When we can live with only one kidney, why do we have two?
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While we can live with only one kidney, it is an essential organ; if your kidneys fail, you will die very quickly without medical intervention. It is a good thing that nature gave us two of them. Having two kidneys allows them to function as backups to one another. It also means that you have excess capacity; if you need to clear a lot of fluid from the body quickly, each kidney can pick up part of the extra load, whereas people with only one kidney can run into trouble under such a physical stress. Also, if an individual develops a kidney stone, the other kidney can continue to function while the stone is passed. If there was only one kidney, stones, which are not uncommon, could kill you.
Another reason for the existence of two kidneys is the body's tendency toward symmetry. When the internal organs are developing, many of them follow a mirror-reflection pattern, which is how we get two eyes, two arms, two legs, and so on. The controlling mechanism that establishes the kidneys is part of the same system that creates symmetry on other areas of the body.
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