Absorption, Storage and Assimilation : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 12 of 25)

The Storage of Nutriment. - For some time after the taking of a meal, food materials are being absorbed more rapidly than they can be used by the cells. Following this is an interval when the body is taking no food, but during which the cells must be supplied with nourishment. It also happens that the total amount of food absorbed during a long interval may be in excess of the needs of the cells during that time; and it is always possible, as in disease, that the quantity absorbed is not equal to that consumed. To provide against emergencies, and to keep up a uniform supply of food to the cells, it is necessary that the body store up nutrients in excess of its needs.

Methods of Storage. - The general plan of storage varies with the different nutrients as follows:

1. The carbohydrates are stored in the form of glycogen. This, as already stated, is a substance closely resembling starch. It is stored in the cells of both the liver and the muscles, but mainly in the liver. It is a chief function of the liver to collect the excess of dextrose from the blood passing through it, and to convert it into glycogen, which it then stores within its cells. It does not, however, separate all of the dextrose from the blood, a small amount being left for supplying the immediate needs of the tissues. As this is used, the glycogen in the liver is changed back to dextrose and, dissolving, again finds its way into the blood. In this way, the amount of dextrose in the blood is kept practically constant. The carbohydrates are stored also by converting them into fat.

2. The fat is stored for the most part in the connective tissue. Certain of the connective tissue cells have the property of taking fat from the blood and of depositing it within their inclosing membranes. When this is done to excess, and the cells become filled with fat, they form the so-called adipose tissue. Most of this tissue is found under the skin, between the muscles, and among the organs occupying the abdominal cavity. If one readily takes on fat, it may also collect in the connective tissue around the heart. The stored-up fat is dissolved again as needed, and enters the blood, where it again becomes available to the active cells.

3. The proteins form a part of all the tissues, and for this reason are stored in larger quantities than any of the other food substances. The large amount of protein found in the blood may also be looked upon as storage material. The proteins in the various tissues are spoken of as tissue proteins, and those in the blood as circulating proteins. The proteins of the tissues serve the double purpose of forming a working part of the cell protoplasm, and of supplying reserve food material. That they are available for supplying energy, and are properly regarded as storage material, is shown by the rapid loss of protein in starving animals. When the proteins are eaten in excess of the body's need for rebuilding the tissues, they are supposed to be broken up in such a manner as to form glycogen and fat, which may then be stored in ways already described.

General Facts Relating to Storage. - The form into which the food is converted for storage in the body is that of solids - the form that takes up the least amount of space. These solids are of such a nature that they can be changed back into their former condition and, by dissolving, reenter the blood.

Only energy-yielding foods are stored. Water and salts, though they may be absorbed in excess of the needs of the body, are not converted into other substances and stored away. Oxygen, as already stated, is not stored. The interval of storage may be long or short, depending upon the needs of the body. In the consumption of stored material the glycogen is used first, then as a rule the fat, and last of all the proteins.

Storage in the Food Canal. - Not until three or four hours have elapsed are all the nutrients, eaten at a single meal, digested and passed into the body proper. The undigested food is held in reserve, awaiting digestion, and is only gradually absorbed as this process takes place. It may properly, on this account, be regarded as stored material. That such storage is of advantage is shown by the observed fact that substances which digest quickly (sugar, dextrin, "predigested foods," etc.) do not supply the needs of the body so well as do substances which, like starch and proteins, digest slowly. Even substances digesting quite slowly (greasy foods and pastry), since they can be stored longer in the food canal, may be of real advantage where, from hard work or exposure, the body requires a large supply of energy for some time. These "stay by" the laborer, giving him strength after the more easily digested foods have been used up. Storage by the food canal is limited chiefly to the stomach.

Regulation of the Food Supply to the Cells. - The storage of food materials is made to serve a second purpose in the plan of the body which is even more important than that of supplying nourishment to the cells during the intervals when no food is being taken. It is largely the means whereby the rate of supply of materials to the cells is regulated. The cells obtain their materials from the lymph, and the lymph is supplied from the blood. Should food substances, such as sugar, increase in the blood beyond a low percent, they are converted into a form, like glycogen, in which they are held in reserve, or, for the time being, placed beyond the reach of the cells. When, however, the supply is reduced, the stored-up materials reenter the blood and again become available to the cells. By this means their rate of supply to the cells is practically constant.

We are now in a position to understand why carbohydrates, fats, and proteins are so well adapted to the needs of the body, while other substances, like alcohol, which may also liberate energy, prove injurious. It is because foods are of such a chemical nature that they are adapted in all respects to the body plan of taking up and using materials, while the other substances are lacking in some particulary.

B. Diagram showing disposition of alcohol if this substance is taken in quantity corresponding to that of the nutrients. The alcohol thrown off as waste is not oxidized and yields no energy.

D.C. Heath and Co. - Publishers
Original copyright 1909