CHAPTER XXVII 

 THE BLOOD AND CIRCULATION 



When the soluble products of digestion diffuse through 

 the walls of the alimentary canal they pass directly or in- 

 directly into the blood, and are carried by this fluid to all 

 parts of the body. The blood comprises about }{ 3 of the 

 weight of the body, although it varies greatly in amount at 

 different times and with different people. It is composed 

 of a fluid called plasma and numerous very minute corpuscles 

 which are so small that as many as 5,000,000 are 

 estimated to occur in a cubic millimeter. The plasma 

 of blood is a very complex fluid. It contains many food 

 products, proteins, fats, sugar and various salts which may 

 be taken up by the cells of the body; it contains also the 

 waste matter derived from the destructive metabolism of 

 cells. And there are various other substances in it which 

 have a number of different functions. Blood is the great 

 medium of transport of food, oxygen and waste. Each 

 cell takes out of it the material needed for its life and gives 

 off into it the broken down products of its vital activity. 

 The blood has to keep in circulation in order to supply all 

 the parts of the body which are dependent on it. And in 

 the normal life of man it never stops for a moment from 

 before birth to old age. 



When blood is withdrawn from its vessels it has the 

 curious property of forming a solid, jelly-like mass, the 

 clot. This clot is composed of a substance called fibrin, 

 a form of protein which is supposed to be derived from 



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