Physiology and Hygiene for Secondary Schools

by Francis M. Walters

Fig. 79—Stored-up fat. The figure shows four connective tissue cells containing small particles of fat. 1. Nucleus. 2. Protoplasm. 3. Fat. 4. Connective tissue fibers.

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 (Fig. 79). 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[pg 179] connective tissue around the heart. The stored-up fat is redissolved as needed, and enters the blood, where it again becomes available to the active cells.

3. The proteids 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 proteid found in the blood may also be looked upon as storage material. The proteids in the various tissues are spoken of as tissue proteids, and those in the blood as circulating proteids. The proteids 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 proteid in starving animals. When the proteids 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, reënter 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 (page 108), 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 proteids.

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 [pg 180]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 proteids, 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 per cent, 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 reënter 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 proteids are so well adapted to the needs of the body, while other substances, like alcohol, which[pg 181] 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 particular.