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Blood Flow Regulating Structures in the Avian Kidney

The vascular supply of the kidney of birds is far more complicated than in mammals. In addition to arterial input and venous drainage, there is a prominent venous portal system, as in all nonmammalians (Fig. 1). The latter delivers blood from the leg (V. iliaca externa -Ua), the lateral pelvis (V. ischiadica – Ub) and the cloacal region (V. hypogastrica and V. pudenda – Uc and Ud). This blood then passes the peritubular capillaries of the kidney, providing the substrate for uric acid excretion, the main avian nitrogenous waste product. However, not all of the portal blood passes through the renal capillaries. A variable proportion bypasses the kidney via the common iliac / posterior caval vein (VCP) and the coccygo-mesenteric vein (VCM).