THE BLOOD 191 



1025. They contain about 90 per cent, of water and 10 per 

 cent, of solids. The chief solids are the proteids serum 

 albumin and serum globulin (with, in the plasma, the addition 

 of fibrinogen). The proportion of the two former proteids 

 to one another varies considerably in different animals, but 

 in the same animal at different times the variations are 

 small. The globulin probably consists of at least two 

 bodies englobulin precipitated by weak acid, and pseudo- 

 globulin not so precipitated. The amount of albumin is 

 generally greater when the body is well nourished. In man, 

 they together form about 7 per cent, of the serum. 



The other organic constituents of the serum are in much 

 smaller amounts and may be divided into 



1. Substances to be used by the tissues. 



Glucose is the most important of these. It occurs only in 

 small amounts about 1 to 2 per mille. Part of it is free, 

 but part is probably combined in organic combinations such 

 as jecorin. It is probably in larger amount in blood going 

 to muscles than in blood coming from muscles, and this 

 difference seems to be specially well marked when the 

 muscles are active. 



Fats occur in very varying amounts, depending upon the 

 amount taken in the food. 



2. Substances given off by the tissues. 



The chief of these is urea, which occurs constantly in 

 very small amounts in the serum about -05 per cent. 

 We shall afterwards see that it is derived from the liver, 

 and that it is excreted in the urine by the kidneys. 



Creatin (p. 43), uric acid (p. 397), and some allied bodies 

 appear to be normally present in traces, and their amount 

 may be increased in diseased conditions. 



Of the inorganic constituents of the serum the most 

 abundant is chloride of sodium, but in addition sodium 

 carbonate, and alkaline sodium phosphate, are also present. 

 Calcium, potassium and magnesium occur in very small 

 amounts. 



