THE BLOOD. 175 



THE TOTAL QUANTITY OF BLOOD IN THE BODY. 



The amount of blood in the body of an animal is ascertained by 

 collecting and measuring the blood obtained by bleeding the animal. 

 To the total thus obtained must be added the blood which does not 

 escape when the animal is bled. This must be dissolved out of the 

 tissues until no more colour is obtained ; the tint of the mixed washings 

 is compared with that of a known dilution of the blood, and in this 

 way the amount of dissolved blood is calculated and added to that 

 found by direct measurement. Two experiments of this kind have 

 been performed on the bodies of guillotined criminals, and from these 

 it was found that the blood in man forms one-thirteenth of the body 

 weight. 



Haldane has devised a method for ascertaining the quantity of 

 blood in the living subject. This method is based upon the affinity 

 of haemoglobin for carbon monoxide. A small quantity of blood is 

 withdrawn and its oxygen capacity is estimated. The subject is then 

 allowed to breathe air containing a known amount of carbon monoxide, 

 which is absorbed by the blood. A certain proportion of the haemo- 

 globin of the blood combines with the carbon monoxide, forming 

 carboxyhsemoglobin, and thus reducing the oxygen capacity of the 

 blood to that extent. A small quantity of blood is again withdrawn 

 and its oxygen capacity estimated. If it is found in this way that 

 100 grams of blood contain ^ of the absorbed carbon monoxide, then 

 100 grams obviously form -g^- of the total blood contained in the body. 

 By this method the total blood has been calculated to be one-twentieth 

 of the body weight, so that there is a discrepancy between the results 

 obtained by the two methods, though the latter method is probably 

 the more accurate. 



