Why are kidneys important and how do they function?

Your kidneys are two bean-shaped organs, each about the size of a fist. They are located just below the rib cage, one on each side of your spine.

Healthy kidneys filter about a half cup of blood every minute, removing waste and extra water to make urine. The urine flows from the kidneys to the bladder through two thin tubes of muscle called ureters, one on each side of your bladder. Your kidneys, ureters and bladder are part of your urinary tract.

Your kidneys remove waste and extra fluid from your body. Your kidneys also remove acid that is produced by the cells of your body and maintain a healthy balance of water, salts and minerals—such as sodium, calcium, phosphorus and potassium—in your blood.

Your kidneys also make hormones that help:

• Control your blood pressure.
• Make red blood cells.
• Keep your bones strong and healthy.

Each of your kidneys is made up of about a million filtering units called nephrons. Each nephron includes a filter, called the glomerulus and a tubule. The nephrons work through a two-step process:

1. The glomerulus filters your blood
2. The tubule returns needed substances to your blood and removes wastes.

The glomerulus filters your blood

As blood flows into each nephron, it enters a cluster of tiny blood vessels—the glomerulus. The thin walls of the glomerulus allow smaller molecules, wastes and fluid—mostly water—to pass into the tubule. Larger molecules, such as proteins and blood cells, stay in the blood vessel. 
 

The tubule returns needed substances to your blood and removes wastes

A blood vessel runs alongside the tubule. As the filtered fluid moves along the tubule, the blood vessel reabsorbs almost all of the water, along with minerals and nutrients your body needs. The tubule helps remove excess acid from the blood. The remaining fluid and wastes in the tubule become urine.