Urinary System

Urine is formed by the kidneys, which filter many substances out of the blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine.

After urine forms in the kidneys, it is transported through the ureters (one per kidney) to the sac-like bladder, which stores the urine until urination.

Waste products removed from the body with the formation and elimination of urine include many water-soluble metabolic products.

Regulating acid-base balance in the body: When blood pH is too low (blood is too acidic), for example, the kidneys excrete less bicarbonate (which is basic) in urine. When pH is too high (blood is too basic), the opposite occurs, and more bicarbonate is excreted in urine.

Controlling the volume of extracellular fluids, including the blood volume, which helps maintain blood pressure: The kidneys control fluid volume and blood pressure by excreting more or less salt and water in urine.

The nephron is the basic structural and functional unit of the kidney, and each kidney typically contains at least a million of them. As blood flows through a nephron, many materials are filtered out of the blood, needed materials are returned to the blood, and the remaining materials form urine.

The urinary bladder is a hollow, muscular, and stretchy organ that rests on the pelvic floor. It collects and stores urine from the kidneys before the urine is eliminated through urination.

The urethra is a tube that connects the urinary bladder to the external urethral orifice, which is the opening of the urethra on the surface of the body.