*Physical Properties of the Blood.*-Experiments such as those described at the close of this chapter reveal the more important physical properties of the blood. It may be shown to be heavier and denser than water; to have a faint odor and a slightly salty taste; to have a bright red color when it contains oxygen and a dark red color when oxygen is absent; and to undergo, when exposed to certain conditions, a change called coagulation.

These properties are all accounted for through the different materials that enter into the formation of the blood.

[Fig. 8]

Fig. 8-Blood corpuscles, highly magnified. _A._ Red corpuscles as they appear in diluted blood. _B._ Arrangement of red corpuscles in rows between which are white corpuscles, as may be seen in undiluted blood.

_C._ Red corpuscles much enlarged to show the form.

*Composition of the Blood.*-To the naked eye the blood appears as a thick but simple liquid; but when examined with a compound microscope, it is seen to be complex in nature, consisting of at least two distinct portions. One of these is a clear, transparent liquid; while the other is made up of many small, round bodies that float in the liquid. The liquid portion of the blood is called the _plasma_; the small bodies are known as _corpuscles_. Two varieties of corpuscles are described-the _red_ corpuscles and the _white_ corpuscles (Fig. 8). Other round particles, smaller than the corpuscles, may also be seen under favorable conditions.

These latter are known as _blood platelets_.

*Red Corpuscles.*-The red corpuscles are cla.s.sed as cells, although, as found in the blood of man and the other mammals (Fig. 9), they have no nuclei.(6) Each one consists of a little ma.s.s of protoplasm, called the _stroma_, which contains a substance having a red color, known as _hemoglobin_. The shape of the red corpuscle is that of a circular disk with concave sides. It has a width of about 1/3200 of an inch (7.9 microns(7)) and a thickness of about 1/13000 of an inch (1.9 microns). The red corpuscles are exceedingly numerous, there being as many as five millions in a small drop (one cubic millimeter) of healthy blood. But the number varies somewhat and is greatly diminished during certain forms of disease.

[Fig. 9]

Fig. 9-Red corpuscles from various animals. Those from mammals are without nuclei, while those from birds and cold-blooded animals have nuclei.

It is the _function_ of the red corpuscles to serve as _oxygen carriers_ for the cells. They take up oxygen at the lungs and release it at the cells in the different tissues.(8) The performance of this function depends upon the hemoglobin.

*Hemoglobin.*-This substance has the remarkable property of forming, under certain conditions, a weak chemical union with oxygen and, when the conditions are reversed, of separating from it. It forms about nine tenths of the solid matter of the red corpuscles and to it is due the colors of the blood. When united with the oxygen it forms a compound, called _oxyhemoglobin_, which has a bright red color; the hemoglobin alone has a dark red color. These colors are the same as those of the blood as it takes on and gives off oxygen. The stroma, which forms only about one tenth of the solid matter of the corpuscles, serves as a contrivance for holding the hemoglobin. The conditions which cause the hemoglobin to unite with oxygen in the lungs and to separate from it in the tissues, will be considered later (Chapter VIII).

*Disappearance and Origin of Red Corpuscles.*-The red corpuscles, being cells without nuclei, are necessarily short-lived. It has been estimated that during a period of one to two months, all the red corpuscles in the body at a given time will have disappeared and their places taken by new ones. The origin of new corpuscles, however, and the manner of ridding the blood of old ones are problems that are not as yet fully solved. The removal of the products of broken down corpuscles is supposed to take place both in the liver and in the spleen.(9)

Regarding the origin of the red corpuscles, the evidence now seems conclusive that large numbers of them are formed in the red marrow of the bones. The red marrow is located in what is known as the spongy substance of the bones (Chapter XIV) and consists, to a large extent, of cells somewhat like the red corpuscles, but differing from them in having nuclei. These appear to be constantly in a state of reproduction. The blood, flowing through the minute cavities containing these cells, carries those that have been loosened out into the blood stream. Nuclei appear in the red corpuscles at the time of their formation, but these quickly separate and, according to some authorities, form the blood platelets.

*White Corpuscles.*-The white corpuscles, or _leucocytes_, are cells of a general spherical shape, each containing one, two, or more nuclei. They are much less numerous than the red, there being on the average only one white corpuscle to about every five hundred of the red ones. On the other hand, the white corpuscles are larger than the red, one of the former being equal in volume to about three of the latter.

[Fig. 10]

Fig. 10-*Escape of white corpuscles from a small blood vessel* (Hall). At _A_ the conditions are normal, but at _B_ some excitation in the surrounding tissue leads to a migration of corpuscles. 1, 2, and 3 show different stages of the pa.s.sage.

