renal portal system

The renal portal system is a circulatory pathway found in most lower vertebrates, including birds, but it has been lost in mammals. This system carries deoxygenated blood from posterior parts of the body directly to the kidneys. In many higher vertebrate forms, this system is enlarged or partially replaced by blood vessels from the hind limbs.12

In contrast, mammals have a highly efficient kidney structure called the nephron, which filters blood under pressure. This mammalian kidney system has rendered the renal portal system obsolete. The portal system in humans, such as the hepatic portal system, is different; it directs blood from organs like the stomach and intestines to the liver before returning to the heart. The kidneys in higher animals function as sophisticated filters to regulate water, salts, and organic materials, excreting nitrogenous wastes like urea.2345