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13 Surprising Facts About the Armpit

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The human body is an amazing thing. For each one of us, it's the most intimate object we know. And yet most of us don't know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

Tucked away in that damp crevice between your arm and torso, the armpit—a.k.a. the axilla—is often the source of unpleasant odors and embarrassing wetness, and a hairy font of annoyance. But it's also an important juncture that protects important lymph nodes and soft tissue. Mental Floss spoke to microbiologist Alex Berezow, a senior fellow of biomedical science with the American Council on Science and Health, about this often overlooked spot. Here are 13 things we learned.

1. YOUR ARMPITS ARE CHOCK FULL OF LYMPH NODES.

In the small hollow of each armpit are a surprisingly large number of lymph nodes, approximately 20, in two clumps, though you can't usually feel them unless they're swollen. (One clump is closer to the surface than the other.) These lymph nodes are actually an important part of your body's immune system and serve to filter toxins out of tissues. They also produce a variety of immune cells known as lymphocytes that fight infection. In some kinds of breast cancer, these affected lymph nodes may have to be surgically removed.

2. THEY PRODUCE A DIFFERENT KIND OF SWEAT FROM OTHER BODY PARTS.

Not all sweat is created equal. In fact, your skin has two types of sweat glands that help to cool you down: eccrine glands and apocrine glands. Eccrine glands cover most of the surface of your body, and are responsible for that thin sheen of sweat on your brow and extremities during heat and exercise. However, your armpits are abundant in apocrine glands (also found in your groin). These glands are copious in places with more hair follicles, and the sweat they secrete tends to be thicker.

3. YOUR PITS ARE TEEMING WITH BACTERIA.

Your skin is home to many different kinds of bacteria, some of which are quite beneficial, collectively known as a microbiome. This microbiome can vary depending on the body part—so the bacteria on your hand can be vastly different from the moist, warm, dank environment of your armpits.

"Because of oil and sweat secretion, the armpit provides a nice home for many different kinds of bacteria," Berezow tells Mental Floss. Compared to other parts of our skin, armpits are rather densely populated, he explains. Not only that, but armpit microbiomes vary from person to person. "One study showed, after sampling nine people, that there were three types of armpit bacterial communities: One was dominated by Betaproteobacteria, a second by Corynebacterium, and a third by Staphylococcus. So one person's armpit bacteria won't necessarily be the same as somebody else's."

4. IT'S NOT YOUR SWEAT THAT STINKS.

"The secretions our armpits make don't stink. Bacteria break down the compounds, and those breakdown products stink," says Berezow. The bacteria that live in the moist crevices of your armpits interact with your sweat, which contains volatile fatty acids and odorous steroids (among other compounds). That creates a product known as thioalcohols, whose oniony, meaty scents you're likely familiar with if you've ever been stuck in a crowded elevator, subway, or gym at peak workout time.

5. SCIENTISTS ARE WORKING ON A DEODORANT THAT WOULD KILL ONLY SOME BACTERIA…

The researchers plan to engineer a deodorant that would kill only the stink-producing bacteria, instead of the entire armpit microbiome. That's because some good bacteria also live under there, like those that help protect you against fungal infections.

6. …BECAUSE REGULAR DEODORANTS CHANGE YOUR ARMPIT MICROBIOME.

…and not necessarily for the better. "Deodorants change the composition of the microbiome," Berezow says. He cites a study that found "antiperspirant reduces the number of bacteria in our armpits, but interestingly seems to encourage a greater diversity of microbes." He adds, "deodorant seems to increase the number of bacteria compared to people who don't wear deodorant."

Scientists have also found that the pits of people who usually use antiperspirants or deodorants, but stopped for a couple of days as part of the study, grew crowded with an overabundance of Staphylococcaceae—the bacteria that causes staph infections. The individuals who habitually did not use products were dominated by the friendlier—and yet stinkier—Corynebacterium. We just can't win. 

