The kidneys are bean-shaped organs, each about the size of a fist. They are just below your ribcage, one on either side of your spine, at the back of the body. Every day the kidneys filter about 200 liters of fluid, removing 2 to 3 liters (more than 3 to 5 pints) of water and waste products from the blood. If the kidneys did not remove them, these wastes would build up in the blood and damage the body.
Blood is moved through the kidneys in blood vessels. Tiny filters within the kidneys act as a sieve. They keep proteins and cells in the blood where they are needed. Extra fluid and waste are passed to the bladder in urine. The bladder stores urine until you go to the toilet. The kidneys also produce a hormone called erythropoietin, which is needed to make red blood cells. Healthy kidneys help keep your blood pressure stable. And, kidneys are important for absorbing the right amount of calcium and phosphate, which helps keep your bones strong.
What Kidneys Do
The main job of kidneys is to help keep your body chemistry in balance all the time. To do this, kidneys:
Make Urine
Your kidneys make urine to get rid of wastes and extra water. Wastes come from some foods, breaking down medicines, and even just moving your muscles.
Balance Minerals
Your muscles, blood vessels, nerves, and bones need precise amounts of minerals in your blood all the time. Kidneys sense the levels of minerals in your blood. They hold onto what you need and send the rest to your bladder, as urine.
Control Your Blood Pressure
Kidneys keep water and salt(s) in balance in your blood. And, they make an enzyme (renin) that helps your blood vessels tense up to raise your blood pressure if it drops too low. High blood pressure can harm kidneys and cause CKD. Or, CKD can cause high blood pressure. Like the chicken and the egg, it can be hard to tell which came first.
Help You Make Red Blood Cells
Each cell in your body needs oxygen to live, and red blood cells bring it. If you have too few red blood cells (anemia), your kidneys send out a hormone (erythropoietin, or EPO). EPO tells your bone marrow to make more red blood cells. Kidneys that don’t work well make less EPO.
Keep Your Bones Strong
You need the right level of calcium in your blood all the time to live. Your bones are a “storage bank” for calcium and phosphorus. When you need more blood calcium, the kidneys send out a hormone. Active vitamin D is a hormone that lets your gut absorb calcium from foods. If the hormone signal does not work, your body will pull calcium out of your bones, which can make them weak and more likely to break.
Help Keep Acid and Base Balance in Your Body
The pH in your body is close to neutral, but may be a little alkaline, or base (7.38 to 7.42). A pH that is too high or too low can be fatal. Kidneys work with your lungs to keep the right pH level.