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Circulation and Transportation— 

 Body Fluids 



However dry the atmosphere may be outside, the cHmate inside an animal 

 is as wet as a rain forest. In the majority of many-celled animals the creation 

 of such an adequate internal environment is due to circulating fluids, primarily 

 the blood. It provides for the needs of cells no matter where they are located; 

 those in the roots of the hairs receive oxygen as freely as those in the lungs; 

 waste products are cleared from the bones as well as from the kidneys. 



Water and Body Fluids 



Water composes the largest part of all body fluids. In lower animals the 

 internal fluid is known as body fluid, in higher ones, as blood with its auxil- 

 iaries, tissue fluid and lymph. The high water content of their bodies has made 

 it possible for animals to travel long distances over parched lands. Even in 

 the desert a lizard is a colony of wet cells watered by streams of blood, an oasis 

 in the sands. 



Balancing their water content, to keep the right amount of it in and out 

 and to make up for what is lost, is a universal problem of plants and animals. 

 Evaporation is prevented by thick skins, shells, and scales; undue loss from 

 excretion is prevented by controls of the sweat glands and kidneys; and loss 

 from various causes throughout the body is offset by shifts in the osmotic 

 pressure of membranes. Wherever animals live, in fresh water, salt water, or 

 on land, their body fluids are similar; all are salty. In marine invertebrates, 

 whether jellyfishes or horseshoe crabs, the body fluids are practically filtered 

 sea water. Even in fresh water and land animals, the saltiness of the body 

 fluids tells of their origin in ancient ancestors that lived in the sea. 



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