13 Fundamental Facts About the Anus

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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.

The anus might be the most underappreciated part of the body. Its very mention generally causes people to giggle or cringe. Yet without it, you'd be unable to eliminate waste and eventually die. In case your childhood wasn't enough to give you an accurate idea of its location, the anus is essentially the valve that stands between your poop and the toilet. It's considered the last piece of your digestive tract, essentially a two-inch long canal comprised of pelvic floor muscles and sphincters, easy to locate below either your vagina or your scrotum.

1. IT HAS TWO SPHINCTERS …

You may have heard talk of your "anal sphincter," which is not just one, but two valves. The internal sphincter is involuntary, meaning you can't squeeze it open—it's always closed up tight until it's time to poop. You can consciously hold closed the external, voluntary sphincter if you have to go to the bathroom but there's no toilet in sight, and consciously choose to open it when you're ready to poop. Thanks to your pelvic floor muscle, your poop normally doesn't come out when you don't want it to.

2. …AND ONE VERY IMPORTANT ROLE.

"The most important role is to eliminate waste," Rafael Lugo, a colorectal surgeon in Houston, tells Mental Floss. "We don't think much of it, but it's very important because if we don't eliminate waste, we have a [serious] problem." In rare cases, long-term constipation can cause illness and even death.

3. YOU REALLY SHOULD TAKE A LOOK AT IT … FOR YOUR HEALTH.

Since some of the health conditions of the anus don't cause any immediate symptoms you can feel, Lugo recommends you get comfortable looking at your anus from time to time. "We have these taboos in this society about looking at our bodies. Your body is your body. Touch it. The best place is in the shower. You have soap, feel the area. If anything is abnormal, take it to a mirror," he says. Once a year, have a professional look at it. He says he finds a lot of abnormalities that people live with and don't even know can be treated, such as fissures, hemorrhoids, and even cancer.

4. A SELF-CHECK CAN CATCH ANAL CANCER IN ITS EARLY STAGES.

"We can't control how [anal] melanoma happens, but it can be addressed early if you keep an eye on your butt. You can detect these changes early on and not die from it," Lugo advises. He recommends you "know your body head to toe. People tend to notice the dent in their car more often than they notice an issue in their body."

5. YOU DON'T HAVE TO SUNTAN TO GET ANAL CANCER.

Anal carcinoma, unfortunately, is not something you can usually feel with your hand. "It doesn't grow a tumor like a melanoma," Lugo says. "There's no bump." This kind of cancer can be genetic and is further reason to see a proctologist once a year, particularly if you have a family history of cancer.

6. ANAL SEX SLIGHTLY INCREASES YOUR RISK OF CANCER FROM THE HPV VIRUS.

Another form of anal carcinoma is caused by the human papilloma virus (HPV) that causes genital warts. While this virus can spread from a penis or vagina to an anus, chances of transmission are greater by anal sex. Unlike the convenient test for the HPV virus in the vagina, there's not a similar test for signs of HPV of the anus. This is another reason why having a professional look at your anus is a good idea. "There might be a little wart, something that looks abnormal. If you're sexually active, have multiple partners, or having high-risk sex, you should be more vigilant every few months," Lugo says.

7. THERE'S NOTHING DIRTY ABOUT THE ANUS.

So long as you handle your basic hygiene, your anus isn't dirty, Lugo insists. He says the anus doesn't require soap to stay clean, just water and wiping. In fact, soaps and shampoos can irritate and dry out the sensitive skin of the anus, so go easy on those. "It's a very complex organ that we basically disregard," says Lugo.

8. HUMANS HAVE WIPED THEIR BUTTS IN A LOT OF WAYS.

Over the centuries, before the advent of toilet paper, humans turned to a wide variety of objects with which to wipe their butts, including "leaves, grass, stones, corn cobs, animal furs, sticks, snow, seashells, and, lastly, hands," as Scientific American reported. Notice that most of these objects are hard, and you might really appreciate that squeezably soft Charmin.

9. BUTT HAIR IS NORMAL.

In your infrequent inspection of your anus, you may discover that hair grows down there. Scientists aren't entirely sure why. That hasn't stopped us laypeople from asking, though. In fact, so many SciShow viewers asked host Hank Green to tackle the subject that he dedicated an episode to it. The prevailing theories can be summed up as: 1. we simply haven't evolved away from hair there, because it doesn't get in the way of elimination or procreation; 2. it helps conduct scents that our ancient selves relied upon for reproductive purposes; or 3. it helps prevent irritation or chafing in that sensitive tissue.

10. HEMORRHOIDS ARE VARICOSE VEINS OF THE ANUS.

The veins in your anus "help the anus seal," like a closing door, says Lugo. But when you get a hemorrhoid, essentially just a swollen, dilated vein, "it's like having a shoe in the door, because there's something uneven there." Not only can hemorrhoids bleed, throb and itch, they can lead to a condition called anal seepage. "There are many modalities to take care of them, but they're not very comfortable, and you may need a surgery to take care of it," Lugo says.

11. IT IS POSSIBLE TO PREVENT HEMORRHOIDS.

The key to preventing hemorrhoids is avoiding constipation and straining on the toilet. "Prevention is important: Keep your stool soft, [take] plenty of fiber, and drink plenty of water," Lugo advises.

