GENERAL ZOOLOGY 



Dorsal aorta 



Hind limb 



Renal 

 portal vein 



Cutaneous 

 artery 



Pulmonary 

 artery 



Fig. 3.5. The circulatory system in a vertebrate having a heart vs'ith two atria and one ven- 

 tricle, as in the frog; diagrammatic. The arrows indicate the direction of circulation. 



The arrangement of the chambers of the heart and its valves controls the 

 direction of blood flow as the heart contracts (Figs. 3.7 and 3.8). Blood 

 leaving the heart enters the large arteries which have walls sufficiently heavy 

 to maintain a fairly constant diameter; their walls contain, in addition to the 

 thin lining of endothelium, elastic connective tissue, and non-striated muscle. 

 As the arteries divide and redivide, the bore of each branch becomes smaller 

 and the wall thinner. Of course, the total capacity or volume of the branches 

 is far greater than that of the original artery. Capillaries are limited only 



^Aorta 



Iliac artery 



Pulmonary 

 artery 



Left ventricle 



Iliac 

 vein 



Fig. 3.6. The circulatory system in a vertebrate having a heart wiih two atria and two ven- 

 tricles, as in mammals; diagrammatic. The arrows indicate the direction of circulation. 



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