CHAPTER XXXIII 

 CIRCULATION, ASSIMILATION AND EXCRETION 



It would do us little good to provide our bodies with 

 the proper food and to digest that food if there were no 

 system for carrying the nourishment, and the oxygen nec- 

 essary to burn it, to the cells where they are needed. 



To meet this need, our wonderful human machine is 

 provided with a force pump that sends a stream of blood 

 to all the parts of our bodies. This blood picks up the 

 digested food and oxygen and distributes them to the cells 

 to repair waste and produce energy. Having done this, the 

 blood gathers up and carries away waste and worn out 

 material to points of discharge from the body. 



The circulatory system is always working to free the 

 body of what endangers it, and to meet special needs. So 

 the heart pump speeds up when exercise demands more 

 energy and therefore more oxygen. It slows down when 

 the need is past. The whole system is a wonderful illus- 

 tration of making every move count for something worth 

 \rhile. We should know more about its structure and use 

 in order to help and not to abuse it. 



Circulation. The circulatory system consists of the heart 

 and of a complicated arrangement of tubes called arteries, veins 

 and capillaries through which the blood is constantly mov- 

 ing. The blood carries food and oxygen to all the cells of 

 the body, and carries away the carbon dioxide, urea and other 

 wastes excreted by the cell's. The heart furnishes the power which 

 causes the blood to move through the arteries to all parts of the 

 body and to return through the veins after passing through the 

 capillaries, which are small connecting tubes that unite the arteries 

 with the veins. 



The Blood. The blood is a red liquid forming about eight 

 per cent of the weight of the body. It is composed of red and 

 of white corpuscles floating in a colorless liquid called plasma. 



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