A Civic Biology, Presented in Problems

by George W. Hunter

XXI. THE BLOOD AND ITS CIRCULATION

Problems.—To discover the composition and uses of the different parts of the blood.

To find out the means by which the blood is circulated about the body.

Laboratory Suggestions

Demonstration.—Structure of blood, fresh frog's blood and human blood. Drawings.

Demonstration.—Clotting of blood.

Demonstration.—Use of models to demonstrate that the heart is a force pump.

Demonstration.—Capillary circulation in web of frog's foot or tadpole's tail. Drawing.

Home or laboratory exercise.—On relation of exercise on rate of heart beat.

Function of the Blood.—The chief function of the digestive tract is to change foods to such form that they can be absorbed through the walls of the food tube and become part of the blood.[45]

If we examine under the microscope a drop of blood taken from the frog or man, we find it made up of a fluid called plasma and two kinds of bodies, the so-called red corpuscles and colorless corpuscles, floating in this plasma.

Composition of Plasma.—The plasma of blood is found to be largely (about 90 per cent) water. It also contains a considerable amount of protein, some sugar, fat, and mineral material. It is, then, the medium which holds the fluid food that has been absorbed from within the intestine. This food is pumped to the body cells where, as work is performed, oxidation takes place and heat is given off as a form of energy. The almost constant temperature of the body is also due to the blood, which brings to the surface of the body much of the heat given off by oxidation of food in the muscles and other tissues. When the blood returns from the tissues where the food is oxidized, the plasma brings back with it to the lungs part of the carbon dioxide liberated where oxidation has taken place. Some waste products, to be spoken of later, are also found in the plasma.

Human blood as seen under the high power of the compound microscope; at the extreme right is a colorless corpuscle.

The Red Blood Corpuscle; its Structure and Functions.—The red corpuscle in the blood of the frog is a true cell of disklike form, containing a nucleus. The red corpuscle of man is made in the red marrow of bones and in its young stages has a nucleus. In its adult form, however, it lacks a nucleus. Its form is that of a biconcave disk. So small and so numerous are these corpuscles that over five million are found in a cubic centimeter of normal blood. They make up almost one half the total volume of the blood. The color, which is found to be a dirty yellow when separate corpuscles are viewed under the microscope, is due to a protein material called hæmoglobin. Hæmoglobin contains a large amount of iron. It has the power of uniting very readily with oxygen whenever that gas is abundant, and, after having absorbed it, of giving it up to the surrounding media, when oxygen is there present in smaller amounts than in the corpuscle. This function of carrying oxygen is the most important function of the red corpuscle, although the red corpuscle also removes part of the carbon dioxide from the tissues on their return to the lungs. The taking up of oxygen is accompanied by a change in color of the mass of corpuscles from a dull red to a bright scarlet.

Clotting of Blood.—If fresh beef blood is allowed to stand overnight, it will be found to have separated into two parts, a dark red, almost solid clot and a thin, straw-colored liquid called serum. Serum is found to be made up of about 90 per cent water, 8 per cent protein, 1 per cent other organic foods, and 1 per cent mineral substances. In these respects it very closely resembles the fluid food that is absorbed from the intestines.

If another jar of fresh beef blood is poured into a pan and briskly whipped with a bundle of little rods (or with an egg beater), a stringy substance will be found to stick to the rods. This, if washed carefully, is seen to be almost colorless. Tested with nitric acid and ammonia, it is found to contain a protein substance which is called fibrin.

Blood plasma, then, is made up of a fluid portion of serum, and fibrin, which, although in a fluid state in the blood vessels within the body, coagulates when blood is removed from the blood vessels. This coagulation aids in making a blood clot. A clot is simply a mass of fibrin threads with a large number of corpuscles tangled within. The clotting of blood is of great physiological importance, for otherwise we might bleed to death even from a small wound.

A small artery (A) breaking up into capillaries (c) which unite to form a vein (V). Note at (P) several colorless corpuscles, which are fighting bacteria at that point.

