METABOLISM IN VERTEBRATES 



known as a closed system. That is, as the blood courses through the body, it 

 is confined to definite channels, in contrast to the situation in many, but not 

 all, invertebrates. The chief parts of the blood-vascular system are the same 

 in all vertebrates- There is a highly muscular region known as the heart, 

 located in the pericardial cavity (Fig. 3.2). By contracting rhythmically, 

 beating as we say, the heart forces the blood into the arteries, or vessels in 

 which blood flows away from the heart. Arteries divide repeatedly as they 

 pass to all parts of the body and finally end in a network of very thin-walled, 

 small vessels known as capillaries, which are found in all organs of the body 

 in close association with the cells (Fig. 3.3). The capillaries are also con- 

 nected with other larger vessels, known as veins, through which the blood 

 flows toward the heart. The arteries and veins that are seen when a verte- 

 brate animal like the frog is dissected are, therefore, continuous with each 

 other in organs all over the body by way of the capillaries. There is one 

 very important exception to this statement in all vertebrates. Blood leaving 

 the capillaries of the stomach and intestine passes into what is known as a 

 portal vein because it carries the blood to another set of capillaries. This 

 portal vein is called the hepatic portal vein because the capillary bed which it 

 feeds is located in the liver. In lower vertebrates there is a second portal 

 system; blood returns from capillaries in the hind legs to capillaries in the 

 excretory organs by way of the renal portal veins. 



The heart connects the veins and arteries, so that the blood flows continu- 

 ously away from the heart in the arteries, into the capillaries, thence into 

 the veins, and so back to the heart. Although this statement describes a very 

 simple circuit, such as that in the fishes, it is essentially true for all verte- 

 brates. The differences that exist in the circuits of the different kinds of 

 vertebrates are related to changes that occur in the heart and in some of the 

 larger arteries and veins (Figs. 3.4, 3.5, and 3.6). In vertebrates with lungs, 

 it is convenient to refer to the arteries and veins that supply regions of the 

 body other than the lungs as systemic arteries and veins, in contrast to 

 pulmonary arteries and veins supplying the lungs. 



Caudal 

 artery 



Anterior cardinal vein 



Caudal 

 vein 



Fig. 3.4. The circulatory system in a vertebrate having a heart with one atrium and one ven- 

 tricle, as in elasmobranch fishes; diagrammatic. The arrov^s indicate the direction of circulation. 



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