7. WHY DON'T YOUNG KIDS' PITS STINK?

While teenagers often exist in a funk so tangible you can almost see it, most children do not begin to have stinky pits until their tweens. A process called adrenarche begins around age eight for some kids (but often even later) in which the adrenal glands start to secrete hormones called androgens. While these are typically thought of as male hormones, both boys and girls produce them in different quantities. At this stage, not only can sweat start to take on its pungent stench, but children can begin to grow armpit and groin hair. Not much is understood about adrenarche, except that it may be a necessary step in order to trigger puberty. Which may explain why middle school locker rooms do tend to get whiffy.

8. WOMEN'S PITS SMELL LIKE ONIONS AND MEN'S LIKE CHEESE.

Researchers from Firmenich, a company in Geneva, set out to understand the subtle nuances in body odor to better market deodorant products to consumers. In their 2009 study, published in Chemical Senses, they discovered that your unique bouquet may be different depending on whether you're a cisgender man or woman. Women's sweat contained higher levels of an odorless sulphur-containing compound that produces a pungent oniony thioalcohol when combined with the bacteria in the underarm. Men's sweat held higher levels of a fatty acid that produced a "cheesy" scent when the bacteria of the armpit came in contact with it.

9. WOMEN DIDN'T ALWAYS SHAVE THEIR ARMPITS.

Since women were socialized to keep most of their bodies covered for centuries, exposing an armpit was an unlikely event in a public place before 1915. However, an ad in Harper's Bazaar changed everything when it suggested that in order to engage in "Modern Dancing," women should first remove their "objectionable" underarm hair. By the Roaring Twenties, many women's pits were as hairless as the day they were born.

10. SOCIAL EXPECTATIONS SHAPE OUR COMFORT WITH ARMPIT HAIR.

Despite armpit hair being as natural as the hair on our heads—and everywhere else it grows—women's armpit hair tends to be controversial. A feminist scholar set out to explore some of the reasons for this in a 2013 study in the Psychology of Women Quarterly and found that social expectations play a huge role in women seeing body hair—on themselves and on other women—as "disgusting" or simply socially unacceptable. Even women who purposely grew their pit-hair out to flout societal expectations felt self-conscious showing armpit hair in social settings.

11. …AND SO MIGHT OUR ANIMAL NATURE.

The 2013 study, conducted by a professor at Arizona State University, suggests that this revulsion with armpit hair may be a Western aversion to our primal roots as animals. Other animals send out chemical signals called pheromones to attract mates. We still don't know whether pheromones exist in humans, but plenty of evidence indicates we are highly sensitive to each other's biochemicals. If pheromones do exist, body hair around the groin and armpits could be a likely place to find them. But as "civilized" people, we believe the process of finding a partner lies in our hearts and minds—not in our armpits. Maybe one day we'll find out it's all of the above. 

12. YOUR ARMPIT LYMPH NODES MAY WARN YOU OF BREAST CANCER.

Most of the time a swollen lymph node in the armpit is little more than a sign of a cold or flu virus attacking your body. However, it can also be an early symptom of inflammatory breast cancer, an aggressive form of cancer that is best treated when caught as early as possible. Other areas that may swell in this cancer are your breast itself, and around your collarbone. If you have these kinds of sudden swellings, it's a good idea to see a doctor.

13. SOME PEOPLE GET THEIR ARMPITS BOTOXED.

A condition known as hyperhidrosis—excessive sweating—can be frustrating for those who'd like to be able to simply wear clothing they don't drench. According to Dr. Sonam Yadav, medical director of a cosmetic dermatology clinic in New Delhi, India, Botox is used to treat underarm sweating (yes, here in the U.S. too). Yadav tells Mental Floss, "It works by regulating the synergy between the neuromuscular junction and the sweat glands."

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Medicine
This Soft Artificial Heart May One Day Shorten the Heart Transplant List
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ETH Zurich

If the heart in the Functional Materials Laboratory at ETH Zurich University were in a patient in an operating room, its vital signs would not be good. In fact, it would be in heart failure. Thankfully, it's not in a patient—and it's not even real. This heart is made of silicone.

