86 THE BLOOD AND THE LYMPH 



by comparing with a suitable standard mixture of dye and serum. These 

 methods give the total amount of blood in the body as from 5 to 8.8 per 

 cent of its weight. Meek has recently developed a method in which gum 

 acacia is used. After mixing with the blood, the concentration of this 

 substance is determined from the calcium content. Being colloid, none 

 of the gum leaves the blood vessels. 



The newer methods have shown that the volume of the circulating 

 fluid is maintained fairly constant in spite of influences tending to alter 

 it. The body accomplishes this by drawing upon the reserve fluid in 

 the tissues and b}^ varying the rate of water excretion, particularly 

 through the kidneys. Years ago the doctrine of an increased amount of 

 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. 



