13 Facts About Skin

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Skin isn't just the outermost layer of our bodies. Without it, we couldn't do most of the things we take for granted, like breathing, moving, and keeping the body's inner workings where they belong. And while skin also evolved to keep pathogens and other bad stuff out of our bodies, consumers spend millions of dollars on products to penetrate that defense (with mixed results). Read on for more fascinating facts about the skin.

1. YOUR SKIN HAS THREE DISTINCT LAYERS.

Skin is considered an organ in its own right. It's comprised of three layers: the waterproof top layer, the epidermis; a middle layer of tougher connective tissue, hair follicles, and glands called the dermis; and the inner layer, the hypodermis, which is mostly fat and connective tissue that supports the skin's structure and attaches it to muscles.

2. SKIN COLOR IS DETERMINED BY CELLS IN THE EPIDERMIS.

Those cells are known as melanocytes, which secrete a pigmented substance called melanin; the more melanin in the cells, the darker the skin. Having too little or too much melanin can lead to some skin color disorders: On one end of the spectrum are conditions like vitiligo—which occurs when some melanocytes lose the ability to produce melanin, resulting in whitish patches on the skin—and albinism, a condition in which melanocytes don't produce any melanin. On the other end is hyperpigmentation—the presence of excess melanin, which can cause darker patches of skin.

3. YOUR SKIN COULD WEIGH MORE THAN 20 POUNDS.

"Your skin accounts for 15 percent of your body weight," says Toral Patel, M.D., a board-certified dermatologist and supervising physician at D&A Dermatology in Chicago and a clinical instructor of medicine at Northwestern University. This makes it your body's largest organ.

According to that calculation and data from the Centers for Disease Control and Prevention, an average American woman weighs 168.5 pounds and carries more than 25 pounds of skin. An average man weighing 195.7 pounds will have nearly 30 pounds of skin.

4. YOUR SKIN RENEWS ITSELF EVERY 28 TO 30 DAYS.

New cells are created in that deep layer of the skin and take about four weeks to rise to the surface. There, they grow hard and then shed. This process, in which old skin is sloughed off and replaced by newer skin, might occur more than 1000 times over the average American's lifespan. But all skin is not created equal: Its thickness varies naturally among all areas of the body. Thickness can also be affected by age, gender, and habits (like smoking) that can change the cells' elasticity and other traits. According to Patel, the skin on the soles of your feet is up to seven times thicker than the skin of your eyelids.

5. TATTOOS STAY PUT, THANKS TO CELLS CALLED MACROPHAGES.

If your skin cells shed every month, how do tattoos stick around? It turns out to be a function of your immune system. The puncture of the tattoo needle causes inflammation in the dermis, the skin's middle layer. In response, white blood cells known as macrophages are sent in to help heal the damage. These macrophages "eat" the dye and can pass it to newer macrophages when they die off, so the pigment is essentially transferred from one cell to another. Any leftover pigment is soaked up by fibroblasts, which are longer-lasting skin cells that don't regenerate as often. Only lasers designed for tattoo removal are strong enough to kill off the macrophages and fibroblasts that hold the dye.

6. YOUR SKIN IS HOST TO BILLIONS OF CREATURES.

Your skin hosts a microbiome that can contain more than 1000 types of bacteria (along with other microbes, viruses, and pathogens). These "tiny ecosystems," as Patel describes them, are mostly friendly bacteria that work in concert with our bodies for many beneficial purposes, including wound healing, reducing skin inflammation, and assisting the immune system to help fight infection. These bacteria were once thought to outnumber your own cells 10 to one, but more recent research has found the ratio is closer to 1:1.

7. ANCIENT EGYPTIANS PUT SALT (AND OTHER FOODS) IN THEIR WOUNDS.

Injuring or breaking the skin's dermis, the layer below the epidermis, can expose the inner tissues to pathogens. To prevent infections from reaching any further into the skin, body fat, or muscle, ancient Egyptians cared for topical wounds with salt (yes, really!), fresh meat, moldy bread, and onions.

