546 



VERTEBRATE LIFE AND ORGANIZATION 



Anterior 

 cardinaJ vaini 



Ce-phal 

 capi" ' 



Dorsal 

 r aorta. 



•Posberior cardinal vein. 

 Coelia-c a-rberj/ 



■Renal artery 



■Renal portal 

 vein 



Gill 



capillar! 



-Ventral 

 aorta. 



Chambers of the heart 



a.rter5f 

 Liver capillaries 



portal 

 vein. 



Tail 

 capillaries" 



Figure 27.2. The major parts oi the cardiovascular system of a primitive fish. 

 1, sinus vcnosus; 2, atrium; 3, ventricle; 4, conns arteriosus of the heart. The aortic 

 arches are numbered with Roman numerals. Only traces of the first aortic arch remain 

 in the adults of most fishes. 



sus) arranged in linear sequence. The heart increases the blood pressure, 

 which is very low in the veins, and sends the blood out through an 

 artery, the ventral aorta, to five or six pairs ot aortic arches that extend 

 dorsally through capillaries in the gills to the dorsal aorta. Carbon 

 dioxide is removed and oxygen is added as the blood flows through 

 the gills, i.e., it changes to arterial blood. The dorsal aorta distributes 

 this through its various branches to all parts of the body. 



Blood pressure decreases as blood flows along because of the friction 

 between the blood and the lining of the vessels. Blood pressure is 

 reduced considerably as the blood passes through the capillaries of the 

 gills, for friction is greatest in vessels of small diameter. The mean 

 blood pressure in the ventral aorta of a dogfish, for example, is 28 

 mm. fig; that in the dorsal aorta is 15 mm. Hg. Thus the IdIoocI dis- 

 tributed by the dorsal aorta is under relatively low pressure, and this 

 will be much lower by the time it reaches the capillaries in the tissues. 

 Circulation in primitive fishes is rather sluggish, and not conducive to 

 great activity. 



Veins drain the capillaries of the body (where blood pressure is 

 further reduced) and lead to the heart, but not all veins go directly 

 to the heart. In primitive fish, blood returning from the tail first passes 

 through capillaries in the kidneys before entering veins leading to the 

 heart. Veins that drain one capillary bed and lead to another are called 

 portal veins, and these particular veins are known as the renal portal 

 system. Another group, known as the hepatic portal system, drain the 

 digestive tract and lead to capillaries in the liver. Since much of 

 the blood returning to the heart has passed through one or the other 

 of these portal systems in addition to the capillaries in the gills and 

 tissues, its pressure is quite low. 



It is not difficult to appreciate the significance of an hepatic portal 

 system, since the liver plays such an important role in the metabolism 

 of foods, but the adaptive significance of a renal portal system in primi- 

 tive vertebrates is less clear. One might postulate that it ensures an 



