Physiology and Hygiene for Secondary Schools :: CHAPTER IV

CHAPTER IV - THE BLOOD

Two liquids of similar nature are found in the body, known as the blood and the lymph. These are closely related in function and together they form the nutrient fluid referred to in the preceding chapter. The blood is the more familiar of the two liquids, and the one which can best be considered at this time.

The Blood: where Found.—The blood occupies and moves through a system of closed tubes, known as the blood vessels. By means of these vessels the blood is made to circulate through all parts of the body, but from them it does not escape under normal conditions. Though provisions exist whereby liquid materials may both enter and leave the blood stream, it is only when the blood vessels are cut or broken that the blood, as blood, is able to escape from its inclosures.

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.

[pg 025]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 classed 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 mass 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 microns7) and a thickness of[pg 026] 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[pg 027] 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 [pg 028]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 passage.

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 pass 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 spaces (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.

[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.

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 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

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 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[pg 035] 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 constituents 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 quantity 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 quantity 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[pg 036] 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 constituents, 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 classed as freight. Certain constituents in the blood destroy disease germs, and other constituents, 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 quantity and composition.

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

2. Show by an outline the different constituents of the blood.

3. Compare the red and white corpuscles with reference to size, shape, number, origin, and function.

4. Name some use or purpose for each constituent of the blood.

5. What constituents of the blood may be regarded as freight and what as agents for carrying this freight?

6. After coagulation, what portions of the blood are found in the clot? What portions are found in the serum?

7. What purposes are served by water in the blood?

8. Show how the blood, though constantly changing, is kept about the same in quantity, density, and composition.

9. In the lungs the blood changes from a dark to a bright red color and in the tissues it changes back to dark red. What is the cause of these changes?

10. If the oxygen and hemoglobin formed a strong instead of a weak chemical union, could the hemoglobin then act as an oxygen carrier? Why?

[pg 037]11. What habits of living favor the development of corpuscles in the blood?

12. Why will keeping the skin clean and active improve the quality of one's blood?

PRACTICAL WORK

To demonstrate the Physical Properties of Blood (Optional).—Since blood is needed in considerable quantity in the following experiments, it is best obtained from the butcher. To be sure of securing the blood in the manner desired, take to the butcher three good-sized bottles bearing labels as follows:

1 Fill two thirds full. While the blood is cooling, stir rapidly with the hand or a bunch of switches to remove the clot.

2 Fill two thirds full and set aside without shaking or stirring.

3 Fill two thirds full and thoroughly mix with the liquid in the bottle.

Label 3 must be pasted on a bottle, having a tight-fitting stopper, which is filled one fifth full of a saturated solution of Epsom salts. The purpose of the salts is to prevent coagulation until the blood is diluted with water as in the experiments which follow.

Experiments.—1. Let some of the defibrinated blood (bottle 1) flow (not fall) on the surface of water in a glass vessel. Does it remain on the surface or sink to the bottom? What does the experiment show with reference to the relative weight of blood and water?

2. Fill a large test tube or a small bottle one fourth full of the defibrinated blood and thin it by adding an equal amount of water. Then place the hand over the mouth and shake until the blood is thoroughly mixed with the air. Compare with a portion of the blood not mixed with the air, noting any difference in color. What substance in the air has acted on the blood to change its color?

3. Fill three tumblers each two thirds full of water and set them in a warm place. Pour into one of the tumblers, and thoroughly mix with the water, two tablespoonfuls of the blood containing the Epsom salts. After an interval of half an hour add blood to the second tumbler in the[pg 038] same manner, and after another half hour add blood to the third. The water dilutes the salts so that coagulation is no longer prevented. Jar the vessel occasionally as coagulation proceeds; and if the clot is slow in forming, add a trace of some salt of calcium (calcium chloride). After the blood has been added to the last tumbler make a comparative study of all. Note that coagulation begins in all parts of the liquid at the same time and that, as the process goes on, the clot shrinks and is drawn toward the center.

4. Place a clot from one of the tumblers in experiment 3 in a large vessel of water. Thoroughly wash, adding fresh water, until a white, stringy solid remains. This substance is fibrin.

5. Examine the coagulated blood obtained from the butcher (bottle 2). Observe the dark central mass (the clot) surrounded by a clear liquid (the serum). Sketch the vessel and its contents, showing and naming the parts into which the blood separates by coagulation.

To examine the Red Corpuscles.—Blood for this purpose is easily obtained from the finger. With a handkerchief, wrap one of the fingers of the left hand from the knuckle down to the first joint. Bend this joint and give it a sharp prick with the point of a sterilized 'needle just above the root of the nail. Pressure applied to the under side of the finger will force plenty of blood through a very small opening. (To prevent any possibility of blood poisoning the needle should be sterilized. This may be done by dipping it in alcohol or by holding it for an instant in a hot flame. It is well also to wash the finger with soap and water, or with alcohol, before the operation.) Place a small drop of the blood in the middle of a glass slide, protect the same with a cover glass, and examine with a compound microscope. At least two specimens should be examined, one of which should be diluted with a little saliva or a physiological salt solution.16 In the diluted specimen the red corpuscles appear as amber-colored, circular, disk-shaped bodies. In the undiluted specimen they show a decided tendency to arrange themselves in rows, resembling rows of coins. (Singly, the corpuscles do not appear red when highly magnified.)

A few white corpuscles may generally be found among the red ones in the undiluted specimen. These become separated by the formation[pg 039] of the red corpuscles into rows. They are easily recognized by their larger size and by their silvery appearance, due to the light shining through them.

To examine White Corpuscles.—Obtain from the butcher a small piece of the neck sweetbread of a calf. Press it between the fingers to squeeze out a whitish, semi-liquid substance. Dilute with physiological salt solution on a glass slide and examine with a compound microscope. Numerous white corpuscles of different kinds and sizes will be found. Make sketches.

To prepare Models of Red Corpuscles.—Several models of red corpuscles should be prepared for the use of the class. Clay and putty may be pressed into the form of red corpuscles and allowed to harden, and small models may be cut out of blackboard crayon. Excellent models can be molded from plaster of Paris as follows: Coat the inside of the lid of a baking powder can with oil or vaseline and fill it even full of a thick mixture of plaster of Paris and water. After the plaster has set, remove it from the lid and with a pocket-knife round off the edges and hollow out the sides until the general form of the corpuscle is obtained. The models may be colored red if it is desired to match the color as well as the form of the corpuscle.

Clyx.com