BLOOD AND H^MOPOIETIC ORGANS 95 



obstruction, in which no food can reach the stomach,* 

 and in diabetic coma it may be present in such qugfhtity 

 that the blood serum becomes milky. 



The most striking thing about the composition of the 

 plasma is its great constancy. The blood maintains in 

 a wonderful way a uniform standard of composition. 

 This it does by means of two mechanisms viz., excretion 

 through the kidney and excretion into the lymph spaces 

 of the tissues. Haemorrhage, for instance, does not con- 

 centrate the blood, for fluid is instantly withdrawn from 

 the tissues to make good the loss. Nor is the blood 

 necessarily more watery in conditions of dropsy, for the 

 excess of fluid is got rid of into the tissues.! 



Injection of saline solution into a vein does not dilute 

 it, for the excess is immediately excreted through the 

 kidneys, and, above a certain point, into the tissues also. 

 As a matter of fact, if f litre of normal solution is 

 injected into a vein, only a slight fall in the specific 

 gravity of the blood is noticeable, and it only lasts for 

 about half an hour (Schmaltz I). 



In the same way it is impossible permanently to 

 increase the proportion of any of the mineral constitu- 

 ents of the plasma, for the kidney immediately excretes 

 any surplus. 



The total amount of blood in the body is about one- 

 twentieth of the body-weight, or on an average 3J litres 

 (about 6 pints). In health this amount is probably as 



* Boninger, Zeit. /. Klin. Med., 1901, xlii. 65. 

 f Askanazy, Deut. Arch. f. Klin. Med., 1897, lix. 385; also 

 Hammerschlag, Zeit. f. Klin. Med., 1892, xxi. 475. 

 I Deut. Arch. f. Klin. Med., 1891, xlvii. 145. 



