Kidneys, eyes, ears, and more: Why do we have reserves?

One of the many underappreciated characteristics of the human body is that it has a lot of excess capacity. That is, our organs have more reserves than most of us will ever need.

It’s as if our bodies were designed with the idea that we might need reserves in case of illness or injury. And voilà: If all goes well, we’re born with two kidneys, not just one!

Of course, the kidneys aren’t the only example. So why are we built with natural redundancy? And which of your body parts can safely fail or be removed without affecting your health?

Why do our organs have so much reserve?

The likely answer is evolution: Early humans with a genetic makeup that produced organs with functional space were better able to survive, thrive, and reproduce than others without such a genetic makeup. As a result, genes associated with excess organ capacity—remember: two kidneys, not one—became more likely to be passed on to future generations.

Kidneys, eyes, ears, and more: Why do we have reserves?

Evolutionary ancestors without so much reserve may not have survived long enough to reproduce, and thus weren’t as successful at passing on their genes. Over thousands of years, this force of natural selection has resulted in modern humans having organs with ample reserves.

Eyes, liver, lungs, and more

Here’s just a partial list of body parts with ample reserves:

Eyes: You can be perfectly healthy with one eye, though you may lack the depth perception and wider field of vision that two eyes give you. Even losing both eyes doesn’t directly lead to poor health, though blindness can of course present challenges and affect quality of life. In addition, studies suggest that significant visual impairment may increase the risk of Alzheimer’s disease.

Ears: Although two ears allow us to locate sounds from all directions, losing hearing in one or both ears doesn’t immediately impact overall health. But as with vision loss, quality of life can be affected by hearing loss. And as with vision loss, recent studies suggest that people with hearing loss are at increased risk of developing cognitive problems.

Intestine: Relatively large portions of the small and large intestines can be removed without much impact on your health. In fact, the entire colon can be removed (a surgery called pancolectomy) without shortening a person’s life, although diarrhea or other digestive symptoms may follow. Removing a section of the intestine is a relatively common surgery (for example, for colon cancer), but removing part of the intestine does not affect health or shorten lifespan.

Kidneys: Most people can live perfectly well with just one kidney. That’s why people can donate a kidney to those in need. However, the remaining kidney has to work harder, and the risk of future kidney failure increases somewhat. In addition, an injury, infection, or other disease affecting the remaining kidney can cause kidney failure more quickly than usual.

Kidneys, eyes, ears, and more: Why do we have reserves?

Lungs: If necessary, an entire lung can be removed and you can rely on the other lung to function quite well. A lung can be removed because of a tumor, but occasionally it is done because of infection or emphysema.

Liver: A relatively large portion of the liver can be removed (assuming the rest of the liver is healthy) because there is so much “reserve” liver tissue and because the liver has the ability to regenerate.

Does this mean that many parts of our bodies are truly expendable?

Maybe. If you only think about survival, you might consider many of our body parts to be expendable. In fact, you could survive without your spleen, much of your liver, your eyes, your ears, a lung, a kidney, and other parts.

But of course there are other factors to consider besides survival, notably quality of life. So no one would suggest parting with the least useful organs without good reason.

The conclusion

It is fortunate that our organs have so much reserve: millions of people around the world owe their survival to the fact that our organs have so much redundancy. And living organ donors can donate a kidney or part of another organ to help others live well and still stay healthy.

Even if some parts are notare absolutely necessary, it is good to know that there is so much in reserve. You never know when you might need it.