9 



Mechanical and Electrical 

 Character of the Heartbeat 



I. Role of the Vertebrate Circulatory System 



All vertebrates possess a closed circulatory system. The blood which 

 circulates through this system is a suspension of various types of single 

 cells in a viscous solution of proteins and inorganic salts. The blood is 

 pumped; that is, it is forced to flow through the closed circulatory system. 

 The organ which does the pumping is called the heart. 



The circulatory system in vertebrates carries oxygen from special 

 exchange organs (lungs or gills) to the other tissues. It also transports 

 carbon dioxide from the tissues back to the lungs or gills. In some 

 amphibia, the skin also serves as an auxiliary gas exchanger. In any 

 case, the blood flows through a special exchange organ in which very 

 thin, moist walls separate the blood from the external environment. 



Besides dissolved gases, foods and metabolic waste products are also 

 carried by the blood. The endocrine secretions likewise are trans- 

 ported from gland to target organ by the blood stream. Finally, the 



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