In man the blood constitutes about 5 per cent, of the total body weight, in rabbits about 5 per cent., in dogs 7 or 8 per cent., in rats 61/2 per cent., in guinea-pigs 4 per cent., and in mice 6 per cent. Young animals tend to have relatively rather more blood than old individuals of the same species. Under most circumstances the volume is kept almost constant, and it is difficult to produce any material, and especially any lasting, alteration in the volume by experimental procedures.
If one, for example, seeks to increase the volume of the circulating blood by the injection of salt solution, most of the added liquid is soon passed out of the vessels, partly by the urine and alimentary canal, partly into the tissues. This takes place very rapidly, even when the kidneys are excluded by ligation of the vessels. Thus, in dogs into whose veins is injected a volume of salt solution equal to half the volume of the blood, one-third to one-half of the injected liquid is found to have already left the bloodvessels during the ten or fifteen minutes occupied by the injection ; most of the rest soon follows, though a long time may elapse before the original volume is quite restored. The result is essentially the same whether hypotonic (O3 per cent.), isotonic (O8 per cent.), or hypertonic (2*6 per cent.) salt solution is injected.* The same readjustment of the blood-volume is found if the liquid injected into the vessels is the animal's own tissue fluid. This can be effected by injecting a large amount of concentrated solution of dextrose intravenously : the volume of the blood is at first much increased by fluid being dragged in from the tissues, but the great bulk of this is, along with the sugar, very soon passed out of the circulation again. Such solutions are, of course, useless for respiratory purposes, and only hamper the intake of oxygen by diluting the concentration of haemoglobin per unit volume of blood. This is, however, evidently not the reason why they are turned out of the blood ; for just the same result is obtained if for salt solution we substitute the fresh blood of another animal of the same species. Transfusion of blood in this way is immediately followed by a rise in the concentration of haemoglobin in the blood of the recipient, and after a few hours it is found that the volume of the blood is again reduced to the normal level. Be it noted that this involves the ejection from the circulation, not only of the injected plasma, but also of a volume of the animal's own plasma equal in bulk to the injected red corpuscles ; these remain inside the vessels. More striking still, perhaps, is the experiment in which the animal is transfused with its own normal blood (" plethora apocoptica "). This can be done by driving all the blood out of, e.g., the legs by firm centripetal bandaging. As a result, it is found that the specific gravity of the blood rises, while that of the extravascular tissues falls, indicating that fluid has passed from the blood to the tissues, and showing the activity of the mechanism whereby the volume of the blood is kept adjusted to the level appropriate to the capacity of the vessels. If, on the other hand, one essays to diminish the volume of the blood, similar difficulties are met with. The obvious experiment here is to withdraw blood from the circulation. This is immediately followed by the intaking of a corresponding bulk of liquid from the tissues. So rapidly does this occur that, in an animal bled to death from a divided carotid artery, the last portions of blood obtained may be definitely less concentrated than the first. If the haemorrhage is prolonged over so short a period as a quarter of an hour, the difference between the first and last portions is usually obvious. This rapid dilution of the blood during and after haemorrhage is the explanation of the apparently remarkable phenomenon that a person with 3 or 4 litres of blood may within a few hours lose by haemorrhage a bulk of blood found by measurement to be perhaps twice as great. The restoration of the volume of the blood after haemorrhage can only occur if a sufficient bulk of fluid can be spared from the extravascular tissues. In animals deprived of water it takes place more slowly than in those allowed to drink freely. Considerable variations are also found both among different individuals and between different species of animals. Rabbits, for example, generally restore the original volume of the blood within two or three hours after being bled to a third or a half of their total haemoglobin. Dogs may do so ; more commonly, however, the concentration of haemoglobin in their blood continues to fall progressively for some time after a severe haemorrhage, and the full restoration of volume may not be complete for several days. Much the same generally occurs in man. Now, in dogs exact experiment has shown that the volume is not restored much, if at all, sooner if, immediately after the haemorrhage, the blood removed is replaced by injecting an equal volume of salt solution into the veins. The salt solution is treated as an essentially foreign substance, and consequently ejected. There is, on the other hand, evidence to show that the restoration of volume in the dog is materially accelerated, and may, in fact, be as quick as in the well-watered rabbit, by placing a quantity of water in the stomach instead of directly into the circulation. This presumably affords a reservoir whence may be put together a plasma sufficiently normal to satisfy the economy. It has long been recognized in man that the immediate effects of a severe haemorrhage on the respiratory, cardiac and other functions may often be corrected by adding water to the body, either by direct intravenous injection or by introducing it under the skin or into the bowel. The results of experiment indieate that the latter methods are probably preferable, since the volume is then made up with plasma rather than with artificial solutions, and it is significant that the practice of direct intravenous injection has now to a considerable extent been replaced by the other methods. These considerations have some bearing on the question of the mechanism whereby the adjustment of the volume of the blood is effected. Considering that any increase of volume by addition of liquid is soon corrected, no matter what the nature of the injected fluid, and that the rate at which the excess leaves the vessels is very quick at first, and becomes slower and slower as the composition of the blood is reduced more nearly to the normal, one is led to suppose that the restoration of volume is due to the disturbed correlation between the volume of blood and the natural capacity of the vascular system. It is certain that the injection of considerable quantities of liquid into the circulation has no material lasting effect on arterial pressure. But there are grounds for believing that under these circumstances some rise of venous pressure, and hence some increase of capillary pressure, is brought about. This may be the immediate cause of fluid being expelled from the vessels. A considerable haemorrhage in the same way does not necessarily cause any but a transitory fall in arterial pressure. The vessels are, however, underfull, and the alteration of the relation between intravascular and extravascular tissue pressures may well result in fluid being sucked into the circulation. The experiments on bleeding and infusion already noted show, however, that these factors are not in all cases paramount, but may be overridden by considerations of the nature of the fluid available for the restoration of the volume. Alterations in the natural capacity of the vascular system bring about corresponding changes in the blood-volume. If the capacity is diminished, as in " plethora apocoptica," the volume is diminished. This can also be brought about by a general rise in arterial pressure (e.g., adrenalin). If the capacity is increased and the blood-pressure falls (e.g., section of the cord, shock), the concentration of the blood is diminished and the volume increased. It is evident, then, that the tissues of the body co-operate in seeing that the blood should be just the right volume to fill the vascular system to which it belongs. No doubt the heart and vessels work most economically if this condition is fulfilled. Otherwise it would appear to be better after a haemorrhage to leave the volume of the blood less than normal, and so keep up the concentration of haemoglobin. In this way the right amount of haemoglobin could be sent through the lungs and round the body every minute without it being necessary for the heart to pump a larger quantity, which it has to do if the volume is restored and the haemoglobin concentration consequently diminished. "We may surmise that, if the volume of blood is much less than normal, the heart's intake on the venous side cannot be properly effected. On the other hand, less trouble is made about the volume being too large. After the injection of salt solution or the animal's own plasma, the haemoglobin concentration is diminished, and there is no good reason why the total volume should not be brought back to normal. But after the injection of blood the haemoglobin concentration is increased under these circumstances the volume may remain above the normal level to avoid the difficulty of excessive viscosity. In thinking, therefore, of the regulation of the volume of the blood, the relative importance of at least three factors must be considered the relation to the volume of the vascular system, the concentration of the oxygen-carrying power and the viscosity, the normal of each one being that which fits best with the possibly conflicting interests of the other two. The capacity of the blood to carry other things than oxygen has probably so wide a margin of safety that it may be neglected.