While these may seem like unsanitary things to put on a cut, modern research has found that there was actually merit in their methods. With its high iron content, meat was a good blood coagulant and recommended for the first day of a wound, according to a 2016 paper in the Journal of the German Society of Dermatology. Salt and onions are both astringent, which can stop blood flow. Moldy bread likely had antibacterial properties—a very early form of penicillin, you might say. Skin wounds would then be sealed with a combination of oils, fats, honey, and plant fibers.

8. YOUR BODY'S FLUID BALANCE DEPENDS ON SKIN.

Your skin is a significant shield against billions of tiny microbes and pathogens. But just as importantly, skin keeps fluids in. Another way to think of this, Patel says, is that your skin resembles a brick and mortar pattern. The bricks are the cells. The mortar is made up of lipids, fatty acids, and other sticky proteins that form the watertight layer. "If you have any ‘holes' in skin where moisture can escape, which are more susceptible to damage, that leads to dryness, cracking, and inflammation," Patel says.

People who have suffered burns often have fluid-balance problems, says Robert T. Brodell, M.D., professor of dermatology at University of Mississippi Medical Center in Jackson, Mississippi. "Fluids are seeping out, and they can't keep them balanced internally," he tells Mental Floss. This can be incredibly dangerous, because fluid loss can cause the heart to stop pumping blood to the rest of the body. Dehydration, hypertension, and other problems may also occur when skin is injured.

9. A SKIN CONDITION CAN PUT YOU AT GREATER RISK OF OTHER DISEASES.

Psoriasis is an autoimmune condition in which the skin cells in an affected area grow rapidly, leading to excess skin buildup, inflammation, and a red and scaly rash. While it can be uncomfortable to live with the condition on its own, studies [PDF] have shown that inflammation of the skin can lead to inflammation of other tissues and internal organs, and eventually certain diseases. For example, psoriasis has been linked to a greater risk for heart disease, as well as diabetes, Crohn's disease, metabolic syndrome, and other conditions thought to be correlated with inflammation.

Patel says that association makes treatment even more important: "If one organ is inflamed, you have to make sure another isn't."

10. YOUR LEGS MAY BE THE DRIEST PART OF YOUR BODY.

Unless you live in the tropics, you've probably noticed that the skin of your lower legs becomes drier in winter—and there's a biological reason for that. "You have fewer oil glands on your legs than any other area of your body," Brodell tells Mental Floss. Oil (or sebaceous) glands, found near the dermis's border with the epidermis, secrete an oily substance called sebum that lubricates skin and hair. As people age, the glands secrete less oil, and that means drier skin. Winter's low humidity and our tendency to spend more time around heat sources dries out skin even more.

The solution is to install a humidifier or apply some moisturizer. Certain skincare products, such as those with emulsifiers like sodium laureth sulfate, can also dry out or irritate your skin, so read your labels carefully.

11. OVERHEATING IS A RISK IF YOU LACK SWEAT GLANDS.

Both types of sweat glands are also located in the dermis. Eccrine glands, found all over the body, emit sweat directly through pores in the epidermis. Apocrine glands release sweat along hair follicles, so it's no surprise that these glands are concentrated in the hairiest parts of the body—head, armpit, and groin. Both types help regulate body temperature: In hot conditions, the glands release water and fatty liquids to cool the skin.

A lack of sweat glands puts people in danger of overheating. Those with a condition known as anhidrotic ectodermal dysplasia have few to no sweat glands, so they can't properly cool off when the body overheats. "They get heatstroke easily," Brodell says. A subset of people with this disorder suffer from immunodeficiency. They produce low levels of antibodies and infection-fighting immune T- and B-cells, so they are more prone to skin and lung infections.

12. YOUR GUT AND YOUR SKIN ARE SYMBIOTIC.

The gut and the skin never come into direct contact with one another, yet research shows that the gut has a profound impact on the skin.

