Physiology and Hygiene for Secondary Schools. Francis M. Walters
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.
Fig. 11—Fibrin threads (after Ranvier). These by contracting draw the corpuscles together and form the clot.
Fibrin Ferment and Calcium.—Most difficult of all to answer have been the questions: What causes the blood to coagulate outside of the blood vessels and what prevents its coagulation inside of these vessels? The best explanation offered as yet upon this point is as follows: Fibrinogen does not of itself change into fibrin, but is made to undergo this change by the presence of another substance, called fibrin ferment. This substance is not a regular constituent of the blood, but is formed as occasion requires. It is supposed to result from the breaking down of the white corpuscles, and perhaps also from the blood platelets, when the blood is exposed to unnatural conditions. The formation of the ferment leads in turn to the changing of the fibrinogen into fibrin.
Another substance which is necessary to the process of coagulation is the element calcium. If compounds of calcium are absent from the blood, coagulation does not take place. These are, however, regular constituents of healthy blood. Whether the presence of the calcium is necessary to the formation of the ferment or to the action of the ferment upon the fibrinogen is unknown.
Purpose of Coagulation.—The purpose of coagulation is to check the flow of blood from wounds. The fact that the blood is contained in and kept flowing continuously[pg 033] through a system of connected vessels causes it to escape rapidly from the body whenever openings in these vessels are made. Clots form at such openings and close them up, stopping in this way the flow that would otherwise go on indefinitely. Coagulation, however, does not stop the flow of blood from the large vessels. From these the blood runs with too great force for the clot to form within the wound.
Time Required for Coagulation.—The rate at which coagulation takes place varies greatly under different conditions. It is influenced strongly by temperature; heat hastens and cold retards the process. It may be prevented entirely by lowering the temperature of the blood to near the freezing point. The presence of a foreign substance increases the rapidity of coagulation, and it has been observed that bleeding from small wounds is more quickly checked by covering them with linen or cotton fibers. The fibers in this case hasten the process of coagulation.
Quantity of Blood.—The quantity of blood is estimated to be about one thirteenth of the entire weight of the body. This for the average individual is an amount weighing nearly twelve pounds and having a volume of nearly one and one half gallons. About 46 per cent by volume of this amount is made up of corpuscles and 54 per cent of plasma. Of the plasma about 10 per cent consists of solids and 90 per cent of water, as already stated.
Functions of the Blood.—The blood is the great carrying, or distributing, agent in the body. Through its movements (considered in the next chapter) it carries food and oxygen to the cells and waste materials from the cells. Much of the blood may, therefore, be regarded as freight in the process of transportation. The blood also carries, or distributes, heat. Taking up heat in the warm parts of the body, it gives it off at places having a lower temperature. This enables all parts of the body to keep at about the same temperature.
In addition to serving as a carrier, the blood has antiseptic properties, i.e., it destroys disease germs. While [pg 034] this function is mainly due to the white corpuscles, it is due in part to the plasma.13 Through its coagulation, the blood also closes leaks in the small blood vessels. The blood is thus seen to be a liquid of several functions.
Fig. 12—A balanced change in water. The level remains constant although the water is continually changing; suggestive of the changes in the blood.
Changes in the Blood.—In performing its functions in the body the blood must of necessity undergo rapid and continuous change. The red corpuscles, whose changes have already been noted, appear to be the most enduring constituents of the blood. The plasma is the portion that changes most rapidly. Yet in spite of these changes the quantity and character of the blood remain practically constant.14 This is because there is a balancing of the forces that bring about the changes. The addition of various materials to the blood just equals the withdrawal of the same materials from the blood. Somewhat as a vessel of water (Fig. 12) having an inflow and