Blood Plates.—In blood within the circulatory system of the body, the fibrin is held in a fluid state called fibrinogen. An enzyme, acting upon this fibrinogen, the soluble protein in the blood, causes it to change to an insoluble form, the fibrin of the clot. This change seems to be due to the action of minute bodies in the blood known as blood plates. Under abnormal conditions these blood plates break down, releasing some substances which eventually cause this enzyme to do its work.

The Colorless Corpuscle; Structure and Functions.—A colorless corpuscle is a cell irregular in outline, the shape of which is constantly changing. These corpuscles are somewhat larger than the red corpuscles, but less numerous, there being about one colorless corpuscle to every three hundred red ones. They have the power of movement, for they are found not only inside but outside the blood vessels, showing that they have worked their way between the cells that form the walls of the blood tubes.

A colorless corpuscle catching and eating germs.

A Russian zoölogist, Metchnikoff, after studying a number of simple animals, such as medusæ and sponges, found that in such animals some of the cells lining the inside of the food cavity take up or engulf minute bits of food. Later, this food is changed into the protoplasm of the cell. Metchnikoff believed that the colorless corpuscles of the blood have somewhat the same function. This he later proved to be true. Like the amœba, they feed by engulfing their prey. This fact has a very important bearing on the relation of colorless corpuscles to certain diseases caused by bacteria within the body. If, for example, a cut becomes infected by bacteria, inflammation may set in. Colorless corpuscles at once surround the spot and attack the bacteria which cause the inflammation. If the bacteria are few in number, they are quickly eaten by certain of the colorless corpuscles, which are known as phagocytes. If bacteria are present in great quantities, they may prevail and kill the phagocytes by poisoning them. The dead bodies of the phagocytes thus killed are found in the pus, or matter, which accumulates in infected wounds. In such an event, we must come to the aid of nature by washing the wound with some antiseptic, as weak carbolic acid or hydrogen peroxide.

Antibodies and their Uses.—In case of disease where, for example, fever is caused by poison given off from bacteria we find the cells of the body manufacture and pour into the blood a substance known as an antibody. This substance does not of necessity kill the harmful germs or even stop their growth. It does, however, unite with the toxin or poison given off by the germs and renders it entirely harmless.

Function of Lymph.—The tissues and organs of the body are traversed by a network of tubes which carry the blood. Inside these tubes is the blood proper, consisting of a fluid plasma, the colorless corpuscles, and the red corpuscles. Outside the blood tubes, in spaces between the cells which form tissues, is found another fluid, which is in chemical composition very much like plasma of the blood. This is the lymph. It is, in fact, fluid food in which some colorless amœboid corpuscles are found. Blood gives up its food material to the lymph. This it does by passing it through the walls of the capillaries. The food is in turn given up to the tissue cells, which are bathed by the lymph.

The exchange between blood and the cells of the body.

Some of the amœboid corpuscles from the blood make their way between the cells forming the walls of the capillaries. Lymph, then, is practically blood plasma plus some colorless corpuscles. It acts as the medium of exchange between the blood proper and the cells in the tissues of the body. By means of the food supply thus brought, the cells of the body are able to grow, the fluid food being changed to the protoplasm of the cells. By means of the oxygen passed over by the lymph, oxidation may take place within the cells. Lymph not only gives food to the cells of the body, but also takes away carbon dioxide and other waste materials, which are ultimately passed out of the body by means of the lungs, skin, and kidneys.

Internal Secretions.—In addition to all the functions given above, the blood has recently been shown to carry the secretions of a number of glands through which it passes, although these glands have no ducts to carry off their secretions. These internal secretions seem absolutely necessary for the health of the body. Several glands, the thyroid, adrenal bodies, the testes, and ovaries, as well as the pancreas, give off these remarkable substances.

The Amount of Blood and its Distribution.—Blood forms, by weight, about one sixteenth of the body. This would be about four quarts to a body weight of 130 pounds. Normally, about one half of the blood of the body is found in or near the organs lying in the body cavity below the diaphragm, about one fourth in the muscles, and the rest in the head, heart, lungs, large arteries, and veins.