"The skin becomes very unhealthy when the microbiome of the gut goes into a state of dysbiosis," meaning when something attacks the gut's good bacteria, says Gregory Maguire, Ph.D., a former professor of neuroscience at UC San Diego and the founder and chief scientific officer of BioRegenerative Sciences, a stem-cell technology company.

Dysbiosis can lead to inflammation, irritation, rashes, and pain. "There's good evidence that eczema [or] atopic dermatitis is partially due to dysbiosis of the gut and skin," he says.

In a 2017 paper published in the Archives of Dermatological Research, Maguire writes that normal gut bacteria can actually calm the body's response to stress. A reduction in the release of the stress hormone cortisol, which is thought to cause breakouts, also reduces the chance of skin irritation—all thanks to microbes in your intestine.

13. RESEARCHERS ARE USING "GOOD" BACTERIA TO TREAT ACNE.

When the skin's pores get clogged with sebum from the sebaceous glands and dead cells, a condition usually associated with hormonal changes, you've got acne. Clogged pores that stay closed are called whiteheads; if the pore opens and reveals the gunk inside, it's a blackhead. (The medical term for a blackhead, an "open comedo," stems from a Latin phrase alluding to "worms which devour the body." But don't worry, blackheads are not actual worms living in your face.)

While acne may seem like a rite of passage associated with puberty, researchers are experimenting with fighting "bad" bacteria (in this case, Propiobacterium acnes, which is linked to acne breakouts) with "good" bacteria, also known as probiotics. "One of the things [probiotics] do is ferment things on the skin like ammonia and nitrites, and metabolize it and turn it into other chemicals that are beneficial to the stem cells in your skin," Maguire explains. A 2015 study in the Journal of Women's Dermatology and other research has found that applying topical probiotics like Streptococcus salivarius and Streptococcus thermophiles inhibits P. acnes and may make skin more resilient against it in the long run.

Why Is Pee Yellow?

Chloe Effron
Chloe Effron

WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Your body is kind of like a house. You bring things into your body by eating, drinking, and breathing. But just like the things we bring home to real houses, we don’t need every part of what we take in. So there are leftovers, or garbage. And if you let garbage sit around in your house or your body for too long, it gets gross and can make you sick. Your body takes out the garbage by peeing and pooping. These two things are part of your body’s excretory system (ECKS-krih-tore-eee SISS-tem), which is just a fancy way of saying “trash removal.” If your body is healthy, when you look in the toilet you should see brown poop and yellow pee.

Clear, light yellow pee is a sign that your excretory system and the rest of your body are working right. If your pee, or urine (YER-inn), is not see-through, that might mean you are sick. Dark yellow urine usually means that you aren’t drinking enough water. On the other hand, really pale or colorless pee can mean you might be drinking too much water! 

Your blood is filtered through two small organs called kidneys (KID-knees). Remember the garbage we talked about earlier? The chemicals called toxins (TOCK-sins) are like garbage in your blood. Your kidneys act like a net, catching the toxins and other leftovers and turning them into pee.

One part of your blood is called hemoglobin (HEE-moh-gloh-bin). This is what makes your blood red. Hemoglobin goes through a lot of changes as it passes through your body. When it reaches your kidneys, it turns yellow thanks to a chemical called urobilin (yer-ah-BY-lin). Urobilin is kind of like food coloring. The more water you add, the lighter it will be. That's why, if you see dark yellow pee in the toilet, it's time to ask your mom or dad for a cup of water. 

To learn more about pee, check out this article from Kids Health. 

12 Facts About Kidney Stones

Illustration by Mental Floss. Images: iStock
Illustration by Mental Floss. Images: iStock

Kidney stones are more common than ever. According to Harvard Medical School, every year more than 3 million people see a doctor for relief from these hard mineral and salt deposits, which form in your kidney when urine becomes too concentrated. Here's what we know about the condition formally called nephrolithiasis.

