Why do humans have two kidneys?

Humans are born with two kidneys, and while one healthy kidney is enough to survive, having two offers important advantages. The reason we have a pair is not just about filtering blood more efficiently—it is also about safety, balance, and adaptability.

Each kidney contains about one million filtering units called nephrons, which remove waste, excess water, and toxins from the blood. Together, both kidneys filter about 180 liters (48 gallons) of blood every day. Having two allows this process to happen quickly and continuously, supporting fluid balance, blood pressure control, and waste removal at all times.

But the real benefit of having two kidneys is built-in backup. If one kidney becomes damaged, injured, or has to be removed, the other can usually take over and continue doing the job. This is why kidney donors can give away one kidney and still live a healthy life. The remaining kidney adapts by working harder, a process called compensatory hypertrophy.

Having two kidneys also adds balance to the body. They sit on either side of the spine and work in harmony to regulate blood flow and filtration. Their location and structure also reduce the risk of total kidney failure in case of trauma or disease affecting one side.

While some people are born with only one kidney—or lose one later in life—they can often live without complications. Still, having two kidneys offers extra protection and flexibility, making the body more resilient.

So while you may only need one kidney to live, having two helps your body handle more, recover faster, and stay balanced more easily.

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