The white corpuscles are found, when studied under favorable conditions, to possess the power of changing their shape and, by this means, of moving from place to place. This property enables them to penetrate the walls of capillaries and to pa.s.s with the lymph in between the cells of the tissues. The white corpuscles are, therefore, not confined to the blood vessels, as are the red corpuscles, but migrate through the intercellular s.p.a.ces (Fig. 10). If any part of the body becomes inflamed, the white corpuscles collect there in large numbers; and, on breaking down, they form most of the white portion of the sore, called the _pus_.

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 cla.s.ses of materials as are taken in the daily fare, _i.e._, proteids, carbohydrates, fats, and salts (Chapter IX).

Three kinds of proteids are found in the plasma, called _serum alb.u.min_, _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 quant.i.ties and appears as fine droplets-the form in which it is found in milk. While several mineral salts are present in small quant.i.ties 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 pa.s.sed 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 const.i.tuents of the plasma, except the wastes, enter the blood from the digestive organs.

*Purposes of Water in the Blood.*-Not only is water the most abundant const.i.tuent of the blood; it is, in some respects, the most important. It is the liquefying portion of the blood, holding in solution the const.i.tuents 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 ma.s.s, 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 const.i.tuents 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 const.i.tuent 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 exceedingly fine and very delicate threads (Fig. 11) _throughout the ma.s.s of blood_ that is coagulating, the fibrin first entangles the corpuscles and then, by contracting, draws them into the solid ma.s.s or clot.(12) The contracting of the fibrin also squeezes out the serum. This liquid contains all the const.i.tuents of the plasma except the fibrinogen.

[Fig. 11]

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 const.i.tuent 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 const.i.tuents 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 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 r.e.t.a.r.ds 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.

*Quant.i.ty of Blood.*-The quant.i.ty 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 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]

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 const.i.tuents of the blood. The plasma is the portion that changes most rapidly. Yet in spite of these changes the quant.i.ty 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 an outflow which are equal in amount may keep always at the same level, the balancing of the intake and outgo of the blood keeps its composition about the same from time to time.

*Hygiene of the Blood.*-The blood, being a changeable liquid, is easily affected through our habits of living. Since it may be affected for ill as well as for good, one should cultivate those habits that are beneficial and avoid those that are harmful in their effects. Most of the hygiene of the blood, however, is properly included in the hygiene of the organs that act upon the blood-a fact which makes it unnecessary to treat this subject fully at this time.

From a health standpoint, the most important const.i.tuents of the blood are, perhaps, the corpuscles. These are usually sufficient in number and vigor in the blood of those who take plenty of physical exercise, accustom themselves to outdoor air and sunlight, sleep sufficiently, and avoid the use of injurious drugs. On the other hand, they are deficient in quant.i.ty and inferior in quality in the bodies of those who pursue an opposite course. Impurities not infrequently find their way into the blood through the digestive organs. One should eat wholesome, well-cooked food, drink freely of _pure_ water, and limit the quant.i.ty of food _to what can be properly digested_. The natural purifiers of the blood are the organs of excretion. The skin is one of these and its power to throw off impurities depends upon its being clean and active.

*Effect of Drugs.*-Certain drugs and medicines, including alcohol and quinine,(15) have recently been shown to destroy the white corpuscles. The effect of such substances, if introduced in considerable amount in the body, is to render one less able to withstand attacks of disease. Many patent medicines are widely advertised for purifying the blood. While these may possibly do good in particular cases, the habit of doctoring one's self with them is open to serious objection. Instead of taking drugs and patent medicines for purifying the blood, one should study to live more hygienically. We may safely rely upon wholesome food, pure water, outdoor exercise and sunlight, plenty of sleep, and a clean skin for keeping the blood in good condition. If these natural remedies fail, a physician should be consulted.

*Summary.*-The blood is the carrying or transporting agent of the body. It consists in part of const.i.tuents, such as the red corpuscles, that enable it to carry different substances; and in part of the materials that are being carried. The latter, which include food and oxygen for the cells and wastes from the cells, may be cla.s.sed as freight. Certain const.i.tuents in the blood destroy disease germs, and other const.i.tuents, by coagulating, close small leaks in the blood vessels. Although subject to rapid and continuous change, the blood is able-by reason of the balancing of materials added to and withdrawn from it-to remain about the same in quant.i.ty and composition.

Exercises.-1. Compare blood and water with reference to weight, density, color, odor, and complexity of composition.