THE BLOOD AND ITS FUNCTIONS 123 



Thus animals in which there is some other means of dis- 

 tributing air and food lack the blood system. We may con- 

 clude, then, that one of the functions of that system is the 

 distribution of food and oxygen. 



THE BLOOD 



Blood makes up about one-thirteenth of the body weight. 

 It is really a very complex fluid, for it contains all of the food 

 materials from the intestine and also receives many waste 

 products from worn-out parts of the body. But leaving aside 

 these complexities of chemical composition, we may learn 

 by a study with the microscope that fresh blood consists 

 of a liquid almost as limpid as water, which has floating in 

 it an immense number of minute, solid bodies. The liquid 

 is called the plasma; the solid bodies are of three kinds: red 

 corpuscles or erythrocytes, white corpuscles or leucocytes, and 

 platelets; Fig. 10. 



The relative proportion of these in the blood is approxi- 



: , mately as follows: water 90%, solids 10%. Of the solids, 



i about 72% is proteid in character, the remainder consisting 



of fats, acids, and salts. Some of these are unutilized food 



1 materials, others the result of "wear and tear" of body pro- 



! topi asm as it ceaselessly, day in and day out, performs its 



vital work. 



Blood Plasma. The plasma is a transparent liquid of a 

 | light straw color. It is the plasma that gives the fluid char- 

 \ acter to the blood and enables it to flow through the vessels. 

 Its chemical composition varies. Into it are absorbed the 

 i foods from the intestine, and into it also are passed the various 

 ' waste products from the body. Its composition will there- 

 fore be different after a meal from what it is after a period of 

 fasting. When one is resting, too, fewer waste products are 

 eliminated to make the blood impure than when one is actively 

 working. One of the constituents of the plasma will be no- 

 ticed later because of its important relation to blood clotting. 