Suspended in a metal frame and connected by tubes to trays of water standing in for blood, the silicone heart pumps water at a beat per second—a serious athlete's resting heart rate—in an approximation of the circulatory system. One valve is leaking, dripping onto the grate below, and the water bins are jerry-rigged with duct tape. If left to finish out its life to the final heartbeat, it would last for about 3000 beats before it ruptured. That's about 30 minutes—not long enough to finish an episode of Grey's Anatomy

Nicolas Cohrs, a bioengineering Ph.D. student from the university, admits that the artificial heart is usually in better shape. The one he holds in his hands—identical to the first—feels like taut but pliable muscle, and is intact and dry. He'd hoped to demonstrate a new and improved version of the heart, but that one is temporarily lost, likely hiding in a box somewhere at the airport in Tallinn, Estonia, where the researchers recently attended a symposium.

Taking place over the past three years, the experimental research is a part of Zurich Heart, a project involving 17 researchers from multiple institutions, including ETH, the University of Zurich, University Hospital of Zurich, and the German Heart Institute in Berlin, which has the largest artificial heart program in Europe.

A BRIDGE TO TRANSPLANT—OR TO DEATH

Heart failure occurs when the heart cannot pump enough blood and oxygen to support the organs; common causes are coronary heart disease, high blood pressure, and diabetes. It's a global pandemic, threatening 26 million people worldwide every year. More than a quarter of them are in the U.S. alone, and the numbers are rising.

It's a life-threatening disease, but depending on the severity of the condition at the time of diagnosis, it's not necessarily an immediate death sentence. About half of the people in the U.S. diagnosed with the disease die within five years. Right now in the U.S., there are nearly 4000 people on the national heart transplant list, but they're a select few; it's estimated that upwards of 100,000 people need a new heart. Worldwide, demand for a new heart greatly outpaces supply, and many people die waiting for one.

That's why Cohrs, co-researcher Anastasios Petrou, and their colleagues are attempting to create an artificial heart modeled after each patient's own heart that would, ideally, last for the rest of a person's life.

Mechanical assistance devices for failing hearts exist, but they have serious limitations. Doctors treating heart failure have two options: a pump placed next to the heart, generally on the left side, that pumps the blood for the heart (what's known as a left ventricular assist device, or LVAD), or a total artificial heart (TAH). There have been a few total artificial hearts over the years, and at least four others are in development right now in Europe and the U.S. But only one currently has FDA approval and CE marking (allowing its use in European Union countries): the SynCardia total artificial heart. It debuted in the early '90s, and since has been implanted in nearly 1600 people worldwide.

While all implants come with side effects, especially when the immune system grows hostile toward a foreign object in the body, a common problem with existing total artificial hearts is that they're composed of hard materials, which can cause blood to clot. Such clots can lead to thrombosis and strokes, so anyone with an artificial heart has to take anticoagulants. In fact, Cohrs tells Mental Floss, patients with some sort of artificial heart implant—either a LVAD or a TAH—die more frequently from a stroke or an infection than they do from the heart condition that led to the implant. Neurological damage and equipment breakdown are risky side effects as well.

These complications mean that total artificial hearts are "bridges"—either to a new heart, or to death. They're designed to extend the life of a critically ill patient long enough to get on (or to the top of) the heart transplant list, or, if they're not a candidate for transplant, to make the last few years of a person's life more functional. A Turkish patient currently holds the record for the longest time living with a SynCardia artificial heart: The implant has been in his chest for five years. Most TAH patients live at least one year, but survival rates drop off after that.

The ETH team set out to make an artificial heart that would be not a bridge, but a true replacement. "When we heard about these problems, we thought about how we can make an artificial heart that doesn't have side effects," he recalls.

USING AN ANCIENT TECHNIQUE TO MAKE A MODERN MARVEL

Using common computer assisted design (CAD) software, they designed an ersatz organ composed of soft material that hews closely to the composition, form, and function of the human heart. "Our working hypothesis is that when you have such a device which mimics the human heart in function and form, you will have less side effects," Cohrs says.

To create a heart, "we take a CT scan of a patient, then put it into a computer file and design the artificial heart around it in close resemblance to the patient's heart, so it always fits inside [the body]," Cohrs says.