1. KIDNEY STONES TYPICALLY CAUSE REALLY PAINFUL SYMPTOMS.

At first you may notice your urine is cloudy, bloody, and foul smelling. Your back may begin to ache, and nausea may come over you. Then, as the stone moves from your kidney into your urinary tract or bladder, sometimes becoming trapped, there’s often an intense, stabbing pain that many people say they wouldn’t wish on their worst enemy.

2. MOST PEOPLE DEVELOP ONE TYPE OF STONE …

What kind of kidney stone you get depends on your diet, fluid intake, genetics, hereditary disorders, and even whether you take certain medications, but the vast majority of people get calcium oxalate stones. They're formed from a mix of calcium in urine and the compound oxalate, which is found naturally in food like nuts, chocolate, and some vegetables, including beets and spinach; oxalate is also produced by your liver. There's some evidence that people who take the seizure medicine topiramate can develop these stones in the form of calcium phosphate.

3. … BUT THERE ARE THREE OTHER KINDS TOO.

Struvite stones are fast-growing mineral deposits that typically develop in response to a urinary tract infection, and can grow large enough to block the kidney, ureter, or bladder before you notice any symptoms; they affect women more than men. Uric acid stones turn up in people who eat a lot of red meat, shellfish, and organ meats, which contain hefty doses of an organic compound called purine that can lead to more uric acid than the kidneys can excrete. Cystine stones are caused by a rare hereditary disorder called cistinuria in which your kidneys excrete excessive amounts of the amino acid cystine.

4. THEY'RE EXTREMELY COMMON—ESPECIALLY IN MEN.

There's a solid chance you could end up with a kidney stone. The National Kidney Foundation notes that one in 10 people will develop one during the course of their life. And if you’re male, take note: Your gender alone is considered a kidney stone risk factor. Men are twice as likely as women to develop them. Another factor is age: Although stones are most common from ages 20 through 50, they tend to peak around age 30.

5. IF YOU’VE HAD A KIDNEY STONE, YOU’LL PROBABLY DEVELOP ANOTHER ONE …

Sorry to say, but simply having a kidney stone puts you at risk for a recurrence. If you’ve had one, the U.S. National Library of Medicine notes that there’s a 30 to 50 percent chance more stones will form within five years.

6. … BUT YOU CAN TAKE STEPS TO PREVENT THEM.

Cutting back on sodium (i.e. deli meats, packaged soups, and processed foods) can help, because a stone can form from excessive salt consumption. You should also avoid too much animal protein—it produces urine containing more acid, which is known to increase your risk for kidney stones—and increase your intake of fruits, vegetables, whole grains, and low-fat dairy. And be sure to drink plenty of fluids, especially water—at least 12 glasses a day. (That's good advice for everyone, not just those prone to kidney stones.)

Don't drink much apple or cranberry juice as both contain oxalates and are linked to an increased risk of developing calcium oxalate stones. High doses of Vitamin C may boost the concentration of oxalate in urine; the Cleveland Clinic recommends a daily maximum of 500 milligrams.

7. IT'S A MYTH THAT CALCIUM CREATES SOME KIDNEY STONES.

Despite the fact that the word calcium is part of the most common kind of kidney stone, you don’t need to treat calcium as the enemy. In fact, having too little calcium can actually increase the odds you’ll get these types of stones. According to the Cleveland Clinic, eating about two or three servings of calcium-rich foods daily reduces oxalate absorption, helping to keep calcium oxalate stones away. So get out the cheese.

8. IF YOU PASS A STONE, CONGRATULATIONS! NOW TAKE IT TO A DOCTOR.

Ninety percent of kidney stones are passed through urination. Getting one out this way may hurt a lot, but once the stone has finished causing you agony, it could provide clues that could help you avoid developing another one. If you’re able to retrieve the stone, bring it to your doctor, who can order an analysis. Identifying its components can reveal the kind of stone it is and potentially point to a treatment or prevention plan.

