78 Basic Structure of Vertebrates 



into the kidney, where it breaks up into branches ultimately passing 

 into the renal capillary network, which is likewise the terminus of the 

 ultimate branches of the incoming renal arteries. Consequently the 

 blood circulating through the renal tissue is a mixture of arterial and 

 venous blood and, as in the case of the hepatic-portal vein, the vein 

 which enters the kidney, known as the renal-portal vein, begins and 

 ends in capillaries. The blood emerging from the renal capillaries passes 

 into outgoing renal veins (venae revehentes: Fig 76) which deliver it 

 into the postcardinals (Fig. 71 A) or postcava (Fig. 71B-D). This 

 renal-portal system, more or less well developed in vertebrate embryos, 

 persists in the adults of fishes and amphibians and, in a somewhat 

 reduced way, in adult reptiles and birds. In adult mammals the caudal 

 and iliac veins do not enter the kidneys, but pass directly into the 

 postcava (Fig. 71E). 



