11 Facts About the Kidneys

Kidneys are kind of like the Brita filters of the human body. Each of these bean-shaped organs is only about the size of a fist, but they serve several vital functions. In addition to ridding your body of waste, the kidneys also help make red blood cells and regulate your blood pressure. If they aren’t kept healthy, though, they can cause a variety of kidney disease symptoms, from kidney stones to infections that could require a kidney transplant. Here are 11 facts you might not know.

1. Your pair of kidneys is lopsided.

Kidneys are located in the lower back—right below the rib cage—and they’re usually asymmetrical. Your right kidney tends to be smaller and sit a little lower than your left one because it needs to make room for the liver, whose bulkiest part is situated on the right side of your body. Your left kidney, on the other hand, has some more room to sprawl out below the spleen, an organ of smaller stature.

2. There’s a reason why you only need one kidney to live.

Most people have two kidneys at birth, but only one kidney is needed to lead a healthy life. Each kidney has about 1.5 million blood-filtering units called nephrons, which help remove a waste product called urea from blood as it flows through the organs. People only need a minimum of 300,000 nephrons to filter blood properly, and one kidney is more than enough to fulfill this purpose. (People without healthy kidneys can survive with dialysis, a process where blood is filtered by machine, or opt for a kidney transplant.)

3. Your kidneys filter about 45 gallons of blood per day.

Although the heart is responsible for pumping blood throughout the body, the kidneys are doing their fair share of work, too. They filter a half-cup of blood every minute, which works out to be 45 gallons of blood per day—or enough to fill a small bathtub.

4. Kidneys make pee.

As part of the body’s urinary system, kidneys create urine from urea, water, and other waste products. The fluid flows from the kidney’s tubules, which are found inside the nephrons, to two tubes called ureters. The ureters then release the urine into the bladder—and you know what happens from there. But if problems occur, the urine can get backed up and cause kidney infections. In addition, in cases where minerals in the urine crystallize, kidney stones can form.

5. Ancient Egyptians may have been the first people to describe kidneys.

Prior to 2018, it was commonly thought that ancient Egyptians had no knowledge of the kidneys, even though their understanding of medicine and the human body was advanced in other ways. That changed when an Egyptian papyrus dating back some 3500 years revealed otherwise. It contained the world’s first known description of the kidneys, among other medical insights.

6. Kidneys are mentioned frequently in the Bible.

The kidneys are cited more than 30 times in the Bible—far more frequently than the heart, which was rarely mentioned. According to a 2005 article in the Journal of the American Society of Nephrology by Baylor College of Medicine professor Garabed Eknoyan, it was not uncommon for symbolic meaning to be ascribed to various organs in ancient Middle Eastern texts. “Unlike most ancient literature, however, the kidneys receive special attention in the Bible as the seat of conscience, emotions, desire, and wisdom,” Eknoyan wrote. “The broader region of the loins, which according to the Oxford English Dictionary is implied in the now archaic term ‘reins,’ is considered the site of physical strength and prowess.”

7. A procedure for removing kidney stones used to be deadly.

These days, if a patient is unable to pass a kidney stone naturally (albeit painfully), laser and high-frequency sound wave treatments can be used to break the hard mineral deposits into smaller pieces. For much of history, though, a patient’s only option was to go under the knife. Kidney stone surgeries were common from the 16th to 18th centuries, and one of the procedures involved cutting open the perineum, inserting a cutting instrument into the bladder, and chopping up the stone manually. Self-taught surgeon Frère Jacques Beaulieu came up with this technique, but it wasn’t without serious risk. In 1698, 25 of the 60 patients he operated on died.

8. A Dutch doctor used sausage casing, orange juice cans, and a washing machine to invent an "artificial kidney" that predated dialysis.

At the start of World War II, not long after Germany invaded the Netherlands, Dutch doctor Willem Kolff got to work inventing an artificial kidney that evolved into modern-day dialysis. Because supplies were limited during the war, he wrapped semipermeable sausage casings around a wooden drum to create his kidney machine. A patient’s blood was pumped into the casings, and the drum was rotated to remove impurities. Later, he improved his invention by adding orange juice cans and a washing machine to the mix. Some of his earliest patients with kidney failure died after a few days, but in 1945, one woman lived seven more years thanks to Kolff’s machine. When Kolff wasn’t busy creating artificial organs, he was saving lives in other ways: He also established Europe’s first blood blank and helped more than 800 people avoid Nazi concentration camps by hiding them in his hospital.

9. Drinking too much water can be bad for kidneys.

Staying hydrated helps keep your kidneys in good working order, but on the flip side, you don’t want to drink too much water. Doing so can cause a condition called hyponatremia, which occurs when the sodium in the blood becomes diluted because the kidneys can’t get rid of the fluid fast enough. The condition can be severe, causing swelling of the cells. It's uncommon, though, and it mainly occurs among athletes who overexert their bodies and drink extra water to compensate. So how much water is the right amount? It varies from person to person, but the Health and Medicine Division of the National Academies of Sciences, Engineering, and Medicine suggests that women drink around 9 cups (2.2 liters) per day, and that men drink about 13 cups (3 liters).

10. Too much ibuprofen and aspirin can also harm kidneys.

All drugs pass through your kidneys, so you want to be careful what you’re feeding them. If used daily for long periods of time, pain medications like ibuprofen, higher-dose aspirin, and naproxen (Aleve) can damage kidneys and potentially cause a disease called chronic interstitial nephritis. However, taking a daily low-dose aspirin to prevent heart attacks has no effect on kidney function.

11. Climate change may be causing an uptick in chronic kidney disease.

Recent research shows that chronic kidney disease is becoming more prevalent in Central America and parts of Asia, especially among manual laborers who spend most of their day outdoors. Although diabetes and high blood pressure are the main causes of impaired kidney function, these factors were ruled out among workers in El Salvador, Sri Lanka, India, and other countries. Other environmental factors may be involved, but researchers say the extreme heat is largely to blame—and climate change is only making it worse. For one, the more someone sweats, the more dehydrated they become. Over time, this can result in severe kidney damage.

“This can be considered the first disease that’s related to climate change,” Dr. Roberto Lucchini, an environmental medicine and public health professor at Mount Sinai’s Icahn School of Medicine in New York City, tells Mental Floss. He says the problem is so severe in Guatemala that job applicants’ creatinine levels are tested before they’re hired to work an outdoor job. Creatinine is a waste product that gets removed from blood by the kidneys, and if those levels are too high, it could signal a greater risk of kidney disease. Two other studies suggest the problem is even starting to affect outdoor workers in warmer parts of the U.S., including California and Florida. “If this continues as a general trend towards increased temperature, this is concerning,” Lucchini says.