[1] Its function is to supply blood to renal tubules when glomerular filtration is absent or downregulated.
[2] The main channel is the renal portal vein, developed from the posterior cardinal vein, which brings venous blood circulation from the tail and groin to the kidney, where it is shunted into a capillary network around the convoluted tubules.
In fishes and salamanders, due to the lack of metarterioles, all the blood passes through the capillaries.
However, in frogs and amniotes, most of the blood usually passes through the metarterioles instead, although it can still be diverted through the capillary networks if need be.
The system is completely abandoned in mammals, with the result that all the blood from the tail (if present), groin, pelvic area, and hind legs is forced to pass directly into the posterior vena cava, and the only blood passing out of the kidney through the renal vein is that that entered it through the renal artery.