Functional unit of the kidney that removes waste and excess substances from the blood to produce urine. Each of the million or so nephrons in each kidney is a tubule 1.2–2.2 in. (30–55 mm) long. At one end it is closed, expanded, and folded into a double-walled cuplike structure (Bowman's capsule) enclosing a cluster of capillaries (glomerulus). Fluid forced out of the blood through the capillary walls of the glomerulus into Bowman's capsule flows into the adjacent renal tubule, where water and nutrients are selectively reabsorbed from the fluid back into the blood, and electrolytes such as sodium and potassium are balanced in several distinct sections along its length. The final concentrated product is urine. See also urinary system.
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