160 The Vertebrate Organ Systems 



THE FUNCTIONS OF THE CIRCULATORY SYSTEM 



The circulatory system has many important functions which may 

 vary somewhat according to the needs of the different animals. In 

 general, the functions can be summarized as follows : 



1. Distribution. The distribution of innumerable materials to 

 points where they are needed is an extremely important function. 

 Digested food is taken from the digestive tract for use by the cells, 

 oxygen is removed from the lungs to help oxidize this food, waste 

 products are transported for disposal, the secretions of the endocrine 

 glands are carried from their points of origin, and the various protec- 

 tive materials of the blood are taken to sites where they are most 

 needed. 



2. Maintenance of a Constant Internal Environment. If 

 the cells of the organism are to function efificiently, they must be in an 

 environment of a constant quality ; that is, the osmotic pressure, the salt 

 and sugar content, and, in many animals, the temperature must always 

 be the same. The blood performs this important function by its ability 

 to bring in and remove materials. The circulatory system is aided 

 in this by the kidneys. 



3. Defense Against Disease. The white blood cells or leuco- 

 cytes are able to destroy infective bacterial cells which in some way may 

 have gained entrance to the body. After many infectious diseases, 

 special protective bodies, antibodies, are produced by the body. These 

 are in the blood at all times and prevent recurrence of these diseases. 



4. Temperature Regulation. The heat produced by the oxi- 

 dation of foods within the cells must be dispersed in some way. This 

 too is accomplished by the circulatory system. The heat is transferred 

 to the body surface and lost. This, of course, is of greater importance 

 in the homoiothermic animals than in the poikilothermic forms. 



5. Acid-Base Balance. Within the blood are chemicals known 

 as buyers. The chief of these in mammals is NaHCOs, sodium bi- 

 carbonate. A buffer is of great service due to the fact that it can 

 neutralize either acids or bases, thus maintaining the pH (hydrogen 

 ion concentration) of the blood at a constant level. The blood is aided 

 in this important function by the lungs which eliminate carbon dioxide, 

 CO2. COi; is carried in the blood as the weak acid, carbonic acid 

 H2CO3, which can easily dissociate to form water and carbon dioxide 



