Renal portal system (RPS) in birds
An unique feature of avian kidney is its Renal Portal System (RPS) which provides an extra branch of blood flow to the renal tubules along with peritubular capillaries.
Venous blood from portal vein gives one branch to the kidney and this branch provides the microcapillaries which perfuse the tubules along with peritubular capillaries.
Both the branches of capillaries are interconnected and drained by the venous system to the posterior venacava. When it leaves these capillary network and enters the renal vein there is a valve which regulates the transition of the blood in these capillary network.