Physiology and Hygiene for Secondary Schools

by Francis M. Walters

[pg 029]New white corpuscles are formed from old ones, by cell-division. Their production may occur in almost any part of the body, but usually takes place in the lymphatic glands (Chapter VI) and in the spleen, where conditions for their development are especially favorable. In these places they are found in great abundance and in various stages of development.

Functions of White Corpuscles.—The main use of the white corpuscles appears to be that of a destroyer of disease germs. These consist of minute organisms that find their way into the body and, by living upon the tissues and fluids and by depositing toxins (poisons) in them, cause different forms of disease. Besides destroying germs that may be present in the blood, the white corpuscles also leave the blood and attack germs that have invaded the cells. By forming a kind of wall around any foreign substance, such as a splinter, that has penetrated the skin, they are able to prevent the spread of germs through the body. In a similar manner they also prevent the germs from boils, abscesses, and sore places in general from getting to and infecting other parts of the body.[10] Another function ascribed to the white corpuscles is that of aiding in the coagulation of the blood (page 31); and still another, of aiding in the healing of wounds.

Plasma.—The plasma is a complex liquid, being made up of water and of substances dissolved in the water. The dissolved substances consist mainly of foods for the cells and wastes from the cells.

1. The foods represent the same classes of materials as are taken in the daily fare, i.e., proteids, carbohydrates,[pg 030] fats, and salts (Chapter IX). Three kinds of proteids are found in the plasma, called serum albumin, serum globulin, and fibrinogen. These resemble, in a general way, the white of raw egg, but differ from each other in the readiness with which they coagulate. Fibrinogen coagulates more readily than the others and is the only one that changes in the ordinary coagulation of the blood. The others remain dissolved during this process, but are coagulated by chemical agents and by heat. While all of the proteids probably serve as food for the cells, the fibrinogen, in addition, is a necessary factor in the coagulation of the blood (page 31).

The only representative of the carbohydrates in the plasma is dextrose. This is a variety of sugar, being derived from starch and the different sugars that are eaten. The fat in the plasma is in minute quantities and appears as fine droplets—the form in which it is found in milk. While several mineral salts are present in small quantities in the plasma, sodium chloride, or common salt, is the only one found in any considerable amount. The mineral salts serve various purposes, one of which is to cause the proteids to dissolve in the plasma.

2. The wastes are formed at the cells, whence they are passed by the lymph into the blood plasma. They are carried by the blood until removed by the organs of excretion. The two waste products found in greatest abundance in the plasma are carbon dioxide and urea.

The substances dissolved in the plasma form about 10 per cent of the whole amount. The remaining 90 per cent is water. Practically all the constituents of the plasma, except the wastes, enter the blood from the digestive organs.

Purposes of Water in the Blood.—Not only is water the[pg 031] most abundant constituent of the blood; it is, in some respects, the most important. It is the liquefying portion of the blood, holding in solution the constituents of the plasma and floating the corpuscles. Deprived of its water, the blood becomes a solid substance. Through the movements of the blood the water also serves the purpose of a transporting agent in the body. The cells in all parts of the body require water and this is supplied to them from the blood. Water is present in the corpuscles as well as in the plasma and forms about 80 per cent of the entire volume of the blood.

Coagulation of the Blood.—If the blood is exposed to some unnatural condition, such as occurs when it escapes from the blood vessels, it undergoes a peculiar change known as coagulation.[11] In this change the corpuscles are collected into a solid mass, known as the clot, thereby separating from a liquid called the serum. The serum, which is similar in appearance to the blood plasma, differs from that liquid in one important respect as explained below.

Causes of Coagulation.—Although coagulation affects all parts of the blood, only one of its constituents is found in reality to coagulate. This is the fibrinogen. The formation of the clot and the separation of the serum is due almost entirely to the action of this substance. Fibrinogen is for this reason called the coagulable constituent of the blood. In the plasma the fibrinogen is in a liquid form; but during coagulation it changes into a white, stringy solid, called fibrin. This appears in the clot and is the cause of its formation. Forming as a network of [pg 032]exceedingly fine and very delicate threads (Fig. 11) throughout the mass of blood that is coagulating, the fibrin first entangles the corpuscles and then, by contracting, draws them into the solid mass or clot.[12] The contracting of the fibrin also squeezes out the serum. This liquid contains all the constituents of the plasma except the fibrinogen.