Kidneys play an important role in urine formation through processes like ultrafiltration, selective reabsorption, and secretion. Ultrafiltration forces small molecules like water, glucose, and salts through the basement membrane into the nephron. Through selective reabsorption, the kidneys regulate the levels of water, salts, and other solutes in the blood to maintain homeostasis. The hypothalamus and pituitary gland control the release of antidiuretic hormone, which regulates the amount of water in the blood. The kidneys also help control glucose and salt levels in the blood.