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.

7 Facts About Blood

Moussa81/iStock via Getty Images
Moussa81/iStock via Getty Images

Everyone knows that when you get cut, you bleed—a result of the constant movement of blood through our bodies. But do you know all of the functions the circulatory system actually performs? Here are some surprising facts about human blood—and a few cringe-worthy theories that preceded the modern scientific understanding of this vital fluid.

1. Doctors still use bloodletting and leeches to treat diseases.

Ancient peoples knew the circulatory system was important to overall health. That may be one reason for bloodletting, the practice of cutting people to “cure” everything from cancer to infections to mental illness. For the better part of two millennia, it persisted as one of the most common medical procedures.

Hippocrates believed that illness was caused by an imbalance of four “humors”—blood, phlegm, black bile, and yellow bile. For centuries, doctors believed balance could be restored by removing excess blood, often by bloodletting or leeches. It didn’t always go so well. George Washington, for example, died soon after his physician treated a sore throat with bloodletting and a series of other agonizing procedures.

By the mid-19th century, bloodletting was on its way out, but it hasn’t completely disappeared. Bloodletting is an effective treatment for some rare conditions like hemochromatosis, a hereditary condition causing your body to absorb too much iron.

Leeches have also made a comeback in medicine. We now know that leech saliva contains substances with anti-inflammatory, antibiotic, and anesthetic properties. It also contains hirudin, an enzyme that prevents clotting. It lets more oxygenated blood into the wound, reducing swelling and helping to rebuild tiny blood vessels so that it can heal faster. That’s why leeches are still sometimes used in treating certain circulatory diseases, arthritis, and skin grafting, and helps reattach fingers and toes. (Contrary to popular belief, even the blood-sucking variety of leech is not all that interested in human blood.)

2. Scientists didn't understand how blood circulation worked until the 17th century.

William Harvey, an English physician, is generally credited with discovering and demonstrating the mechanics of circulation, though his work developed out of the cumulative body of research on the subject over centuries.

The prevailing theory in Harvey’s time was that the lungs, not the heart, moved blood through the body. In part by dissecting living animals and studying their still-beating hearts, Harvey was able to describe how the heart pumped blood through the body and how blood returned to the heart. He also showed how valves in veins helped control the flow of blood through the body. Harvey was ridiculed by many of his contemporaries, but his theories were ultimately vindicated.

3. Blood types were discovered in the early 20th century.

Austrian physician Karl Landsteiner discovered different blood groups in 1901, after he noticed that blood mixed from people with different types would clot. His subsequent research classified types A, B and O. (Later research identified an additional type, AB). Blood types are differentiated by the kinds of antigens—molecules that provoke an immune system reaction—that attach to red blood cells.

People with Type A blood have only A antigens attached to their red cells but have B antigens in their plasma. In those with Type B blood, the location of the antigens is reversed. Type O blood has neither A nor B antigens on red cells, but both are present in the plasma. And finally, Type AB has both A and B antigens on red cells but neither in plasma. But wait, there’s more! When a third antigen, called the Rh factor, is present, the blood type is classified as positive. When Rh factor is absent, the blood type is negative.

Scientists still don’t understand why humans have different blood types, but knowing yours is important: Some people have life-threatening reactions if they receive a blood type during a transfusion that doesn’t “mix” with their own. Before researchers developed reliable ways to detect blood types, that tended to turn out badly for people receiving an incompatible human (or animal!) blood transfusion.

4. Blood makes up about 8 percent of our total body weight.

Adult bodies contain about 5 liters (5.3 quarts) of blood. An exception is pregnant women, whose bodies can produce about 50 percent more blood to nourish a fetus.)

Plasma, the liquid portion of blood, accounts for about 3 liters. It carries red and white blood cells and platelets, which deliver oxygen to our cells, fight disease, and repair damaged vessels. These cells are joined by electrolytes, antibodies, vitamins, proteins, and other nutrients required to maintain all the other cells in the body.

5. A healthy red blood cell lasts for roughly 120 days.

Red blood cells contain an important protein called hemoglobin that delivers oxygen to all the other cells in our bodies. It also carries carbon dioxide from those cells back to the lungs.

