There are two blood vessels leading off from the abdominal aorta, which is located in the abdominal section of the heart's major artery, and go to the kidneys. The renal artery is one of these two blood vessels. Where the kidney curves inwards in a concave shape, the hilum is found, which is an opening where the renal artery enters. Under normal circumstances, once the renal artery enters through the hilum, it splits into two main branches, which each then split into numerous smaller arteries, which deliver blood to different areas of the kidneys, known as nephrons. Once the blood has been processed here, it is sent back through the renal vein to the inferior vena cava and to the right side section of the heart. A normal person's kidneys receive approximately a quarter of the hearts blood output, or 1.2 litres of blood each minute. The body has self-regulating mechanisms in place, which increase or decrease the flow of blood to adapt to stress. Receptors located in the smooth muscle wall of the renal artery allow the arteries to expand or contract to compensate for high or low blood pressure.