CHAPTER 11 

 TRANSPORTATION SYSTEM 



Only in small animals can oxygen be taken in, digested food distri- 

 buted, and carbon dioxide and other wastes eliminated by mere diffusion. 

 In large animals the distances are too great for these slow-moving pro- 

 cesses. In such animals there must be a system of transportation con- 

 necting all parts of the body. This communication is furnished b}^ the 

 circulatory system. 



Open and Closed Circulatory Systems. — In crayfishes, insects, and 

 their allies there is a heart which forces blood into a small number of 

 major blood vessels. These vessels or their branches open into small or 

 great spaces among the ceils and organs, so that the blood comes into 

 contact with the tissues directly. Food is carried to the cells, and wastes 

 are removed, by direct contact. From the intercellular spaces the blood 

 is passed through the gills, and finally returns to the heart. Circulation 

 in such an open system must be slow because of the resistance offered by 

 the tissues. 



Any system of fluid communication must, like that of the crayfish, 

 reach the cells rather directly. To retain this necessary direct contact 

 and at the same time speed up the circulation, the vertebrate animals have 

 evolved two separate yet cooperating systems: (1) a blood system in which 

 there are smooth, closed tubular vessels in which the flow is very rapid, 

 and (2) a lymph system in which movement is slow but the cells are 

 reached directly. These systems are connected, and the fluid in the latter 

 is derived largely from the former. 



The Blood System. — A closed blood system consists of a set of tubes 

 which branch so extensively as to bring all parts of the body very near to 

 the circulating liquid. The blood is propelled through these tubes by a 

 contractile organ, th(i heart. In some animals the walls of the blood 

 vessels are contractile, and waves of contraction pass along them in 

 the direction of circulation. When these vessels arc especially large, and 

 when their contraction is more mai'ked than those; of other vessels, as are 

 those at the sides of the esophagus in the earthworm, they may properly 

 be called hearts. In the higher animals, vessels conducting blood away 

 from the heart are called arteries; those returning it to the heart are veins; 

 and the fine tubes leading from the arteries to the veins are called capil- 

 laries. The arteries have strong walls capable of withstanding consider- 



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