BLOOD: ITS GENERAL PROPERTIES 87 



blood in the body (plethora) gave rise to the therapeutic use of bleeding. 

 Especially was this thought to be useful in conditions which we now 

 recognize as chronic hypertension, and which show no increase in blood 

 volume. Indeed variation in blood volume is not common, although 

 plethora may occur in polycythemia, chlorosis, and anemias, and there 

 may be a temporary reduction in the amount of blood in diseases in 

 which there is a great depletion of water, as in Asiatic cholera, and fol- 

 lowing very severe hemorrhage. 



While the total quantity of the blood in the body does not vary greatly, 

 the concentration of its various constituents is subject to distinct, change. 

 The volume percentages of the corpuscles and the plasma can be approx- 

 imately determined by allowing oxalated blood to sediment or by cen- 

 trifuging in a graduated cylinder by the use of the hematocrit. Such 

 methods are not very reliable, but may yield some important information. 

 Normally 45 to 50 per cent of the volume of blood is composed of cor- 

 puscles. It varies more or less directly with the number of red blood 

 cells. 



THE WATER CONTENT OF THE BLOOD 



Since the blood plasma is essentially a watery solution, some idea of 

 its water content can be obtained by a determination of the specific 

 gravity. The most accurate method for accomplishing this is to deter- 

 mine directly the weight of a given volume of blood and compare it 

 with the weight of the same volume of water. Since this method re- 

 quires a rather large amount of blood, indirect methods using smaller 

 amounts have been devised. One of these (Hammerschlag's) uses a 

 solution of chloroform and benzol of a specific gravity of about 1.050, 

 in which a drop of blood is suspended by delivering it cautiously from 

 a pipette bent at right angles near its tip. If the drop sinks, chloroform 

 is added; if it rises, benzol is added until the drop remains suspended. 

 The specific gravity of the benzol-chloroform mixture is then determined, 

 and this value is supposed to give the specific gravity of the blood. 



The specific gravity of the blood determined in this way varies be- 

 tween 1.040 and 1.065. It is somewhat less after eating and increases 

 after exercise; it is slightly lower during the day than at night, and 

 the variation in individuals is considerable. The changes which occur 

 in the specific gravity of the blood in disease are chiefly due to variation 

 in the percentage of protein, since the salt content of the blood is rela- 

 tively fixed. It is only when great changes occur in the concentration 

 of the noncolloidal salts that they markedly affect the specific gravity. 



From 90 to 92 per cent of the plasma and from 59.2 to 68.7 per cent of 

 the corpuscles consist of water. Of the whole blood, from 60 to 70 per cent 