9. IF YOU CAN’T PASS A STONE, TREATMENTS ARE AVAILABLE …

In an attempt to exit the body, a stone travels from the kidney to the bladder through a narrow tube called the ureter. If the stone is larger than a quarter-inch, it's simply too big to pass through the ureter, and will get trapped there. (If it can make it through to your bladder, it's small enough to pass out out of your body through the urethra.) This causes intense pain, blocked urine flow, and possible bleeding from urinary tract walls. That's when it's time for treatment.

There are several methods for getting rid of a kidney stone, all of which aim to break the stone into smaller pieces so they can leave the body. In an extracorporeal shock wave lithotripsy (from the Greek for "crushed stone"), high-frequency sound waves are applied externally to break stones up, allowing them to pass when you pee. Laser lithotripsy takes a similar approach: Stones in the ureter are broken up with a laser and also leave the body naturally. More invasive is percutaneous ultrasonic lithotripsy, which involves passing narrow instruments (including a fiberoptic camera) through your back to your kidney; ultrasound breaks the stones up, and then fragments are removed by an instrument. Finally, a ureteroscopy is a treatment option in which a small scope is inserted in the ureter towards the bladder to determine the stone's location. Then it's broken up for natural passage or removed altogether. Luckily, you're unconscious under general anesthesia during the last procedure.

10. … AND THEY'RE FAR SUPERIOR TO THOSE USED IN THE PAST.

Kidney stones are nothing new—mentions of the painful formations go back more than 5000 years, to Mesopotamian medical texts—and medical interventions have occurred for just as long. Stones made it into the Hippocratic Oath, in which physicians swore they would "not use the knife, not even on sufferers from stone," leaving the procedure to "such men as are engaged in this work" [PDF]. Surgeons in ancient Greece and India were attempting stone removal as far back as the 7th century BCE.

The 16th to 18th centuries were a heyday for stone surgeons, who were largely self-taught. The most notorious of them was Frere Jacques Beaulieu. He pioneered the lateral perineal lithotomy—which involved making an incision in the perineum, inserting a terrifying cutting instrument into the bladder, cutting up the stone, and then extracting the pieces with the instrument or his fingers—in the late 17th century. Unfortunately for his patients, he had no technical training, and his method was often deadly; in 1698, after 25 of his 60 patients died, he was banned from doing the procedure—but he didn't stop. He's thought to have performed more than 5000 lithotomies. (And no, the song doesn't seem to be about him.)

11. IF ALL ELSE FAILS, TRY RIDING A ROLLER COASTER.

If you’re a thrill seeker who happens to have kidney stones (and some vacation time), you may be in luck. After a "notable number" of patients reported that riding the Big Thunder Mountain roller coaster at Walt Disney World in Orlando helped them to pass their kidney stones, Michigan State University urologist David Wartinger decided to investigate. He created a kidney replica—complete with kidney stones—put it in a backpack, and let it ride the roller coaster 60 times. It worked—but passing the stones depended on where the backpack was placed in the coaster. Rides in the last car were the most effective, with the stones passing 64 percent of the time, while the front few cars yielded only a 16 percent success rate.

Big Thunder Mountain was the only ride in the theme park that was effective. Neither Space Mountain nor Aerosmith's Rock 'n' Roller Coaster did the trick, likely because they were too fast, with a G-force that pinned the stones in place. Of course, while this is an interesting finding, if you suspect you have kidney stones, speak to your doctor before you high-tail it to Walt Disney World.

12. A KIDNEY STONE THE SIZE OF A MOUSE WAS REMOVED FROM A MAN IN 2004.

The stone measured 5.11 inches at its widest point—a world record. Five years later, a whopping 2.5-pound stone was surgically removed from a man in Hungary in 2009. Perhaps seeing a bunch of kidney stones in one place other than originating from your own body will put you at ease. If that’s the case, check out the International Museum of Surgical Science in Chicago, where a collection of stones is on display in glass jars.

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