The mammalian kidney helps regulate water balance and removes wastes from the blood. The functional unit of the kidney is the nephron, which consists of the glomerular capsule and a long twisted tubule. Each human kidney has about 1 million nephrons. Blood flows through a twisted capillary bed called a glomerulus, which is surrounded by the glomerular capsule. Blood in the glomerulus is under high pressure so that water and various small molecules leak out through its porous walls and move into the capsule. This liquid filtrate contains many valuable molecules as well as a great deal of water. As the filtrate moves through the tubule, nutrients such as glucose and amino acids are transported back into the bloodstream. Some substances, like calcium and potassium, are secreted into the filtrate across the tubule walls. Most of the water returns to the bloodstream as the filtrate moves through the collecting duct. Urine travels from each kidney to the bladder through the ureter.