But though it's modeled on a patient's heart and looks eerily like one, it's not identical to the real organ. For one thing, it can't move on its own, so the team had to make some modifications. They omitted the upper chambers, called atria, which collect and store blood, but included the lower chambers, called ventricles, which pump blood. In a real heart, the left and right sides are separated by the septum. Here, the team replaced the septum with an expansion chamber that is inflated and deflated with pressurized air. This action mimics heart muscle contractions that push blood from the heart.

The next step was to 3D-print a negative mold of the heart in ABS, a thermoplastic commonly used in 3D printing. It takes about 40 hours on the older-model 3D printers they have in the lab. They then filled this mold with the "heart" material—initially silicone—and let it cure for 36 hours, first at room temperature and then in an oven kept at a low temperature (about 150°F). The next day, they bathed it in a solvent of acetone, which dissolved the mold but left the printed heart alone. This process is essentially lost-wax casting, a technique used virtually unchanged for the past 4000 years to make metal objects, especially bronze. It takes about four days.

The resulting soft heart weighs about 13 ounces—about one-third more than an average adult heart (about 10 ounces). If implanted in a body, it would be sutured to the valves, arteries, and veins that bring blood through the body. Like existing ventricular assist devices and total artificial hearts on the market, it would be powered by a portable pneumatic driver worn externally by the patient.

FROM 3000 TO 1 MILLION HEARTBEATS

In April 2016, they did a feasibility test to see if their silicone organ could pump blood like a real heart. First they incorporated state-of-the-art artificial valves used every day in heart surgeries around the world. These would direct the flow of blood. Then, collaborating with a team of mechanical engineers from ETH, they placed the heart in a hybrid mock circulation machine, which measures and simulates the human cardiovascular system. "You can really measure the relevant data without having to put your heart into an animal," says Cohrs.

Here's what the test looked like.

"Our results were very nice," Cohrs says. "When you look at the pressure waveform in the aorta, it really looked like the pressure waveform from the human heart, so that blood flow is very comparable to the blood flow from a real human heart."

Their results were published earlier this year in the journal Artificial Organs.

But less promising was the number of heartbeats the heart lasted before rupturing under stress. (On repeated tests, the heart always ruptured in the same place: a weak point between the expansion chamber and the left ventricle where the membrane was apparently too thin.) With the average human heart beating 2.5 billion times in a lifetime, 3000 heartbeats wouldn't get a patient far.

But they're making progress. Since then, they've switched the heart material from silicone to a high-tech polymer. The latest version of the heart—one of which was stuck in that box in the Tallinn airport—lasts for 1 million heartbeats. That's an exponential increase from 3000—but it's still only about 10 days' worth of life.

Right now, the heart costs around $400 USD to produce, "but when you want to do it under conditions where you can manufacture a device where it can be implanted into a body, it will be much more expensive," Cohrs says.

The researchers know they're far from having produced an implantable TAH; this soft heart represents a new concept for future artificial heart development that could one day lead to transplant centers using widely available, easy-to-use design software and commercially available 3D-printers to create a personalized heart for each patient. This kind of artificial heart would be not a bridge to transplantation or, in a few short years, death, but one that would take a person through many years of life.

"My personal goal is to have an artificial heart where you don't have side effects and you don't have any heart problems anymore, so it would last pretty much forever," Cohrs says. Well, perhaps not forever: "An artificial heart valve last 15 years at the moment. Maybe something like that."

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9 Interesting Facts About the Ribs
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The human body is an amazing thing. For each one of us, it's the most intimate object we know. And yet most of us don't know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

Ribs are not just an incredibly tasty order on the menu at the nearest steak house: These bony spindles in your torso attach to your sternum (breast bone) to protect your lungs, heart, spleen, and most of the liver and help with giving shape to your chest cavity, which assists in breathing. Ribs are protective on the one hand, but if badly broken, your ribs can turn lethal to you, piercing your organs. Mental Floss spoke to John Martinez, MD, an urgent care provider with Dignity Health Medical Foundation in California for these nine fascinating facts about the ribs.

