Each of your two kidneys is only about the size of a fist. But this pair of organs is vital for a healthy life. Day in and day out, the bean-shaped kidneys continuously filter your blood--as much as 200 quarts of it. They remove waste materials and excess fluid and produce 1 or 2 quarts of urine. They send the filtered blood back through your body. To filter your blood, each kidney has as many as a million tiny filters, called nephrons. It's the individual nephrons that remove waste and excess fluid from your blood. The waste goes into urine made in the kidneys. Urine then flows to the bladder and eventually passes out of your body. While your kidneys are extracting waste, they're also saving any needed minerals in your blood, like potassium. Clean, filtered blood goes back into circulation with these nutrients. The kidneys also play a role in other important jobs: Regulating blood pressure. Your kidneys help control your blood pressure by adjusting the fluid levels in your blood or by making a hormone that causes your blood vessels to tighten. Making red blood cells. Red blood cells carry oxygen from your lungs. They're essential for your body to function. The kidneys produce erythropoietin, a hormone that signals the bone marrow to make more red blood cells. Keeping bones strong. Vitamin D works with calcium and phosphorus to keep your bones healthy and strong. The kidneys make vitamin D available to your body for this process. The kidneys also work to make sure you have the right balance of key electrolytes. These include sodium, potassium and phosphate. Controlling acid levels. Cells make acid when they break down. Different foods either increase or decrease the amount of acid in your body. Your kidneys help control this acid level so you keep a healthy chemical balance. With these critical functions, your overall health depends on your kidneys being in good working order. At your next health checkup, ask your doctor about ways to monitor your kidney function and prevent kidney disease.