Blood fresh from the lungs, whether still in the pulmonary veins or in the systemic arteries, is scarlet in colour. Venous blood is darker and purple-red, the depth of its tint varying with the extent to which it has parted with its oxygen. It looks less opaque than arterial blood. With this exception, the physical properties and chemical composition of blood are remarkably constant in all parts of the body. Arterial blood contains more oxygen, venous blood more carbonic acid. Other chemical differences can be recognized, but they are relatively very small. The constancy in the constitution of blood is its most notable character. Bleeding, unless excessive, does not greatly affect it. The number of corpuscles is of course diminished, but even these are replaced with great rapidity. The plasma, after bleeding, soon recovers its proteins and salts. A similar readjustment occurs if normal saline solution (water containing 0·9 per cent. sodic chloride), or even a strong solution of salt, is injected into the blood. Within certain limits it is very difficult to disturb the balance of its constituents. It gets rid of substances added in excess, or replaces substances removed, with remarkable facility. If sugar (glucose) be injected into a vein, it escapes through the capillary walls into the lymph. After a short interval the lymph contains more sugar than the blood. If an excess of protein, whether of a kind foreign to the blood or its own serum-albumin, be injected, it is removed by the kidneys. The blood has various sources from which it can draw out reserves of anything that is lacking, and various ways of getting rid of anything that is in excess. It draws upon the lymph in the tissue-spaces for water. It discharges salts into the lymph. It also takes salts from the lymph. It draws upon the liver for sugar, and probably for proteins also. In a starving animal the blood still contains sugar long after fresh supplies have ceased to reach it from the intestines. The lungs remove its carbonic acid. The kidneys free it from everything which cannot be otherwise removed. It is essential to the well-being of the organism as a whole that a uniform standard of composition should be maintained by the blood.
Fig. 4.—Red Blood-Corpuscles presenting, some the Surfaces, others the Edges, of their Discs, together with Single Representatives of Four Types of Leucocyte.
A, the most common type, highly amœboid and phagocytic. Its protoplasm is finely granular, its nucleus multipartite. B, a leucocyte closely similar to the last, but larger, and containing an undivided nucleus. It is shown with a cluster of particles of soot in its body-substance. C, a young leucocyte, or “lymphocyte.” D, a coarsely granular leucocyte. Its granules stain brightly with acid dyes—e.g., eosin or acid fuchsin.
Composition.—The structural composition of the blood, and the relation of its several constituents to each other, is best studied under the microscope. A thin transparent membrane in which blood is circulating through small vessels—the web between the toes of a frog’s foot, the mesentery, the membrane of a bat’s ear—affords an opportunity of observing blood in circulation. In any of the smaller vessels, whether artery or vein, a column of red corpuscles is seen moving in the axis of the stream. This column is surrounded by a layer of clear plasma. Amongst the red corpuscles a few leucocytes may be detected floating placidly down the current. Others are seen in the peripheral layer of plasma, tending to creep along the wall of the vessel rather than submit to be moved forward, as passive objects, by the current. If an irritant be applied to the membrane, the vessels dilate; yet, notwithstanding their wider calibre, the current becomes slower. The red corpuscles mass together. Apparently their constitution is slightly altered by this commencing inflammation, in such a manner that they cease to be clean, independent discs which slide past each other like small boats on a river; they exhibit a tendency to stick one to another. In the capillary vessels leucocytes may now be observed, not merely creeping along the inner surface of the endothelium, but squeezing themselves between its scales; making their way out of the vessel into the tissue-spaces through which the vessel passes. Such an observation gives the clue to the functions of the several constituents of the blood. The red corpuscles carry oxygen in chemical combination with their colouring matter. From them it passes into solution in the plasma; from the plasma through the walls of the capillary vessels into lymph; the tissues take it from the lymph as they require it. As fast as it is removed from lymph it is renewed from plasma. Carbonic acid excreted by tissue cells is dissolved in lymph. From lymph it is transferred to plasma. The reception of carbonic acid by these fluids is not quite so simple as the transference of oxygen from blood to lymph. It is aided by the presence of alkaline carbonates which are always ready to form “acid” salts: not acid to litmus-paper—the blood is always alkaline—but containing more than one unit of acid to one of base. Sodic carbonate has the formula Na₂CO₃. With an additional molecule of carbonic acid it becomes Na₂CO₃CO₂(HO)—bicarbonate. When in solution it can hold still more carbonic acid. If carbonic acid were merely dissolved in lymph and plasma, it would be impossible for the blood to carry it away with sufficient rapidity; just as it would be impossible for blood to bring sufficient oxygen were it not for the colouring matter (hæmoglobin) which forms a temporary, easily divorced union with it. But from a physical point of view it comes to the same thing. As the tension of oxygen in plasma falls, it dissolves more from the hæmoglobin. When the tension of oxygen in lymph is less than its tension in plasma, the former borrows from the latter. If the tension of carbonic acid in lymph is higher than in blood, it passes to the blood. The rapidly circulating blood at frequent intervals traverses the lungs. The whole blood of the body is exposed to air in the lungs once every minute. Oxygen tension being higher in pulmonary air than in venous blood, this gas is taken up. Carbonic acid tension being higher in venous blood than in pulmonary air, this gas escapes. The plasma in the capillary vessels which traverse the tissues exchanges gases with the lymph with very great rapidity.
