QUANTITY AND COMPOSITION OF THE BLOOD IN MAN 907 



since neither of these substances is ionised in solution. The conductivity 

 of blood serum is therefore determined almost entirely by its content in 

 salts. Since this is approximately constant, the conductivity of serum 

 varies within very narrow limits. The conductivity of blood varies how- 

 ever within wide limits, since the outer limiting layer of the corpuscles 

 is impermeable to many of the ions of the salts of the serum. The corpuscles' 

 present a resistance to the passage of the charged ions and therefore of the 

 electric current through them, so that the larger the number of corpuscles 

 contained in a given specimen of blood the lower will be the conductivity 

 of the latter. Stewart has made use of this fact as a basis for a method of 

 determining the relative volume of corpuscles and plasma. 



The relative amount of serum can be given by the formula : 

 p-M (1W-AM) 



where p is the number of c.c. of serum in 100 c.c. of blood ; A. (6), X (s), the conductivity 

 respectively of the blood and serum (both measured at or reduced to 5 C. and expressed 

 in reciprocal Ohms X 10 8 ). A reciprocal Ohm is the conductivity of a mercury column 

 1-063 metres long and 1 square millimetre in section. 



THE GENERAL COMPOSITION OF THE BLOOD 



The general composition of the blood has been determined by Karl 

 Schmidt in man, and by Abderhalden in the horse and bullock. The 

 results are given in the Tables on pages 908 and 909. 



The important points to be drawn from these analyses may be sum- 

 marised as follows. Human blood contains from rather over one-third to 

 one-half of its weight of corpuscles. It contains from 20 per cent, to 25 per 

 cent, solids. Blood plasma is resolved by clotting into serum and fibrin. 

 The fibrin forms only 0-2 to 04 per cent, of the total weight' of blood. The 

 serum contains in 100 parts 8 to 9 parts of solids, of which 7 to 8 parts 

 consist of proteins, while the salts make up about 1 part. The chief salt 

 present in the serum is sodium chloride, which constitutes 60 per cent. 

 of the ash. Next to this comes sodium carbonate, about 30 per cent., 

 and besides these two we find traces of potassium, sodium, and calcium 

 chlorides and phosphates. Traces of fats, cholesterin, lecithin, dextrose, 

 urea, and other nitrogenous extractives are constantly found in the serum. 

 The fats are much increased after a meal rich in them and may give the 

 serum a milky appearance. The red corpuscles contain from 30 to 40 per 

 cent, total solids. Of the solid constituents haemoglobin forms nine-tenths ; 

 the other tenth corresponds to the stroma consisting of stroma protein 

 (nucleo-protein), lecithin, cholesterin, and salts. There is a striking con- 

 trast between the salts of the corpuscles and those in the serum, the former 

 consisting chiefly of potassium phosphate, the latter of sodium chloride 

 which in some animals is entirely wanting in the corpuscles. 



