BLOOD, the bright red to very dark red alkaline fluid that circulates through the heart, arteries and veins, carrying nutriment and oxy gen to the bodily tissues, and conveying away from them waste material. The composition and character of the blood varies very .widely in different animals, and hence this description is confined more particularly to the human blood. From the standpoint of cell-structure the blood is a tissue made up of a liquid plasma and solid cells or corpuscles. It contains at least four separate and important ingredients, the plasma, or blood serum; red cells, or erythrocytes; white cells, or leucocytes; and blood plates. one-tenth to one-twelfth of the entire body is blood, of which nine tenths is water.
Plasma.— The Treater portion (56 per cent) of the blood is plasma. This plasma is composed of 90 per cent water containing gases, mineral salts, fats, nitrogenous bodies and carbohydrates in solution. It is a clear yellowish fluid. The mineral salts are sodium chloride, common salt, the most abundant; sodium carbonate, which renders the blood alkaline; potassium chloride, potassium sul phate, calcium phosphate, sodium phosphate, magnesium phosphate and calcium chloride. Traces of other inorganic salts are fre quently found. The gases in the blood plasma are oxygen, nitrogen and carbon dioxide. Of the organic constituents the non-nitrogenous ones are the fats and carbohydrates, with small amounts of fatty coloring matters, lipochromes, cholesterin and sarcolactic acid. The fats are present in variable quantities, being particularly abundant following a meal. They are the glycerides of steanc, oleic and palmitic acids. The carbohydrates are at least three, glycogen, dextrose, or grape sugar, and animal gums. The non-protend nitrogenous constituents of the plasma consist largely of the waste extrac tives. The most important of these are urea, kreatin, kreatinin, uric acid and hippuric acid. Three ferments or enzymes are thought to be present in the plasma — a diastatic ferment, converting starches into sugars; a glycolytic enzyme, breaking up sugar, and a lipase, or fat splitting enzyme. In addition, there is the fer ment that causes coagulation. Whether this is present in the serum or in the white cells is a matter of inquiry. The proteids of the plasma are serum albumins, globulins (serum globulin and fihrinoaen) and nucleo-proteids.
Red Cells.— These are the most abundant of the formed elements of the blood, making up 99 per cent of the corpuscles. There are thought to be in man as least 5,000,000 red blood-cells to every cubic millimetre of blood; their size, therefore, is very small, averaging in man 7.8 m.mm. They are flattened circular discs, with double depressed centres, one-fourth as thick as broad. In the embryo and in cer tain diseased states the red blood-cells have a nucleus, but the normal red blood-cell in man has lost this cell structure. Practically all of the mammals, save the camel tribe, have circu lar red blood-cells; the camels and most of the lower animals have oval red blood-cells; in the lower animals they are mostly nucleated. There is also great variation in size in the red cells of the various animals, being largest among the amphibia (Amphiuma 75 m.mm.).
The red blood-cells are mostly manufactured in the marrow of the long bones. The chem ical structure of the red cells is complex, but they contain an iron compound, hamoglobin, which is the most important constituent of the blood in the process of respiration and oxidation; by it the complex processes of chem ical interchange in the body (metabolism) are made possible. Poisoning of the hamoglobin and the loss of its function means death by asphyxia. The hamoglobin gives the reddish yellow tinge to the blood, and the differences in shade between the venous blood and arterial blood are due to the state of oxidation of the hamoglobin.
White Cells — Lencocytes.— These are much less numerous than the red cells, varying in number from 5,000 to 20,000 to the cubic millimetre. At least five different forms of white cells are normally present in human blood. These are large and small lympho cytes, polymorpho-nuclear neutrophiles, eosi nophiles and transitional forms. Mast cells are another form of varying occurrence. The polymorph neutrophiles are the most numer ous of the leucocytes and make up the greater mass in pus-cells. In shape and size these white cells differ, but all are spherical, some smaller than the red cells (6.7 m.mm.) but mostly larger (about 10 m.mm.), and all have one or more nuclei. The leucocytes are formed in a number of lymphatic tissues, the hamo lymph glands, the spleen, etc., and are among the most interesting of the constituents of the blood, since one of their chief functions is to protect the body from disease-producing micro organisms. They may be aptly termed the human body's of the interior)) in the fight with disease-causing agents. They are useful both physically (by eating, as it were, the bodies of invading bacteria — phagocytosis, q.v.) or chemically (in the elaborating of cer tain counter-poisons — antitoxins, q.v.) or in the manufacture of specific immunizing bodies for the blood-serum. (See Immuzirry). Their careful study in diseased conditions is very helpful in arriving at a diagnosis of the disease process.
Blood These are of frequent oc currence, but as yet little is known of their function. They are thought to be globulin like in their nature and of use in the phenom enon of coagulation; others claim them as nucleo-proteids, made from the white , blood cells.
Functions of the Blood.—These, as al ready indicated, are numerous. Through the haemoglobin, blood is the great oxidizing me dium. It carries products for anabolism and products of katabolism, and is the great equal izer, by arterial pressure, of the osmotic pres sures of all the cells of the body. As a means of defense in the struggle with parasitic in vaders the blood is the most important of the body's bulwarks. See ANEMIA; BLEEDING; CHLOROSIS ; CIRCULATION; HEMOPHILIA; LEU CIEMIA ; PERNICIOUS ANEMIA.
Bibliography.— Bayliss