CHAPTER X 



CIRCULATION OF THE BLOOD 



93. Review. We have learned that the human body 

 is made up of millions upon millions of very tiny cells, 

 each of which must have food and oxygen. The cells 

 derive their nourishment from the blood as it circulates 

 through the various tissues. Oxygen from the air, 

 water, and solid foods changed and dissolved by the 

 digestive fluids, all get into the blood and form the part 

 which the cells need to nourish them. 



The cells cannot move about in search of food, and 

 so blood must be carried to every organ and tissue of 

 the body. The amoeba, you remember, is a single cell 

 and has the power to move about freely in the water 

 from which it can obtain its food. But all higher ani- 

 mals, and plants, as well, have a fluid of some kind 

 circulating within them for the purpose of carrying 

 food materials to each and every cell and removing 

 waste substances and worn-out parts. In the tree the 

 sap forms this circulating medium, and in animals it is 

 the blood. 



Neither plants nor animals can live without such a 

 circulating medium, which is truly a " river of life," 

 carrying to every cell life-giving material, and remov- 



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