BLOOD. The general principle on which the chemical life of the body is conducted is that each living cell carries out in its own substance all those chemical processes necessary to its ex istence. Therefore all the materials which it requires must be carried to it and all those which it discards must be removed. Throughout the whole body therefore a system of transport is necessary, with which every living cell is in intimate contact. That system, very primitive in the case of the more humble creatures, has become highly specialized in the vertebrate creation.
The principal materials which a living cell, be it a muscle fibre, a nerve cell or a gland cell, requires are (1) sugar, (2) the basis of albuminous material, (3) fat, (4) vitamines, (5) oxygen, (6) salts, (7) hormones, (8) water. The blood picks these up where they may be had, in the lung or in the alimentary canal or else where, it being the province of the organs of digestion to reduce the solid constituents of the food to such a form that the blood can absorb them. The principal substances of which the cell must be rid are carbonic acid and simple soluble compounds of nitro gen—compounds of ammonia etc., or, in the case of the liver, urea.
In all the higher animals blood consists of a fluid, the plasma, in which are suspended corpuscles of various kinds adapted for special purposes.
This fluid is nearly but not quite colourless and is clear, unless a meal containing fat has recently been eaten, when the plasma is somewhat milky, because of the minute globules of fat which it transports. In anaemia also the plasma may be milky. The two materials dissolved in greatest quantity are albuminous sub stances (proteins) and common salt.
The general nature of plasma resembles that of raw egg white, diluted with a 0.9% solution of salt. In detail, however, plasma differs in every respect ; its composition is roughly as follows:— Water . . . . . . go% Proteins (Fibrinogen, paraglobulin, serum albumen) . . g o Salts o•g% Sugar, urea, uric acid, kreatin . . . . . . . traces Water is primarily present in order to dissolve the other sub stances and to give the blood a degree of fluidity sufficient to secure its easy propulsion through the minute capillaries.
Protein.—The chemical basis of all life is protein in solution. All the cells with which the blood is in intimate relation are made of it. The protein cannot get in or out of these cells, and among its properties is that of attracting water to itself. If therefore there were proteins in the cells and not in the blood, water would always be passing from the blood into the cells, till the latter became dropsical. The protein in the blood balances that in the cells and so the water equilibrium is maintained. The case is different with salts; they can pass to and fro with the water and therefore they do not set up any permanent stream of water in any direction.
A second purpose of one protein at least, fibrinogen, is to con fer on blood the power of clotting. The clot is the first aid to the healing of a wound; it at once plugs the wound and forms a scaffold on which new tissue is built. Thus if the chin be cut in shaving, the solidification of the blood is not due to drying or exposure but to a chemical process in the plasma which causes the fibrinogen (hitherto in solution) to separate out as a solid sponge of fibrin connecting the edges of the wound and through which the corpuscles of the blood cannot pass. The blood of some persons does not clot readily. Such are called "haemophils." For these, wounds, even such as those occasioned by the pulling of teeth, are very serious on account of the difficulty of stopping the bleed ing. Haemophilia is a hereditary complaint (see HEREDITY). It does not appear in women and is never transmitted from one gen eration to the next through men. Thus, if in the following family tree M stands for man and W for woman, those persons only marked with a star could be haemophilic:— If therefore and had bred only boys, the haemophilic taint would have disappeared. What precisely occurs during the formation of the clot has been the matter of endless research. Microphotographs show a sort of net of material forms which is rendered obvious by bright illumination, as is the mote in the sunbeam. This net is of fibrin. Fibrin is the result of some trans formation of the fibrinogen into solid form. The transformation is wrought by minute quantities of a substance, fibrin ferment or thrombin, which is scarcely present at all in circulating blood but which is formed when the blood is injured or is in contact with injured tissue.
From the chemist's point of view the protein molecule consists of a great number of organic acids held together in a particular way by a link of which ammonia is the basis. When digestion takes place the protein is broken in the alimentary canal into the individual amino-acids. These form molecules small enough to pierce the walls of the capillary vessels surrounding the intestine and so they are picked up by the blood and conveyed to the tissues. If the protein molecule of a cell in one of the tissues has lost a particular amino-acid, the appropriate one can be acquired from the assortment which the blood carries. Sim ilarly, when the protein molecule of a cell loses an amino-acid, the lost matter does not appear in the blood as such, but as a salt of ammonia. It is carried as such to the liver, where it is turned into urea and thrust back into the blood. Hence the presence of ammonium salts and urea in the plasma.
Salts.—Sodium chloride in blood serves primarily to dissolve the protein. Two out of the three proteins in blood are insoluble in pure water. Protein can only form the basis of living material if it is in solution. Many salts might serve equally well to dissolve the protein but they are not innocuous in other respects. Potas sium chloride would stop the heart, ammonium chloride would cause convulsions and so on.
The body is capable of regulating the percentage of sodium chloride in the blood with great accuracy. If it is not eaten in sufficient quantity the kidney ceases to excrete it, and the urine, which normally contains a considerable quantity, is almost free from it. The result of a deficiency of sodium chloride may be felt by persons who carry on sustained and heavy work in very hot surroundings (deep mines or ships' furnaces). Such may perspire 18 pounds of sweat (which is practically sodium chloride solution) in a few hours. By drinking water they make up the deficiency of fluid but not of salt, and the percentage of sodium chloride in the blood sinks, which causes agonizing cramp in their abdominal muscles. These cramps do not occur if the loss is made up by the drinking of salt and water or -even of beer.
Although the elements calcium, potassium and magnesium are present in much smaller quantities than sodium, it is not to be supposed that they are correspondingly valueless. Especially it appears that the proper working of the heart depends upon the balance of calcium and potassium. Calcium also has a special relation to the clotting of blood, which does not take place in its absence. Among the salts in plasma, sodium bicarbonate holds a special place (1) because of its peculiar relation to the transport of carbonic acid from the tissues in which that gas is produced to the lungs and (2) because of its intimate connection with the fact that the blood is, and is maintained within very narrow limits, just on the alkaline side of absolute neutrality. The concentration of hydrogen ions in normal plasma equals o•4X 1 grams of hydro gen per litre (pH=7.4). As any considerable increase in this causes laboured respiration, the increase is checked automatically, for the laboured respiration augments the volume of carbonic acid expelled from the plasma into the air, so tending to reduce the concentration of hydrogen ions in the plasma. (See RESPIRATION.) On the other hand, should the blood become unduly alkaline, the kidney will redress the reaction by secreting an increased quantity of alkali in the urine. This balance of acid and alkali, the main tenance of which preserves the reaction of the blood at pH of 7.4, is called the "acid-base equilibrium" of the blood. To give a single instance of the way in which it is maintained : when a considerable meal is eaten acid is secreted in the stomach for the purpose of effecting its digestion. This acid is withdrawn from the blood which in itself would make the blood more alkaline, the kidney then secretes sodium ; hence the so-called alkaline tide in the urine.
Among fluids blood is remarkable in that addition of acid and alkali produce only a trifling alteration in the hydrogen ion con centration as compared with what they would in water if added to it. This fact is expressed by the phrase "blood is very highly buff ered." The buffering of blood is achieved principally by the interaction of the sodium bicarbonate in the plasma and the haemoglobin in the corpuscles. If carbonic acid is added to the plasma, sodium is withdrawn from the sodium chloride liberating chlorine ions; these tend to pass into the corpuscles which are very permeable to them and there unite with some of the sodium which had hitherto been in combination with the haemoglobin.
(See HORMONES and ENDOCRINOLOGY.)