22 BLOOD 



fore - X V c.c. But we already know that 1 c.c.'of blood 



X 



combines with a c.c. of oxygen. 

 The volume of blood is therefore - 



''x V 



2. Vital-red Method 



Vital-red is a non-toxic dye, which on injection colours 

 the plasma, but does not to any extent affect the corpuscles, 



A known volume of dye, say 15 c.c, is injected into a 

 vein, a sample of blood being drawn before and after the 

 injection. Both samples are centrifugahsed, and the 

 plasma separated from the corpuscles. Two solutions 

 are now made up as follows — 



r 1 part plasma before injection of dye. 

 „, , , 1 part dye solution diluted 200 times with 

 Standard. I isotonic NaCl. 



2 parts isotonic NaCl. 



rp f / 1 part plasma after injection of dye. 



^ ^^^- \ 3 parts isotonic NaCl. 



The intensity of the coloration of the two solutions is 

 then compared, that of the test being expressed as a per- 

 centage of that of the standard. 



It is clear that if the two colours are of equal strength, 

 the total volume of plasma must be 15 X 200 c.c. = 3 htres. 



If K. is the percentage reading of the test solution, the 



volume of plasma — -^ x No. of c.c. of dye injected x 



200. 



From the volume of plasma, the volume of the blood is 

 obtained by means of the hsematocrit. 



Prior to the discovery of these methods, the only estima- 

 tions of the amount of human blood were derived from 