Blood Temperature.—The temperature of blood in the human body is normally about 98.6° Fahrenheit when tested under the tongue by a thermometer, although the temperature drops almost two degrees after we have gone to sleep at night. It is highest about 5 P.M. and lowest about 4 A.M. In fevers, the temperature of the body sometimes rises to 107°; but unless this temperature is soon reduced, death follows. Any considerable drop in temperature below the normal also means death. Body heat results from the oxidation of food, and the circulation of blood keeps the temperature nearly uniform in all parts of the body.

Cold-blooded Animals.—In animals which are called cold-blooded, the blood has no fixed temperature, but varies with the temperature of the medium in which the animal lives. Frogs, in the summer, may sit for hours in water with a temperature of almost 100°. In winter, they often endure freezing so that the blood and lymph within the spaces under the loose skin are frozen into ice crystals. This change in body temperature is evidently an adaptation to the mode of life.

Circulation of the Blood in Man.—The blood is the carrying agent of the body. Like a railroad or express company, it takes materials from one part of the human organism to another. This it does by means of the organs of circulation,—the heart and blood vessels. These blood vessels are called arteries where they carry blood away from the heart, veins where they bring blood back to the heart, and capillaries where they connect the larger blood vessels. The organs of circulation thus form a system of connected tubes through which the blood flows.

The Heart; Position, Size, Protection.—The heart is a cone-shaped muscular organ about the size of a man's fist. It is located immediately above the diaphragm, and lies so that the muscular apex, which points downward, moves while beating against the fifth and sixth ribs, just a little to the left of the midline of the body. This fact gives rise to the notion that the heart is on the left side of the body. The heart is surrounded by a loose membranous bag called the pericardium, the inner lining of which secretes a fluid in which the heart lies. When, for any reason, the pericardial fluid is not secreted, inflammation arises in that region.

Diagram showing the front half of the heart cut away: a, aorta; l, arteries to the lungs; la, left auricle; lv, left ventricle; m, tricuspid valve open; n, bicuspid or mitral valve closed; p and r, veins from the lungs; ra, right auricle; rv, right ventricle; v, vena cava. Arrows show direction of circulation.

Internal Structure of Heart.—If we should cut open the heart of a mammal down the midline, we could divide it into a right and a left side, each of which would have no internal connection with the other. Each side is made up of an upper thin-walled portion with a rather large internal cavity, the auricle, which opens into a lower smaller portion with heavy muscular walls, the ventricle. Communication between auricles and ventricles is guarded by little flaps or valves. The auricles receive blood from the veins. The ventricles pump the blood into the arteries.

The Heart in Action.—The heart is constructed on the same plan as a force pump, the valves preventing the reflux of blood into the auricle when it is forced out of the ventricle. Blood enters the auricles from the veins because the muscles of that part of the heart relax; this allows the space within the auricles to fill. Almost immediately the muscles of the ventricles relax, thus allowing blood to pass into the chambers within the ventricles. Then, after a short pause, during which time the muscles of the heart are resting, a wave of muscular contraction begins in the auricles and ends in the ventricles, with a sudden strong contraction which forces the blood out into the arteries. Blood is kept on its course by the valves, which act in the same manner as do the valves in a pump. The blood is thus made to pass into the arteries upon the contraction of the ventricle walls.

The heart is a force pump; prove it from these diagrams.

The Course of the Blood in the Body.—Although the two sides of the heart are separate and distinct from each other, yet every drop of blood that passes through the right heart likewise passes later through the left heart. There are two distinct systems of circulation in the body. The pulmonary circulation takes the blood through the right auricle and ventricle, to the lungs, and passes it back to the left auricle. This is a relatively short circulation, the blood receiving in the lungs its supply of oxygen, and there giving up some of its carbon dioxide. The greater circulation is known as the systemic circulation; in this system, the blood leaves the left ventricle through the great dorsal aorta. A large part of the blood passes directly to the muscles; some of it goes to the nervous system, kidneys, skin, and other organs of the body. It gives up its supply of food and oxygen in these tissues, receives the waste products of oxidation while passing through the capillaries, and returns to the right auricle through two large vessels known as the venæ cavæ. It requires only from twenty to thirty seconds for the blood to make the complete circulation from the ventricle back again to the starting point. This means that the entire volume of blood in the human body passes three or four thousand times a day through the various organs of the body.[46]

