Mark A.W. Andrews, associate professor of physiology at the Lake Erie College of Osteopathic Medicine, provides the following explanation:

This is an excellent question, especially because kidney disease and kidney transplants are so common (approximately 10,000 to 15,000 Americans receive kidney transplants each year). Most humans are born with two kidneys as the functional components of what is called the renal system, which also includes two ureters, a bladder and a urethra. The kidneys have many functions, including regulating blood pressure, producing red blood cells, activating vitamin D and producing some glucose. Most evidently, however, the kidneys filter body fluids via the bloodstream to regulate and optimize their amount, composition, pH and osmotic pressure. Excess water, electrolytes, nitrogen and other wastes get excreted as urine. These functions maintain and optimize the "milieu interieur" (internal environment) of the body--the fluids in which our cells live.

Life is incompatible with a lack of kidney function (though hemodialysis can act as a substitute). But unlike the case with most other organs, we are born with an overabundant--or overengineered--kidney capacity. Indeed, a single kidney with only 75 percent of its functional capacity can sustain life very well.

This overengineering supplies us with 1.2 million of the basic functional filtering element, the microscopic nephron, in each kidney. Nephrons are tiny tubes that filter the blood plasma, adjust and then return optimized fluid to the body. Under most conditions, though totaling only a few pounds, the kidneys receive about 20 percent of all the blood pumped from the heart. Each day, about 120 liters of fluid and particles enter into the nephron to be filtered.

If only one kidney is present, that kidney can adjust to filter as much as two kidneys would normally. In such a situation, the nephrons compensate individually by increasing in size--a process known as hypertrophy--to handle the extra load. This happens with no adverse effects, even over years. In fact, if one functional kidney is missing from birth, the other kidney can grow to reach a size similar to the combined weight of two kidneys (about one pound).

The kidneys filter this large amount of fluid on a daily basis because nephrons are fairly indiscriminant filters, removing all contents from the blood except for larger proteins and cells. The nephrons, however, are extremely accomplished in processing the filtrate and substances critical to survival--such as water, glucose, amino acids and electrolytes, which are actively reabsorbed into the blood. The water and waste (including urea and creatinine, acids, bases, toxins and drug metabolites) that remain in the nephrons become urine.

In addition to being able to support life with only one kidney, the renal system has other safeguards. Although nephrons stop functioning at a rate of 1 percent per year after 40 years of age, the remaining nephrons tend to enlarge and fully compensate for this demise. Evidence strongly suggests that living kidney donors are highly unlikely to develop significant long-term detrimental effects to their health, as illustrated by donors whose renal function has been assessed for up to 30 years following donation. The main problems with donors are rare instances of complications having to do with the surgery, not the lack of the kidney.

2 Comments

1. 1. onekidney 03:49 PM 6/20/10

   I found out a couple of years ago that I was born with only one kidney. This was found by sonogram while investigating possible appendicitis. Lab results reported that there is compensatory hypertrophy of my right kidney. However, there also was something noted about hydronephrosis. I never thought to ask about this since my appendix was the main focus. I am now 41 years old and wondering if I should be addressing this with a doctor or being monitored? I have started to have some back pain which may just be due to muscle pain--I've been seeing a chiropractor for this. Just wondering if I need to be pursuing something different?

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2. 2. kumard1 06:54 AM 4/2/12

   My brother is 41 years old. he was having an regular helthcheckup and today we came to know that he is having single kidney since birth. the 2nd kidney is not grown after 2.5". He never have any pain in stomach till now but he had high blood pressure in past which is now under control. there is no problem detected in single kidney. his work is related to buildings and road construction and which also involve lots of travelling and self car driving. He also take wine occassionaly but not in excessive amount.
   
   1. We wanted to know that what are the risks of having single kidney in normal life.
   2. What routine test needs to be done in order to make sure that everything is normal.
   3. what are the things to avoid in day to day life.
   4. Can it be possible to cure the 2nd Kidney and what are the options for this.
   5. can transplantation be done for 2nd kidney to make 2 kidney in working status, just to avoid risk of having single Kidney. How successful it is?
   
   

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