1. YOUR RIBS MOVE LIKE A BUCKET HANDLE.

The ribs allow chest expansion for breathing, Martinez explains. "They function similarly to the bucket handle on a bucket and swing upwards as we take a breath, allowing the thoracic cavity to expand." This increase in the thoracic cavity makes it easier to take a breath.

2. YOU HAVE THREE TYPES OF RIBS.

The human skeleton has 12 pairs of ribs. Working from the top of the torso down, ribs 1 to 7 are considered "true ribs," as they connect directly from the spine to the sternum, Martinez says. Ribs 8 to 10 are called "false ribs" because they don't connect directly, but have cartilage that attaches them to the sternum. Ribs 11 and 12 are called "floating ribs" because they only connect to the spine in back. These, he says, "are much shorter."

3. THIS MYTH ABOUT WOMEN'S RIBS PERSISTS.

In an effort to prove the Bible story of Eve as wrought from Adam's rib "true," pastors and Sunday School teachers sometimes pass along a tale that women have more ribs than men. It's not true (and that story is sexist, anyway). Gender plays no part in the number of ribs you have: It's 12 ribs for everyone. However, women's ribs are about 10 percent smaller in volume on average than men's ribs.

4. IN RARE INSTANCES, HUMANS CAN HAVE A "GORILLA RIB."

In rare cases, which have nothing to do with gender, a human might turn up sporting extra lumbar ribs, for a total of 13 pairs of ribs, much like our distant cousins, the gorillas. Thus, it's colloquially known as a "gorilla rib."

5. RIBS ARE THE REASON NEANDERTHALS DIDN'T NEED BELTS.

Neanderthals had wider, thicker rib cages than we modern slim-waisted humans, which would have made them terrible models for skinny jeans. A 2016 study in American Journal of Physical Anthropology found that the Ice-Age diet is likely responsible for the larger ribcage and wider pelvis in Neanderthals. Essentially, carbs were scarce and fat was abundant. This led to an enlarged liver, kidneys and "their corresponding morphological manifestations," the authors write. In other words, they needed more space to house bigger organs.

6. TWO VERY DIFFERENT KINDS OF ATHLETE SHARE ONE COMMON INJURY. 

"Rowers and baseball pitchers are the most common athletes to suffer from stress fractures of the ribs," Martinez says. This is caused by the serratus anterior muscle pulling on the delicate ribs. "Other athletes that may be more likely to suffer from rib stress fractures include golfers, dancers, weightlifters and volleyball players," he adds.

7. YOU CAN SNEEZE YOUR WAY TO A RIB FRACTURE.

"True" rib fractures—where the bone breaks all the way through—are usually from traumatic events such as "a hard football tackle [or] car accident," Martinez says. However, sometimes a rib fracture can occur "from sneezing or coughing due to the force of the contracting chest wall muscles on the ribs." Treatment for true rib fractures is the same as rib stress fractures.

8. WAIST TRAINING USED TO BE ALL THE RAGE … AND STILL SOMETIMES IS.

Women have historically worn corsets, undergarments that cinch the torso in, particularly at the waist, bringing ribs and organs closer together for a smaller waist and more prominent bust. Despite corsets having gone out of fashion by the 1920s, when women began to prefer the looser, more flowing garments of the Flapper era, a number of contemporary women still wear them for reasons ranging from aesthetics to performance art, spawning a practice known as tight lacing or waist training. In this movement, women actively whittle their waists down to exceptionally small circumferences.

Doctors warn that there is risk of permanent damage to squished organs, as well as such uncomfortable syndromes as acid reflux syndrome and back pain. But it doesn't stop those who love the look.

9. THE GUINNESS RECORD HOLDER FOR A TINY WAIST IS 15 INCHES.

2011 Guinness Book of World Records winner Cathie Jung got her waist down to 15 inches through tight lacing by wearing corsets 24 hours a day, and moving down to smaller and smaller sizes. Her waist now has the same circumference as a regular jar of mayonnaise.

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