I. Circulation in a fish. G, gills; C, capillaries of the body. Notice the two-chambered heart.

II. The circulation in a frog. L, the lungs; C, the capillaries. Notice the heart has three chambers. What is the condition of blood leaving the ventricle to go to the cells of the body?

III. The circulation in man. H, head; A, arms; L, lungs; S, stomach; Li, liver; K, kidney; S.I., small intestine; L.I., large intestine; Le, legs; 1, right auricle; 2, right ventricle; 3, left ventricle; 4, left auricle; 5, dorsal aorta; 6, vein to lungs.

Portal Circulation.—Some of the blood, on its way back to the heart, passes to the walls of the food tube and to its glands. From there it is sent with its load of absorbed food to the liver. Here the vein which carries the blood (called the portal vein) breaks up into capillaries around the cells of the liver, when it gives up sugar to be stored as glycogen. From the liver, blood passes directly to the right auricle. The portal circulation, as it is called, is the only part of the circulation where the blood passes through two sets of capillaries on its way from auricle to auricle.

Capillary circulation in the web of a frog's foot, as seen under the compound microscope. a, b, small veins; c, pigment cells in the skin; d, capillaries in which the oval corpuscles are seen to follow one another in single series.

Circulation in the Web of a Frog's Foot.—If the web of the foot of a live frog or the tail of a tadpole is examined under the compound microscope, a network of blood vessels will be seen. In some of the larger vessels the corpuscles are moving rapidly and in spurts; these are arteries. The arteries lead into smaller vessels hardly greater in diameter than the width of a single corpuscle. This network of capillaries may be followed into larger veins in which the blood moves regularly. This illustrates the condition in any tissue of man where the arteries break up into capillaries, and these in turn unite to form veins.

Structure of the Arteries.—A distinct difference in structure exists between the arteries and the veins in the human body. The arteries, because of the greater strain received from the blood which is pumped from the heart, have thicker muscular walls, and in addition are very elastic.

Cause of the Pulse.—The pulse, which can easily be detected by pressing the large artery in the wrist or the small one in front of and above the external ear, is caused by the gushing of blood through the arteries after each pulsation of the heart. As the large arteries pass away from the heart, the diameter of each individual artery becomes smaller. At the very end of their course, these arteries are so small as to be almost microscopic in size and are very numerous. There are so many that if they were placed together, side by side, their united diameter would be much greater than the diameter of the large artery (aorta) which passes blood from the left side of the heart. This fact is of very great importance, for the force of the blood as it gushes through the arteries becomes very much less when it reaches the smaller vessels. This gushing movement is quite lost when the capillaries are reached, first, because there is so much more space for the blood to fill, and second, because there is considerable friction caused by the very tiny diameter of the capillaries.

Capillaries.—The capillaries form a network of minute tubes everywhere in the body, but especially near the surface and in the lungs. It is through their walls that the food and oxygen pass to the tissues, and carbon dioxide is given up to the plasma. They form the connection that completes the system of circulation of blood in the body.

Function and Structure of the Veins.—If the arteries are supply pipes which convey fluid food to the tissues, then the veins may be likened to drain pipes which carry away waste material from the tissues. Extremely numerous in the extremities and in the muscles and among other tissues of the body, they, like the branches of a tree, become larger and unite with each other as they approach the heart.

Valves in a vein. Notice the thin walls of the vein.