Red blood cells are produced in bone marrow, but not everyone produces healthy ones. People with sickle cell anemia, a hereditary condition, develop malformed red blood cells that get stuck in blood vessels. These blood cells last about 10 to 20 days, which leads to a chronic shortage of red blood cells, often causing to pain, infection, and organ damage.

6. Blood might play a role in treating Alzheimer's disease.

In 2014, research led by Stanford University scientists found that injecting the plasma of young mice into older mice improved memory and learning. Their findings follow years of experiments in which scientists surgically joined the circulatory systems of old and young mice to test whether young blood could reverse signs of aging. Those results showed rejuvenating effects of a particular blood protein on the organs of older mice.

The Stanford team’s findings that young blood had positive effects on mouse memory and learning sparked intense interest in whether it could eventually lead to new treatments for Alzheimer’s disease and other age-related conditions.

7. The sight of blood can make people faint.

For 3 to 4 percent of people, squeamishness associated with blood, injury, or invasive medical procedures like injections rises to the level of a true phobia called blood injury injection phobia (BII). And most sufferers share a common reaction: fainting.

Most phobias cause an increase in heart rate and blood pressure, and often muscle tension, shakes, and sweating: part of the body’s sympathetic nervous system’s “fight or flight” response. But sufferers of BII experience an added symptom. After initially increasing, their blood pressure and heart rate will abruptly drop.

This reaction is caused by the vagus nerve, which works to keep a steady heart rate, among other things. But the vagus nerve sometimes overdoes it, pushing blood pressure and heart rate too low. (You may have experienced this phenomenon if you’ve ever felt faint while hungry, dehydrated, startled, or standing up too fast.) For people with BII, the vasovagal response can happen at the mere sight or suggestion of blood, needles, or bodily injury, making even a routine medical or dental checkup cause for dread and embarrassment.

What Purpose Does the Belly Button Serve?

misuma/iStock via Getty Images
misuma/iStock via Getty Images

While your eyelashes are protecting your eyes, your lungs are letting you breathe, and virtually every other part of your body—inside and out—is performing its own relatively well-known task, your belly button is just sitting there collecting lint. And while it’s true that your navel served its most important purpose before you were born, it’s not totally useless now.

According to ZME Science, back when you were a fetus, your belly button was more of a belly portal: Your umbilical cord extended from it and connected you to the placenta on your mother’s uterine wall. That way, the placenta could channel nutrients and oxygen to you through the cord, and you could send back waste.

Your umbilical cord was cut when you were born, creating a tiny bulge that left behind some scar tissue after it healed. That scar tissue is your belly button, navel, or umbilicus. Though you may have heard that the shape of your belly button is a direct result of the scissor skills of the doctor who delivered you, that’s not true. Dr. Dan Polk, a neonatologist in the Chicago area, told the Chicago Tribune that a belly button's shape “has to do with how much baby skin leads onto the umbilical cord from the baby’s body. Less skin makes an innie; more skin makes an outie.” About 90 percent of people have innies.

Regardless of how your belly button looks, you probably don’t use it on a daily basis. However, if you’ve studied anatomy, medicine, or a related field, you might recognize it as the central point by which the abdomen is divided into the following quadrants: right upper, left upper, right lower, and left lower. Another way of classifying that area is into nine regions—including the hypochondriac, lumbar, iliac, epigastric, and hypogastric regions—with the umbilical region at the very center.

Abdominopelvic regions diagram
Blausen Medical, Wikimedia Commons // CC BY 3.0

Your belly button can also serve as the opening for laparoscopic surgery, which can save you from having a scar elsewhere on your abdomen.

The navel is a great central landmark outside of medicine, too. If you’ve taken yoga or Pilates classes, you may have heard it referred to as the center of balance or center of gravity. Because it sits right on top of your abdominal muscles, your belly button is an easy marker for your instructor to mention when they want you to access your core, which helps you balance.

And, of course, belly buttons are notorious for storing quite a bit of lint, which always seems to be blue (you can learn more about that here).

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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