Renal portal system is absent in mammals. Give Reason.

Explanation

1. Fishes and amphibians have a well developed renal portal system. This is reduced in reptiles. In mammals it ultimately dissappears.
2. In mammals there is complete separation of oxygenated and deoxygenated blood. Posterior portion of the body gets oxygenated blood from the heart and after oxidation etc the blood does not contain much of impurities. Hence it is not sent to kidneys for filtration.