If the wall of a vein is carefully examined, it will be found to be neither so thick nor so tough as an artery wall. When empty, a vein collapses; the wall of an artery holds its shape. If you hold your hand downward for a little time and then examine it, you will find that the veins, which are relatively much nearer the surface than are the arteries, appear to be very much knotted. This appearance is due to the presence of tiny valves within. These valves open in the direction of the blood current, but would close if the direction of the blood flow should be reversed (as in case a deep cut severed a vein). As the pressure of blood in the veins is much less than in the arteries, the valves thus aid in keeping the flow of blood in the veins toward the heart. The higher pressure in arteries and the suction in the veins (caused by the enlargement of the chest cavity in breathing) are the chief factors which cause a steady flow of blood through the veins in the body.

Lymph Vessels.—The lymph is collected from the various tissues of the body by means of a number of very thin-walled tubes, which are at first very tiny, but after repeated connection with other tubes ultimately unite to form large ducts. These lymph ducts are provided, like the veins, with valves. The pressure of the blood within the blood vessels forces continually more plasma into the lymph; thus a slow current is maintained. On its course the lymph passes through many collections of gland cells, the lymph glands. In these glands some impurities appear to be removed and colorless corpuscles made. The lymph ultimately passes into a large tube, the thoracic duct, which flows upward near the ventral side of the spinal column, and empties into the large subclavian vein in the left side of the neck. Another smaller lymph duct enters the right subclavian vein.

The lymph vessels; the dark spots are lymph glands: lac, lacteals; rc, thoracic duct.

The Lacteals.—We have already found that part of the digested food (chiefly carbohydrates, proteins, salts, and water) is absorbed directly into the blood through the walls of the villi and carried to the liver. Fat, however, is passed into the spaces in the central part of the villi, and from there into other spaces between the tissues, known as the lacteals. The lacteals carry the fats into the blood by way of the thoracic duct. The lacteals and lymph vessels have in part the same course. It will be thus seen that lymph at different parts of its course would have a very different composition.

The Nervous Control of the Heart and Blood Vessels.—Although the muscles of the heart contract and relax without our being able to stop them or force them to go faster, yet in cases of sudden fright, or after a sudden blow, the heart may stop beating for a short interval. This shows that the heart is under the control of the nervous system. Two sets of nerve fibers, both of which are connected with the central nervous system, pass to the heart. One set of fibers accelerates, the other slows or inhibits, the heart beat. The arteries and veins are also under the control of the sympathetic nervous system. This allows of a change in the diameter of the blood vessels. Thus, blushing is due to a sudden rush of blood to the surface of the body caused by an expansion of the blood vessels at the surface. The blood vessels of the body are always full of blood. This results from an automatic regulation of the diameter of the blood tubes by a part of the nervous system called the vasomotor nerves. These nerves act upon the muscles in the walls of the blood vessels. In this way, each vessel adapts itself to the amount of blood in it at a given time. After a hearty meal, a large supply of blood is needed in the walls of the stomach and intestines. At this time, the arteries going to this region are dilated so as to receive an extra supply. When the brain performs hard work, blood is supplied in the same manner to that region. Hence, one should not study or do mental work immediately after a hearty meal, for blood will be drawn away to the brain, leaving the digestive tract with an insufficient supply. Indigestion may follow as a result.

The Effect of Exercise on the Circulation.—It is a fact familiar to all that the heart beats more violently and quickly when we are doing hard work than when we are resting. Count your own pulse when sitting quietly, and then again after some brisk exercise in the gymnasium. Exercise in moderation is of undoubted value, because it sends the increased amount of blood to such parts of the body where increased oxidation has been taking place as the result of the exercise. The best forms of exercise are those which give as many muscles as possible work—walking, out-of-door sports, any exercise that is not violent. Exercise should not be attempted immediately after eating, as this causes a withdrawal of blood from the digestive tract to the muscles of the body. Neither should exercise be continued after becoming tired, as poisons are then formed in the muscles, which cause the feeling we call fatigue. Remember that extra work given to the heart by extreme exercise may injure it, causing possible trouble with the valves.

Stopping flow of blood from an artery by applying a tight bandage (ligature) between the cut and the heart.

