a. Urine is formed by the kidneys in millions of tiny filtration plants called nephrons. The nephrons receive and filter all the blood in the body about once every 12 minutes.

b. Each nephron (figure 2-3) is composed of 3 parts: the glomerulus, the capsule, and the tubule. Each part is essential in the filtration, re-absorption, and excretion processes.

Figure 2-3. The kidney nephronc. Circulating blood enters the cluster of capillaries known as the glomerulus. Water, electrolytes, and small organic molecules are filtered from the capillary blood and pass into the capsule. Blood cells and large protein molecules cannot pass through the glomerulus into the capsule.

d. The “filtrate” (water and solutes filtered from the blood) passes from the capsule into the tubule. In the tubule, water and usable chemical products are reabsorbed. e. The final waste product, urine, drains from the last loop of the nephron’s tubule into a collecting tubule for drainage into the renal pelvis.