THE STRUCTURAL AND FUNCTIONAL SYSTEMS 



53 



at its outer end into the ureter. In the frog, the arrangement 

 of arteries and veins in the kidneys is such that blood from the 

 hind hmbs enters by the two renal portal veins (Fig. 31 A), 

 and leaves by several renal veins which unite to form the 

 single post-caval vein, lying medially between the kidneys. Blood 

 also enters the kidneys from a nmnber of small renal arteries 

 which are given off by the dorsal aorta or principal artery 

 in this region of the body. Fig. 31 B shows how the blood from 



d.a. 



r.a. 



r.p.v. 



A 



Fig. 3L — Blood vessels and circulation of kidney. 



A, kidney and its principal blood vessels. B, schematic representation of circulation 

 through capillaries of kidney, c.k., capillary of kidney; d.a., dorsal aorta; g, glomerulus; 

 r.a., iliac artery; k, kidney; p.c, post-caval vein; r.a., renal artery; r.p.v., renal portal 

 vein; r. v., renal vein; u, ureter; u.t., urinary tubule. 



the renal portal veins and the renal arteries passes through 

 the kidneys in relation to the kidney tubules. The branches of the 

 renal portals break up into capillaries, which are the smallest 

 divisions of the blood vessels, and these form a network about the 

 tubules before uniting in the renal veins which pass to the post- 

 caval vein. The branches of the renal arteries form the coil of 

 minute arteries called the glomerulus before they join the more 

 general capillary system. The course of the blood through the 

 microscopic vessels of the kidneys is shown by the arrows in Fig. 

 31 B. As seen in sections under the microscope, the kidney shows 



