CHAPTER 10 



THE CIRCULATORY SYSTEM 



The food that is digested by the digestive system and the oxygen 

 that is taken in by the respiratory system are of no vahie to the organ- 

 ism until they are made available to the cells. The distribution of these 

 materials is taken care of by the circulatory system which has often 

 been termed the "middleman" of the body. In many respects this is 

 an apt comparison, for materials are moved from their points of origin 

 to areas where they are needed ; waste materials are taken to disposal 

 points ; and a constant internal environment is maintained. 



The problem of distribution within the body is a simple matter 

 among the smaller invertebrates. Here the various materials can dif- 

 fuse in and out across cell membranes and no specialized structures are 

 needed. Within the cell, the circulating cytoplasm is an adequate 

 means of distribution ; however, larger and more complex animals 

 need specialized distribution channels to take care of the cells which 

 are not in immediate or near contact with the outer environment. 



Among the coelenterates and some worms, a central cavity serves 

 both for digestion and circulation. A separate circulatory system is 

 first seen in the earthworm. Here there are distinct blood vessels as 

 well as a special mechanism for pumping the blood. Many other in- 

 vertebrates have a similar closed type of circulatory system, that is, 

 the blood circulates in closed vessels. Others such as the insects have 

 an open circulatory system, that is, the blood circulates in body spaces 

 and is not confined to closed vessels. Vertebrates, with their complex 

 arrangement of blood vessels, have a closed circulatory system. 



Essentially, then, a circulatory system must have at least three 

 main features : a central pumping mechanism, some sort of channels 

 for the circulating medium, and, last, the circulating medium itself. 



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