for her question:
WHAT HAPPENS TO THE FOOD WE EAT?
Our bodies are made up of millions of individual cells. Each cell is a collection of chemicals in a liquid material, and nourishment for them must be presented in a proper liquid form. The bulky foods we eat must be converted.into smooth liquid paste, and the job of our digestive system is to make the conversion. Of course, the complete process is quite complicated, but it is possible to take a quick tour.
The food we eat goes on a long, winding excursion that begins in the mouth. On the way it is chomped to bits, mixed and churned about and showered with digestive enzyme juices. The original meal is an assortment of sugars, starches, fats, proteins, plus a wide variety of chemicals. During digestion most of these are broken apart and some are reassembled to form substances the body can use.
The first part of the digestive system's job is to break up the bite size chunks of food into smaller and smaller bits. Our teeth perform this service by tearing and crunching the food to shreds. The tongue, which is a powerful muscle, helps by mixing and turning the food so that the teeth can break it up. At the same time saliva is added to moisten it and begin the digestive process. Saliva contains a digestive juice called ptyalin which changes some of the starches into sugars.
When each well-chewed mouthful is swallowed, it jogs down a tube called the esophagus to the stomach. Because food enters the stomach in a semi-liquid form, it needs no grinders. However, powerful muscles mix and mash the food into a still finer paste, while more digestive juices go to work on the proteins.
At the proper time a valve in the lower portion of the stomach opens and the thick liquid enters into the first portion of the small intestine Here, more digestive juices from the liver and pancreas complete the job of breaking down partly digested proteins and starches. Pats are also broken down into materials the body can use.
What started out at the dinner table as food is now a liquid formula containing simple sugars, amino acids and fatty acids. These are the basic foods that the body can use as fuel and for growth. Also present are bulky indigestible materials, such as cellulose from stringy vegetables.
The usable foods are absorbed by tiny blood and lymph vessels in the walls of the small intesting. They are whisked away to the farthest corners of the body to be used as they are needed. The indigestibles pass down to the large intestine where they are removed from the body as waste materials.
The bloodstream carries the properly digested ingredients to the busy cells. The fats and sugars are used right away for energy. If they are not needed, they may be stored in fatty tissues for future use. The proteins are used to build new cells and repair tissue in case of a cut or a broken bone.