Q & A: Why Do We Have Two Kidneys?

Welcome to the first of our Q & A sessions with Alan S. L. Yu, M.D.  Dr. Yu is a practicing nephrologist as well as a former professor and scientist at USC Keck School of Medicine.  He is now at The University of Kansas Hospital in Kansas City, Kansas.

Q: So we decided to start with a really simple question.  Can you tell me why we have two kidneys?

A: Actually that is a great question!

Having two kidneys is not essential for adult life. Adults who donate one kidney for transplantation, or who undergo unilateral nephrectomy for some reason, live a normal lifespan and have no increased risk of complications. Thus, we assume that the reason we have two kidneys is so that there is some extra reserve of kidney mass in case we sustain some renal insult (e.g. toxic injury, or traumatic injury).

Having said that, about 1 in 1000 individuals are born with one kidney (unilateral renal agenesis). The limited data available suggest that these individuals have an increased risk of hypertension, proteinuria and focal segmental glomerulosclerosis in later life. Whether this is because of the reduced renal mass since birth, or because of some other factor is unclear.

Q: Why exactly is the kidney mass important?

A: The kidney is made up of nephrons, whose job is to filter, secrete and reabsorb solutes and water. More kidney mass = more nephrons.

So there you have it. If you have any questions or comments regarding this Q & A, please post them on our blog or on Facebook and we will get them answered. Special thanks to my friend Karen for suggesting this first question! And if you have a new question you’d like answered, just ask.  We can’t dispense medical advice, but we can certainly enlighten you on the workings of the kidney and kidney disease.

2 thoughts on “Q & A: Why Do We Have Two Kidneys?

  1. Just one small request – when the Doc busts out with something impenetrable like:
    “these individuals have an increased risk of hypertension, proteinuria and focal segmental glomerulosclerosis in later life”
    Perhaps you could ask him to break that down into lay terms!
    Otherwise very interesting. I think the real answer as to why I have two kidneys is so that I could donate one to a loved one should the need arise.

  2. Great point! Hypertension refers to high blood pressure. And here’s Dr. Yu’s breakdown of proteinuria and FSG. Hope this helps.

    “Our blood normally contains a large number of essential proteins, of which the most abundant is called albumin. The glomerulus is a structure in the kidney that acts as a filter. It normally allows water and salts to pass through but not proteins. In glomerular diseases, this filter fails and proteins leak through and end up in the urine, which we call proteinuria. Many systemic diseases affect the glomerulus, of which the commonest known cause is diabetes. Some glomerular diseases are idiopathic, meaning that we do not know the underlying cause. Focal segmental glomerulosclerosis is an example of this.”

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