• Question: Why do we have two kidneys if we only need one?

    Asked by NotSally to Ceri, Marikka, Matt, Rob, Sally on 18 Nov 2014.
    • Photo: Ceri Dare

      Ceri Dare answered on 18 Nov 2014:

      Probably because in evolution, right the way back to early ancestors which looked a lot like fish, humans have always been symmetrical down the middle, so it’s easier to make a symmetrical pair of kidneys than just one. Good question!

    • Photo: Sally Cutler

      Sally Cutler answered on 18 Nov 2014:

      Hi, probably as it is great to have a spare! Transplantation technology is pretty dependent upon this.

    • Photo: Robert Hampson

      Robert Hampson answered on 19 Nov 2014:

      Great question!

      We only need one third of our liver to stay alive, we can survive with only one lung, we definitely don’t really need two eyes and two ears for survival…

      Actually having two of most of these things is fairly useful. Having two eyes allows us to have much better perception of distance and depth, also if one eye is damaged the other can compensate. Having two lungs allows us to much better perform daily tasks, although survival is possible with only one it reduces lung capacity and therefore general capacity for exercise. You can survive with only a third of a lung but it will struggle to process toxins and alcohol until it grows back to its full size (its one of the only internal organ which can regenerate like this).

      The same is true with kidneys. If you remove one kidney, the other one struggles to keep up. It actually puts more pressure on the blood so it can filter it more quickly but in the process pushes a protein from the blood called albumin into the urine. Also, because of the way blood pressure is regulated, more kidney donors than usual suffer with high blood pressure.

      Essentially, it comes down to the fact that you can survive with only one of them (legs, eyes, lungs, kidneys,…) but everything just works better and you are more able to thrive when there are two. Also if anything goes wrong you still have a backup!

      Interesting medical paper I found: http://ndt.oxfordjournals.org/content/26/7/2076.full