Treatment of Cuts and Bruises.—Blood which oozes slowly from a cut will usually stop flowing by the natural means of the formation of a clot. A cut or bruise should, however, be washed in a weak solution of carbolic acid or some other antiseptic in order to prevent bacteria from obtaining a foothold on the exposed flesh. If blood, issuing from a wound, gushes in distinct pulsations, then we know that an artery has been severed. To prevent the flow of blood, a tight bandage known as a tourniquet must be tied between the cut and the heart. A handkerchief with a knot placed over the artery may stop bleeding if the cut is on one of the limbs. If this does not serve, then insert a stick in the handkerchief and twist it so as to make the pressure around the limb still greater. Thus we may close the artery until the doctor is called, who may sew up the injured blood vessel.

The Effect of Alcohol upon the Blood.—It has recently been discovered that alcohol has an extremely injurious effect upon the colorless corpuscles of the blood, lowering their ability to fight disease germs to a marked degree. This is well seen in a comparison of deaths from certain infectious diseases in drinkers and abstainers, the percentage of mortality being much greater in the former.

Dr. T. Alexander MacNichol, in a recent address, said:—

"Massart and Bordet, Metchnikoff and Sims Woodhead, have proved that alcohol, even in very dilute solution, prevents the white blood corpuscles from attacking invading germs, thus depriving the system of the coöperation of these important defenders, and reducing the powers of resisting disease. The experiments of Richardson, Harley, Kales, and others have demonstrated the fact that one to five per cent of alcohol in the blood of the living human body in a notable degree alters the appearance of the corpuscular elements, reduces the oxygen bearing elements, and prevents their reoxygenation."

Alcohol weakens Resistance to Disease.—In acute illnesses, grippe, fevers, blood poisoning, etc., substances formed in the blood termed "antibodies" antagonize the action of bacteria, facilitating their destruction by the white blood cells and neutralizing their poisonous influence. In a person with good "resistance" this protective machinery, which we do not yet thoroughly understand, works with beautiful precision, and the patient "gets well." Experiments by scientific experts have demonstrated that alcohol restrains the formation of these marvelous antibodies. Alcohol puts to sleep the sentinels that guard your body from disease.

The Effect of Alcohol on the Circulation.—Alcoholic drinks affect the very delicate adjustment of the nervous center's[TN7] controlling the blood vessels and heart. Even very dilute alcohol acts upon the muscles of the tiny blood vessels; consequently, more blood is allowed to enter them, and, as the small vessels are usually near the surface of the body, the habitual redness seen in the face of hard drinkers is the ultimate result.

"The first effect of diluted alcohol is to make the heart beat faster. This fills the small vessels near the surface. A feeling of warmth is produced which causes the drinker to feel that he was warmed by the drink. This feeling, however, soon passes away, and is succeeded by one of chilliness. The body temperature, at first raised by the rather rapid oxidation of the alcohol, is soon lowered by the increased radiation from the surface.

"The immediate stimulation to the heart's action soon passes away and, like other muscles, the muscles of the heart lose power and contract with less force after having been excited by alcohol."—Macy, Physiology.

Alcohol, when brought to act directly on heart muscle, lessens the force of the beat. It may even cause changes in the tissues, which eventually result in the breaking of the walls of a blood vessel or the plugging of a vessel with a blood clot. This condition may cause the disease known as apoplexy.

Effects of Tobacco upon the Circulation.—"The frequent use of cigars or cigarettes by the young seriously affects the quality of the blood. The red blood corpuscles are not fully developed and charged with their normal supply of life-giving oxygen. This causes paleness of the skin, often noticed in the face of the young smoker. Palpitation of the heart is also a common result, followed by permanent weakness, so that the whole system is enfeebled, and mental vigor is impaired as well as physical strength."—Macy, Physiology.

[45] This change is due to the action of certain enzymes upon the nutrients in various foods. But we also find that peptones are changed back again to proteins when once in the blood. This appears to be due to the reversible action of the enzymes acting upon them. (See page [307].)

[46] See Hough and Sedgwick, The Human Mechanism, page 136.