Kidney anatomy. Historical artwork of a kidney that has been sliced vertically in half to show its internal anatomy. This is a left-hand kidney, seen from the front. The kidney is one of a pair of organs that form urine by filtering out waste products from the blood. The three tubes (down left) for the kidney are, from top: the renal artery; the renal vein; and the ureter. The renal artery brings blood to the kidney, and the renal vein removes it. Blood circulating in a kidney is filtered in convoluted tubular units (nephrons) that remove water and soluble substances, and then reabsorb what is needed. Waste products (such as urea) and excess water form urine, which drains to the bladder through the ureter.