Due to its primary function as a filtration organ, the kidney has two vascular systems, similar to the heart and lungs. However, the two vascular systems are not separated: vasa privata and vasa publica. The vascular columns in this vascular system are perfused by arterial blood and a certain proportion of the blood is "squeezed out", passing as primary urine into Bowman's capsule and further into the tubular system. The membrane of these vascular columns, also called renal corpuscles, is essentially built up by the protein nephrin.
In addition, the kidney has an endocrine function by synthesizing the hormone erythropoietin (EPO), which stimulates the production of red blood cells (erythrocytes) and renin, a hormone-like enzyme regulating the blood flow to the kidney via systemic blood pressure.