214 PHYSIOLOGICAL CHEMISTRY. 



later. Beyond the liver the hepatic veins lead to the general 

 circulation. In this general way the nutriments reach the 

 blood which is the main channel of distribution, but this fluid 

 is far from being a simple solution or mixture of these nutri- 

 ments in the condition in which they leave the alimentary 

 canal. The most important of the blood constituents are, in 

 fact, chemically quite distinct from anything produced in the 

 course of digestion. Certain organs of the body have the 

 important function of working over these digestive products 

 and converting them into the things required in the blood. 

 To do this several synthetic reactions are necessary ; how these 

 are carried out we do not know, and in some cases we are 

 ignorant also of where they take place. In what follows some 

 of the main facts in this connection will be given. 



COMPOSITION OF THE BLOOD. 



Quantitative Variations. It is evident that only an aver- 

 age composition can be in general considered since the fluid 

 is in a state of constant change. Soon after a meal certain 

 constituents would naturally be found increased, and after a 

 period of fasting a deficiency in the same would follow. From 

 what has been said it is further apparent that the blood of the 

 portal vein would be found much richer in some substances 

 than that of the hepatic vein or the arteries. It must also be 

 remembered that the blood is not a homogeneous fluid but con- 

 sists of a true solution in which are suspended certain cell 

 structures. We may therefore consider the average composi- 

 tion of the blood as a whole, or of the corpuscles on the one 

 hand and the fluid portion or plasma on the other. The spe- 

 cific gravity of normal blood varies between 1.05 and 1.07; the 

 average specific gravity of the serum is about 1.03. 



Approximately the blood makes up 7 to 8 per cent of the 

 body weight; therefore in an individual weighing 70 kilo- 

 grams the blood weight would be 4.9 to 5.6 kilograms. Of 

 this blood weight about 60 per cent belongs to the plasma and 

 40 per cent to the corpuscles. Among the various recorded 



