CHAPTER XIII 

 CIRCULATION 



I finally saw that the blood, forced by the action of the left ventricle 

 into the arteries, was distributed to the body at large, and its several 

 parts, in the same manner as it is sent through the lungs, impelled by 

 the right ventricle into the pulmonary artery, and that it then passed 

 through the veins and along the vena cava, and so round to the 

 left ventricle . . . which motion we may be allowed to call circular. 



— Harvey, 1628. 



In the Protozoa and many of the lowest Metazoa, the transport 

 of materials to and from the various parts of the organism is ob- 

 viously a simple problem compared to that presented by animals 

 with deeply-hidden tissues, each and every cell of which must be 

 served. Indeed a complex body is impossible without a complex 

 circulatory system, and we now proceed to a summary of how 

 the problem is met, particularly in the Vertebrates. 



The crucial points of contact between the higher animal and its 

 environment, in so far as the intake of matter and energy is con- 

 cerned, are the membranes which line the digestive tract and a 

 large diverticulum from it, the lungs. Through the former must 

 pass all the materials which are to be assembled as integral parts of 

 the organism and the fuel which is to supply the energy for the vital 

 processes, while through the latter must pass the oxygen which is 

 to release this energy. Only when these membranes have been 

 passed are the materials really within the body and at its disposal 

 for distribution by the circulatory system to the individual cells 

 of the various organs which are to use them. 



In addition to carrying the fuel and the oxygen, the circulatory 

 system must remove the waste products of metabolism from the 

 cells and deliver them to the proper excretory organs, such as 

 the lungs or kidneys, to be passed to the outside world. The cir- 

 culatory system is therefore the essential connecting link between 

 the points of intake, utilization, and outgo of materials. 



And incidentally, as it were, the circulatory system is also a 

 coordinating agent of crucial importance, because it distributes 



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