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.

10 Smart Facts About Your Gut

Colorized scanning electron micrograph of E. coli, a common gut bacteria
Colorized scanning electron micrograph of E. coli, a common gut bacteria
National Institute of Allergy and Infectious Diseases, Flickr // CC BY 2.0

Gut feelings get all the press, but your gut may be more of a thinker than you know. Some scientists now consider it a second brain. While it won’t necessarily help you study for an exam or get a promotion, your gut can influence the chemistry of your mood, emotions, immune system, and long-term health. Research even suggests the gut can “learn” new tricks through conditioning. These powerful connections are part an emerging field of science called neurogastroenterology designed to study the gut-brain link. Here are 10 facts you may not know about your gut.

1. THE GUT DOESN'T NEED THE BRAIN'S INPUT. 

You might think of your gut as a rebel against authority. It doesn’t wait for your brain’s impulses to do the important work of digestion, because it doesn’t need to—it acts as its own “brain.” No other organ, not even the all-powerful heart, can pull that off.

2. THERE ARE MORE THAN 100 MILLION BRAIN CELLS IN YOUR GUT.

Your gut’s power to think for itself is no surprise; there are millions of neurons in its lengthy coils (9 meters of intestines, from esophagus to anus). That’s more neurons than are found in the spinal cord or peripheral nervous system.

3. YOUR GUT HAS ITS OWN NERVOUS SYSTEM.

The enteric nervous system—the controlling mechanism of digestion and elimination—is the overlord of your gut, and functions all on its own. Some scientists see it as part of the central nervous system, while others consider it its own entity. It likely evolved to give the gut the go-ahead when the “got to go” impulse strikes, without requiring the brain’s sign-off, particularly when you consider the helplessness of an infant with its brand-new brain.

4. THERE'S AN INFORMATION HIGHWAY FROM YOUR GUT TO YOUR BRAIN.

There’s one big visceral nerve embedded in your gut—the vagus nerve. Research has revealed that up to 90 percent of its fibers carry information from the gut to the brain, rather than the other way around. In other words, the brain interprets gut signals as emotions. So you really should trust your gut.

5. MOST OF YOUR SEROTONIN IS IN YOUR GUT.

Some 95 percent of your body’s serotonin, that marvelous mood molecule that antidepressant drugs like Prozac keep in your body, can be found in the gut. So, it’s no wonder that diet, medications, and antibiotics can wreak havoc on one’s mood.

6. A HEALTHY GUT MAY PROTECT YOUR BONES.

In a study of the serotonin-gut relationship, scientists discovered an unexpected link between the gut and the bones. Inhibiting the gut’s release of serotonin counteracted the bone-density reduction of osteoporosis in mice. This research is going into studies on new osteoporosis-fighting drugs.

7. RESEARCH SHOWS LINKS BETWEEN AUTISM AND HAVING FEWER STRAINS OF GUT BACTERIA. 

In as many as nine out of 10 cases, autistic people have common gut imbalances such as leaky gut syndrome, irritable bowel syndrome, and fewer strains of “good” bacteria. Research on mice is looking at possible treatments of some of the behavioral disorders of autism by balancing microbes in the guts, though many warn that such treatments can’t produce a “cure” for autism.

8. FOOD REALLY DOES AFFECT YOUR MOOD. 

Different foods, when introduced to the gut via feeding tubes, have been shown to change a person's moods without the person’s awareness of what they were "eating." Fat, for instance, increased feelings of happiness and pleasure (no surprise there) because appeared to trigger the release of dopamine—the brain’s natural opiate. Carbohydrate consumption stimulated the release of serotonin, the “feel good” neurotransmitter.

9. YOUR GUT IS YOUR BEST FRIEND IN COLD AND FLU SEASON.

Not only does your gut hold brain cells, it also houses the bulk of your immune cells—70 percent—in the form of gut associated lymphoid tissue, or GALT, which plays a huge part in killing and expelling pathogens. GALT and your gut microbiome—the trillions of bacteria that live, like an immense microbial universe, in your gut—work hard to help you get over what ails you. That’s all the more reason to be careful with the use of antibiotics, which wipe out the good bacteria along with the bad.

10. YOUR GUT CAN BECOME ADDICTED TO OPIATES.

Inside your gut are opiate receptors, which are also found in the brain. The gut is just as susceptible to addiction as the brain and may contribute to the intense difficulty some addicts have trying to kick the habit.

What Causes Hiccups?

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iStock/damircudic

The cause of hiccups depends on whom you ask. The ancient Greek physician Galen thought hiccups were violent emotions erupting from the body, while others thought they were a sign of liver inflammation. Today, evidence points to spasms in the diaphragm, the large muscle between the chest and abdomen that aids airflow during breathing. This involuntary contraction can be brought on by a number of things that might irritate the nerves that control the movement of the muscle. A full stomach, heavy boozing, rapid shifts in temperature either inside or outside of the stomach, and certain emotions like shock or excitement are all common culprits.

No matter the cause, the result is the same: The diaphragm spasms and causes us to take a quick breath. The sudden rush of air causes the epiglottis (the flap that protects the space between the vocal cords) to shut and interrupt the breath, which makes the familiar "hic" sound.

WHAT CURES THEM?

The best cure for hiccups also depends on the person you ask. Almost all cures are based on one of two principles: One type works its magic by overwhelming the vagus nerve with another sensation. The vagus nerve is a cranial nerve that innervates the stomach and conveys sensory information about the body's organs to the brain. When distracted by overwhelming information of another sort, it basically tells the brain that something more important has come up and the hiccuping should probably be stopped (vagus nerve stimulation is also used to control seizures in epileptics and treat drug-resistant cases of clinical depression). The other method for curing hiccups is to interfere with the breathing, increasing the amount of carbon dioxide in the blood, and causing the body to focus on getting rid of the of the CO2 and not making hiccups.

Swallowing a spoonful of sugar is probably the most commonly prescribed hiccup cure and falls into the first category. A teaspoon of sugar is usually enough to stimulate the vagus nerve and make the body forget all about the hiccups. Even ardent supporters of the sugar cure disagree if the sugar should be taken dry or washed down with water, though.

If this home remedy doesn't work, and your hiccups are both severe and persistent, you may need to bring out the big guns. For chronic cases like this, doctors sometimes use a cocktail of Reglan (a gastrointestinal stimulant) and Thorazine (an anti-psychotic with sedative properties) to quiet things down. In some cases that resist these drugs, Kemstro, an anti-spasmodic, is also used. Other doctors have used vagus nerve stimulators implanted in the upper chest of patients. The pacemaker-like devices send rhythmic bursts of electricity through the vagus nerve to the brain to keep the hiccup cycle in check.

Many people prefer home remedies to battle their hiccups, which may include holding your breath, gargling ice water, or breathing into a paper bag. While the same people will swear by the treatment they've been using all these years, there's no firm scientific consensus that any of them actually work. But if it helps you, isn't that all that matters?

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

An earlier version of this article appeared in 2012.

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