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Size comparison of normal pig kidney (left) and solitary pig kidney (right)
We recently posted a sheep kidney dissection video (which you can check out here), where we explained the various structures within a kidney, and how these structures work together to carry out the kidney’s functions. As we explained in the video, kidneys come in pairs of two in mammals, and have many functions such as producing glucose, activating vitamin D, regulating blood pressure, producing red blood cells, and most importantly, filtering bodily fluids via the bloodstream.
Interestingly, unlike any other organ in the human body, kidneys come in excess. Although we have 2 kidneys, did you know that we could technically survive with just one? And that sole kidney doesn’t even need to be functioning at 100%- humans can survive just fine with just one kidney working at 75% of its functional capacity.
So you may be wondering why an under-performing single kidney can compensate for the activity of 2 whole kidneys. Research shows that in situations where one kidney is removed, or is absent for any other reason, the single kidney left behind adjusts to filter just as much bodily fluids as 2 kidneys would normally do together. In a process called renal hypertrophy, the nephrons (functional unit of the kidney) within the sole kidney compensate for the removed kidney by increasing in size, allowing it to take on the extra load. It has even been observed that if a baby is born with only one kidney, that sole kidney over time can grow to a size that is equivalent to the size of two normal kidneys (~1 pound)
It is also interesting to note that renal hypertrophy can occur in instances where both kidneys are present. As we age (typically after the age of 40), our nephrons stop functioning at a rate of 1% of total nephrons per year. However, the kidneys are able to function just fine until we die, because the functional nephrons increase in size to compensate for the nephrons that lost function.
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Photo credit: https://en.wikipedia.org/wiki/Compensatory_growth_(organ)