How the Kidneys Work

The kidneys are:

  • In the upper back—one on each side of the spine
  • About the size of your fist and weigh about 4 to 6 ounces each—most people have two kidneys
  • Shaped like a kidney bean and are reddish-brown in color

The Kidneys and Renal Blood Vessels

How Kidneys Work

Location of kidneys

What Do the Kidneys Do?

The kidneys are complex organs with many functions. You can live a normal, healthy life with just one kidney. You cannot live without one working kidney. Some of the main functions of the kidneys are listed in the chart below.

What do the kidneys do?

What does this do for the body?

Filter the body’s blood of wastes

Removes excess sodium, potassium and urea (waste). They also reabsorb useful substances for the body.


Maintain fluid balance


Removes or reabsorbs water to maintain hydration (keep the right amount of fluids in the body).

Excrete acid

Filters out acids to help maintain the body’s acid and base balance.


Produce the hormone erythropoietin

Erythropoietin helps the bone marrow make red blood cells. Lack of this hormone can cause anemia.


Help activate vitamin D

Activated vitamin D is needed to help with calcium balance and bone health. Lack of vitamin D causes a disease called rickets.