The specific gravity of blood varies from 1·056 to 1·059. The corpuscles are heavier than the plasma. Its reaction to test-paper is alkaline, owing to the presence of bicarbonate of soda and disodic phosphate. The alkalinity is greatest when the body is at rest; it is diminished by severe muscular exercise. Blood contains about 5,000,000 red corpuscles, and 7,000 or 8,000 leucocytes, to a cubic millimetre. Red blood-corpuscles are biconcave discs destitute of nucleus, and, so far as can be seen, devoid of any investing membrane. Seen in profile they appear biscuit-shaped, because the centre is hollowed out. Their largest diameter is 7·5 micromillimetres (¹/₃₂₀₀ inch)—a measurement of great importance to anyone who works with a microscope, because it serves as a standard by which to estimate the size of other objects. They are soft, but fairly tough and highly elastic. In circulating blood a corpuscle may occasionally be seen to catch on the point where two capillary vessels unite. It bends almost double under the pressure of the column of corpuscles behind it, and then springs forward.
A red corpuscle is a vehicle for hæmoglobin. If blood is diluted with water, or if it is alternately frozen and thawed, the hæmoglobin separates from the corpuscles, which can then be seen as colourless discs. Hæmoglobin constitutes 40 per cent. of the weight of a moist corpuscle, or 95 per cent. of its weight after it has been dried. This is an enormous charge for a corpuscle to carry, and the question of how it carries it has been much discussed. It is not in a crystalline state. A corpuscle examined by polarized light is not doubly refractive. Microscopists know that if there were any crystals in the corpuscle it would appear bright on a dark ground when the Nicholl prisms are crossed. It cannot be in solution, since the water which the corpuscle contains would not suffice to dissolve it. It must be combined with some constituent of the corpuscle. But whether it is uniformly distributed throughout the disc, or in a semifluid form enclosed in spaces in a sponge-work; or whether the corpuscle is a hollow vesicle enclosing fluid hæmoglobin—a view which was long ago maintained, and has recently been revived—are questions which still await further evidence.
Red blood-corpuscles, properly so called, are found only in vertebrate animals, although invertebrate animals, from worms upwards, possess genuine blood, and in some of them it contains hæmoglobin, or a similar pigment in the form of globules. These might be likened to the non-nucleated corpuscles of mammals, but it must be remembered that the non-nucleated cells of mammals have been evolved from the nucleated blood-corpuscles of birds, reptiles, amphibians, and fishes. Below fishes red blood-cells are not found. Hæmoglobin is usually dissolved in the blood of invertebrate animals. It is impossible to trace any relationship between the coloured globules of invertebrates and the blood-cells of fishes. The coloured globules must be regarded as deposits or accretions of hæmoglobin held together by a proteid substance.
The nucleated red corpuscles of submammalian vertebrates multiply by cell division while circulating in the blood-stream. A good subject in which to look for dividing corpuscles is the blood of a newt in spring-time, when rapidly increasing activity calls for an additional supply. There is nothing to distinguish the method of division of a nucleated blood-corpuscle from that of any other cell.