It is present in fishes and amphibians, is reduced in reptiles and birds, and is absent in mammals. It supplies the blood from the posterior region of the body to the kidneys by way of renal portal veins, hence its name. It enables the kidneys to remove waste products from the blood before sending it to the heart via renal veins and postcaval.
gradual reduction of the renal portal system in reptiles and birds and its disappearance in mammals is an evolutionary change in which the blood from the posterior region of the body is progressively carried directly to the postcaval.
