Your kidneys are two kidney-bean-shaped organs, each about the size of a fist, located just below the rib cage on either side of the spine. They are vital organs that perform many life-sustaining functions. They keep the blood clean and chemically balanced, and they produce several important hormones.
The Kidneys and Your Blood Approximately 50 gallons of blood enter the kidneys through the renal arteries each day. The kidneys process the blood, removing waste products, toxins and excess water. These waste products are what your kidneys use to make the half-gallon (or more or less) of urine that they produce each day.
Most of the waste products that find their way into the blood come from normal breakdown of active muscle and dietary protein. The waste products that are most commonly measured by blood tests are called creatinine and blood urea nitrogen. Water-soluble toxins (poisonous substances) are removed from the blood by the kidneys. If the kidneys didn't remove waste products and toxins, the waste products and toxins would build up in the blood and cause damage to the body.
If the kidneys didn't remove excess water, the water would accumulate in the blood and cause blood pressure to rise, the heart to become overloaded and the chemicals in the blood to become diluted or imbalanced. When there's not enough water in the body, such as during dehydration, the kidneys can switch gears and conserve salt and water by excreting less of it into the urine. Dehydration can lead to low blood pressure, shock and even death.
Chemicals in the blood, such as electrolytes, acids and bases, are kept balanced by the kidneys through several complex mechanisms. In general, these mechanisms involve excretion of excess chemicals via the urine or conservation of the chemicals when there are not enough in the blood.
After the kidneys clean the blood, the blood exits through the renal veins and returns to the circulation. Newly produced urine flows out of each kidney through a narrow tube called the ureter and empties into the bladder. When the bladder is filled with urine, the bladder walls contract to push the urine through the urethra and out of the body. Together, the ureters, bladder and urethra are the "urinary tract."
The Kidneys and Hormone Production The kidneys release three important hormones: renin, erythropoietin and calcitriol. Renin is released when the blood pressure in the body is low. This hormone causes blood vessels in the body to constrict, raising the blood pressure back toward normal. Erythropoietin stimulates the bone marrow to make more red blood cells, which carry oxygen throughout the body. Calcitriol, also known as the active form of vitamin D, or vitamin D-3, helps maintain calcium for bones and for normal chemical balance in the body.
To see a "point and click" example of how a kidney works, visit our interactive kidney illustration.