CIRCULATION AND RESPIRATION 



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capillary network is intercalated in the arterial system, as in 

 the gill capillaries of a fish, to and from which blood is carried by 

 arteries. A similar situation may also occur in the venous cir- 

 culation. Thus the hepatic portal vein arises from capillaries 

 in the intestine and terminates in capillaries in the liver, forming 

 what is known as a portal circulation. Since the major functions 

 of the blood are carried out in the capillary bed, arteries and veins 

 serve principally as conduits for conveying blood to and from 

 the capillaries. In the capillaries the important exchanges take 

 place between blood and tissues 

 upon which metabolism depends. 

 The heart, the walls of the larger 

 arteries and veins, as well as 

 all other parts of the vertebrate 

 body except some epidermal struc- 

 tures are provided with a capillary 

 circulation. 



Arteries. — A cross section of a 

 large artery shows three regions: 

 the intima, the media, and the ex- 

 terna (Fig. 74). The intima con- 

 sists of a flattened layer of cells 

 known as endothelium, lining the 

 vessel. External to the endothe- 

 lium, the intima contains a layer 

 of white and elastic fibers forming 

 a meshwork elongated in the direction of the length of the vessel. 

 The media consists of a thick layer of circular, nonstriated or plain 

 muscle fibers, spread through wide-meshed elastic tissue. The 

 externa, the outer covering of the artery, is composed of some- 

 what denser connective tissue, part of which is elastic. 



Veins.- — The wall of a vein is thinner than that of a correspond- 

 ing artery, but otherwise the plan of structure is the same for 

 both. In larger veins the media is reduced or absent. Unless 

 distended with blood, veins appear collapsed or wrinkled in 

 sections. At the same level veins appear larger than correspond- 

 ing arteries in sections because the arteries contract markedly when 

 treated with a killing fluid such as is used in preparing sections. 



Capillaries. — As arteries approach capillaries, the outer layers 

 of the artery are lost, until only the endothelial layer with a few 



Fig. 74. — Section of wall of 

 medium sized artery of cat. E, 

 externa; I, intima; M, media. 



