BLOOD: ITS GENERAL PROPERTIES 87 



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 

 by volume or about 55 per cent by weight consists of plasma; and from 

 40 to 30 per cent by volume or 45 per cent by weight consists of cor- 

 puscles. 



THE PROTEINS OF THE BLOOD 



The plasma obtained by centrifuging the blood rendered noncoagula- 

 ble by oxalates, hirudin or other means (see page 99), contains 5 to 8 

 per cent of coagulable proteins. These proteins are serum albumin, 

 serum globulin, and fibrinogen. They can be separated from each other 

 by the use of acids and neutral salts. Their proportion varies under dif- 

 ferent conditions, but is approximately as follows: 



Fibrinogen 0.15-0.6% 



Serum globulin 3.8% 



Serum albumin 2.5% 



The amount of fibrinogen is subject to the greatest variation (Mathews). 



Fibrinogen 



The least soluble of the blood proteins is fibrinogen. The plasma is 

 almost freed of it by half-saturation with sodium chloride, or with a 

 small amount of acetic acid. It is precipitated as fibrin in the process 

 of blood coagulation (see page 99), and is estimated by weighing the 

 amount of fibrin which it produces. 



Serum Globulin and Serum Albumin 



Globulins are ordinarily defined as being insoluble in distilled water, 

 and albumins as being soluble. It is, however, impossible to separate 

 serum globulin and albumin satisfactorily in this manner. The globu- 

 lin obtained by dialysis can be returned to solution by the addition of 



