Illustration of kidney anatomy. Blood flows into the kidney through the renal artery (large red tube). This artery branches into smaller and smaller blood vessels until the blood reaches the nephrons (structural and functional unit of kidneys). At the beginning of each nephron is a Bowman's capsule (spherical, right), a cup-like sac that contains the glomerulus (network of small blood vessels). The glomerulus filters the blood and the filtrate enters the tubules (yellow). Waste products remain in the tubule while needed substances are returned to the blood. The filtered blood exits through the renal vein (large blue tube) and waste products in the form of urine exit through the ureter (large yellow tube), which carries urine to the bladder.