Kidneys are a pair of organs in the urinary system. They are located in the abdomen on either side of the spine, and they filter blood to remove wastes and excess water. The left kidney is usually slightly smaller than the right one.

Each kidney is about 4 or 5 inches long, 2 or 3 inches wide, and 1 or 2 inches thick. Kidneys weigh about half a pound each.

The outer layer of the kidney is called the cortex. The cortex contains renal pyramids, which are cone-shaped structures. The renal pyramids empty urine into the minor calyces.

The inner medulla contains the renal pelvis, a bowl-shaped structure that collects urine from the calyces and drains it into the ureters.

The kidneys filter about 120 to 150 quarts of blood every day. They remove about 2 quarts of waste and excess water to form urine. Urine is composed of water, urea (a waste product), and other dissolved substances. The renal arteries carry blood to the kidneys, and the renal veins carry blood away from the kidneys.

The ureters are tubes that drain urine from the kidneys to the bladder. The bladder is a muscular sac that stores urine until it is expelled through the urethra.

Kidneys perform many functions, including:

  • Regulating the concentration of water and dissolved substances in the blood.
  • Regulating the acid-base balance of the blood,
  • Regulating blood pressure.
  • Producing hormones that help control blood pressure, red blood cell production, and calcium metabolism.

The kidneys are essential for life. A person can live without one kidney, but not without both. Kidneys can be donated by living donors or from cadavers.

Kidney transplants are usually successful, but the transplanted kidney may not function properly and may eventually need to be replaced.

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