Kidney

Kidney pairs sit on either side of the spine just above the waist. Each kidney receives blood through the renal artery, filters it, and sends clean blood back through the renal vein. Inside are about a million tiny filtering units called nephrons.

A nephron begins with a tuft of capillaries named the glomerulus. Water and small particles leave the blood there and enter a twisting tube. As the liquid travels through the tube, the body reuses what it needs, such as glucose, certain salts, and water, while waste and extra minerals stay in the forming urine.

Hormones help the kidneys make careful choices. Antidiuretic hormone opens channels so more water returns to the blood on hot days, while aldosterone adjusts sodium and potassium. The final drops of urine collect in the renal pelvis, flow down the ureters, and wait in the bladder until it is time to use the restroom.

Kidneys also release erythropoietin, a hormone that tells the bone marrow to make more red blood cells when oxygen is low. They activate vitamin D so intestines can absorb calcium. When kidneys struggle, waste builds up, ankles may swell, or blood pressure can rise. Dialysis machines or transplants may be needed if damage is severe.

Protecting kidneys means drinking water, limiting salty snacks, staying active, and seeing doctors to control diabetes or high blood pressure. Helmets or seat belts guard them during accidents. Simple tests such as urinalysis and blood work can spot problems early, giving treatments time to work.