Kevin Hensley, age 12, of Des Moines, Iowa, for his question:
How is the process of digestion accomplished?
A dinner tray of salad, meat and vegetables, dessert and beverage looks like food, smells like food and tastes like food. And, of course, it is food. But in its present form it is not useable food for the body. Before it can be used it must be chomped and pummeled, broken apart, remodeled and mixed with liquid. In other words, every meal must go through the complicated process of digestion.
The food we eat goes on a long winding excursion from the mouth, through the stomach to the intestines. Along the way, it is shredded to fragments and showered with enzyme juices that spurt in through the walls. The original meal is an assortment of chemicals. During digestion, most of these are broken apart and some are assembled to make quite different chemicals.
The digestive process starts in the mouth. The teeth begin the job of tearing the solids to shreds. Saliva contains a digestive juice called ptyalin, which starts the work of changing starchy foods into sugary chemicals. When each well chewed mushy mouthful is swallowed, it jogs down a tube to the stomach.
This muscular pouch is where the main job of digestion is done. There the food is mashed and bashed to a pulpy liquid. We know that meats and milk products provide needed proteins and most of us assume that these proteins are extracted and used by the body. Not so. Proteins are made of chemicals called amino acids and in the stomach the proteins we eat are broken apart into their more than 20 different amino acids. This chemical wonder is performed mainly by hydrochloric acid and the enzymes in pepsin, digestive juices that pour in through the stomach walls.
When this stage is accomplished, the soupy liquid leaves the stomach and enters the small intestine. As it winds through this long tube, the remaining starches are converted to sugars. Fats are converted to glycerol and usable fatty acids. The remaining protein portions are converted to amino acids.
What started out on the dinner table as food is now a liquid formula containing chemicals such as amino acids, fatty acids and simple sugars. At this stage, the mixture also contains a residue of bulky, indigestible materials, such as cellulose from stringy vegetables.
The usable foods, in the form of a liquid formula, pass through the intestine walls into the bloodstream. The indigestibles pass on down to the large intestine, the colon. From there, with other waste materials, they are sent outside the body.
The bloodstream carries the properly digested ingredients to the busy cells. The fats and sugars may be used right away to yield energy. Or they may be stored in fatty tissues for future use. The cells use more than 20 of the digested amino acids to build a large assortment of proteins. Thus the meat proteins we eat are broken down and reassembled to build the special proteins needed by the human body.