Blood

Blood, the viscid red fluid which circulates through the heart, arteries, capillaries, and veins of the body. It ministers to the wants of the several animal tissues, which all draw upon it according to their needs; it takes up oxygen from the lungs and nutrient materials from the capillaries of the alimentary canal, and it receives the contents of the thoracic duct (q.v.) and the lymph of the right lymphatic duct. Again, from the blood the secreting glands elaborate their several secretions, and the kidney, lungs, and cutaneous glands remove certain excretory substances. Thus each portion of the animal body takes up nutrient material from the blood and discharges waste products into it, and the maintenance of the circulation ensures the distribution of suitable nourishment to all the tissues, and the final elimination from the body of such substances as are of no further use in the animal economy. In spite of this continual interchange of such varied materials between the blood and the tissues the composition of the blood remains singularly uniform. In the matter of oxygen, it is true, there is a noticeable difference. The bright red arterial blood coming from the lungs is in striking contrast to the bluish red venous blood which has given up a part of its oxygen to the tissues. But while the carrying of oxygen forms the most obvious and most important function of the blood, it must not be forgotten that the debtor and creditor account of the circulating fluid is concerned with innumerable other substances; and it is not a little remarkable that the chemical composition of the blood should remain so constant in spite of variations in diet, climate, habit of life, and other external conditions. It must be borne in mind that blood is not the only fluid which circulates in vessels within animal bodies. The lymph (q.v.) also plays an important part in transferring the products of tissue change from place to place. The lower we descend in the animal scale the more insignificant becomes the part played by circulating fluids; and, indeed, among invertebrates there are but few types in which there exists a fluid corresponding to the blood of backboned animals. In all the vertebrata, however, a circulating medium exists which is made up of two parts: first, the plasma or liquor sanguinis, and secondly, the blood corpuscles. The plasma is well-nigh colourless; it is a faintly alkaline fluid containing certain albuminous substances, fats, extractive bodies, and mineral salts. The corpuscles are of two kinds, the red corpuscles and the white corpuscles or lencocytes. The former are much more numerous than the latter; roughly speaking, about 400 red corpuscles are found for every white corpuscle in human blood, but this ratio is by no means a constant one; considerable deviations from it are met with at times in healthy persons. After a meal, in particular, the white corpuscles are found to be present in greater numbers. When, however, the lencocytes are so numerous as to nearly equal in quantity the red corpuscles present, the blood is diseased, and is said to be leucocythaemic.

It has been estimated that a cubic millimetre of human blood contains on an average 5,000,000 red corpuscles. In anaemia (q.v.) the number present is much less than this. The red corpuscle of human blood is a circular, biconclave disc measuring 1/3000 to 1/4000 in. in diameter and about 1/12000 in. in thickness. It is made up of a colourless elastic framework or stroma, the substance of which is infiltrated with the remarkable colouring matter called haemoglobin (q.v.). An individual corpuscle seen under the microscope is of a pale yellowish or straw colour; when, however, light passes through plasma containing large numbers of corpuscles, i.e. when several layers of these pale yellow bodies are traversed by the light before it reaches the eye, the deep red colour which we ordinarily associate with blood appears. It is noteworthy that the limitation of the haemoglobin to the stroma of the blood corpuscles explains the opacity of blood. For the light is scattered by the multitude of minute coloured bodies which lie in the colourless plasma, If the haemoglobin be diffused uniformly throughout the substance of the blood, instead of remaining confined to the corpuscles, a much more transparent fluid results. Blood in which this change has been effected is called "laky." Shaking with alcohol or ether and alternate freezing and thawing reduce blood to this "laky" condition.

The human red corpuscle possesses no nucleus. Speaking generally of the five groups into which backboned animals are divided, four, viz. fish, reptiles, amphibia, and birds, have nucleated red corpuscles; in the highest group, mammals, no nucleus is present. Moreover, while in mammals, with the exception of the camel tribe, the red corpuscles are circular, in the other four groups they are oval discs. In a drop of blood viewed under the microscope the coloured corpuscles usually adhere together, like coins piled one on another, in little heaps, which are called rouleaux. The addition of saline solution to human blood makes the discs swell up, producing the horse chestnut-shaped or "crenate" condition.

Red corpuscles are formed in the red marrow of bones, and perhaps in the liver and spleen; their term of existence is a limited one; after a time they are destroyed, mainly, it is supposed, in the spleen.

The white corpuscle is a nucleated cell; its protoplasm is possessed of that form of mobility which is known as "amoeboid," and its shape is consequently continually changing. In size it is a little larger, as a rule, than a red corpuscle.

The great constituent of the coloured corpuscles is haemoglobin (q.v.), and their chief function is to carry oxygen. The functions of the white corpuscles are less clearly understood; probably they play an important part in coagulation, and their number is largely increased in inflammatory conditions; indeed, many theories have been put forward with respect to the influence of leucocytes in disease processes.

Coagulation. Living blood, it has been said, consists of plasma and corpuscles; on removal from the body, however, an important change occurs in it. A new body, fibrin, appears as a network of delicate fibres, which entangle the corpuscles and hold them as in a meshwork; and thus a jelly-like, semi-solid substance is formed, the crassamentum or clot, and the blood is said to have coagulated. The fluid in which the clot floats is called serum; thus while living blood consists of plasma and corpuscles, clotted blood is made up of serum and clot, This coagulation is of the first importance in the prevention of bleeding from injured vessels; were it not for this remarkable phenomenon the slightest scratch or surface abrasion would be attended with most serious consequences. Again, clot formation plays a part in certain diseases. [Phlebitis, Aneurism.] Many attempts have been made to explain how coagulation comes about. The modern view is that there exists in the plasma a complex substance allied to albumen, which is the antecedent of the fibrin, and that under certain circumstances this fibrin generator, "fibrinogen," as it is called, is converted by the agency of another substance, the fibrin ferment, into fibrin. Coagulation may be delayed by cold, by exclusion of air, by contact with living tissues, by addition of solutions of neutral salts, and by introducing certain substances into the circulation before the blood is shed, ft is hastened by access of air, moderate warmth, and contact with foreign substances.

Tests for blood, (i) Microscopic examination of suspected fluids with a view to detecting the presence of corpuscles, (ii) Guaiacum reaction. A few drops of freshly prepared tincture of guaiacum are shaken up with the solution to be tested, and some ozonic ether added; the latter floats at the top, and at the line of junction of the lighter and heavier fluids a blue ring appears if blood be present, (iii) Formation of haemin crystals. (iv) Spectroscopic test.