The mammals have no renal portal system due to the following facts
(i) It is an evolutionary trend that fishes and amphibians have well developed renal portal system, while, in reptiles and birds this system gets reduced. Finally in mammals it ultimately disappears.
(ii) The heart of mammals is four-chambered, due to which there is a total separation of oxygenated and deoxygenated blood.
(iii) Posterior portion of the body gets oxygenated blood from the heart and after oxidation process
etc., the blood does not contain, so much impurities that it should go to the kidneys for filtration.