12. TRY NOT TO SIT ON THE TOILET FOR MORE THAN FIVE MINUTES.

Don't spend more than five minutes on the toilet, Lugo advises. "You shouldn't be sitting there having a social event." When you sit on the toilet seat, gravity exerts its pull on tissues, including hemorrhoids. "When your butt is hanging there, all the blood [flows] into there. Hemorrhoids are like balloons, they stretch and contract."

13. BURNING SENSATION? IT MAY NOT BE WHAT YOU THINK.

A burning or hot sensation of the anus is not always a hemorrhoid. It could be an anal fissure, or a tiny tear in the lining of the anus. "This can happen after a large bowel movement, especially if you already suffer constipation, which can stretch the anus," Lugo says. These can be treated by sitting in a hot bath for 20 minutes a day until the tissue heals. In cases where the fissure recurs and won't go away, surgery may be necessary.

25 Amazing Facts About the Human Body

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The human body is an amazing piece of machinery—with a few weird quirks.

  1. It’s possible to brush your teeth too aggressively. Doing so can wear down enamel and make teeth sensitive to hot and cold foods.

  2. Goose bumps evolved to make our ancestors’ hair stand up, making them appear more threatening to predators.

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  1. Wisdom teeth serve no purpose. They’re left over from hundreds of thousands of years ago. As early humans’ brains grew bigger, it reduced space in the mouth, crowding out this third set of molars.

  2. Scientists aren't exactly sure why we yawn, but it may help regulate body temperature.

  3. Your fingernails don’t actually grow after you’re dead.

  4. If they were laid end to end, all of the blood vessels in the human body would encircle the Earth four times.

  5. Humans are the only animals with chins.

    An older woman's chin
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    1. As you breathe, most of the air is going in and out of one nostril. Every few hours, the workload shifts to the other nostril.

    2. Blood makes up about 8 percent of your total body weight.

    3. The human nose can detect about 1 trillion smells.

    4. You have two kidneys, but only one is necessary to live.

    5. Belly buttons grow special hairs to catch lint.

      A woman putting her hands in a heart shape around her belly button
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      1. The satisfying sound of cracking your knuckles comes from gas bubbles bursting in your joints.

      2. Skin is the body’s largest organ and can comprise 15 percent of a person’s total weight.

      3. Thumbs have their own pulse.

      4. Your tongue is made up of eight interwoven muscles, similar in structure to an elephant’s trunk or an octopus’s tentacle.

      5. On a genetic level, all human beings are more than 99 percent identical.

        Identical twin baby boys in striped shirts
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        1. The foot is one of the most ticklish parts of the body.

        2. Extraocular muscles in the eye are the body’s fastest muscles. They allow both of your eyes to flick in the same direction in a single 50-millisecond movement.

        3. A surgical procedure called a selective amygdalohippocampectomy removes half of the brain’s amygdala—and with it, the patient’s sense of fear.

        4. The pineal gland, which secretes the hormone melatonin, got its name from its shape, which resembles a pine nut.

        5. Hair grows fast—about 6 inches per year. The only thing in the body that grows faster is bone marrow.

          An African-American woman drying her hair with a towel and laughing
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          1. No one really knows what fingerprints are for, but they might help wick water away from our hands, prevent blisters, or improve touch.

          2. The heart beats more than 3 billion times in the average human lifespan.

          3. Blushing is caused by a rush of adrenaline.

12 Intriguing Facts About the Intestines

When we talk about the belly, gut, or bowels, what we're really talking about are the intestines—long, hollow, coiled tubes that comprise a major part of the digestive tract, running from the stomach to the anus. The intestines begin with the small intestine, divided into three parts whimsically named the duodenum, jejunum, and ileum, which absorb most of the nutrients from what we eat and drink. Food then moves into the large intestine, or colon, which absorbs water from the digested food and expels it into the rectum. That's when sensitive nerves in your rectum create the sensation of needing to poop.

These organs can be the source of intestinal pain, such as in irritable bowel syndrome, but they can also support microbes that are beneficial to your overall health. Here are some more facts about your intestines.

1. The intestines were named by medieval anatomists.

Medieval anatomists had a pretty good understanding of the physiology of the gut, and are the ones who gave the intestinal sections their names, which are still used today in modern anatomy. When they weren't moralizing about the organs, they got metaphorical about them. In 1535, the Spanish doctor Andrés Laguna noted that because the intestines "carry the chyle and all the excrement through the entire region of the stomach as if through the Ocean Sea," they could be likened to "those tall ships which as soon as they have crossed the ocean come to Rouen with their cargoes on their way to Paris but transfer their cargoes at Rouen into small boats for the last stage of the journey up the Seine."

2. Leonardo da Vinci believed the intestines helped you breathe.

Leonardo mistakenly believed the digestive system aided respiratory function. In 1490, he wrote in his unpublished notebooks, "The compressed intestines with the condensed air which is generated in them, thrust the diaphragm upwards; the diaphragm compresses the lungs and expresses the air." While that isn't anatomically accurate, it is true that the opening of the lungs is helped by the relaxation of stomach muscles, which does draw down the diaphragm.

3. Your intestines could cover two tennis courts ...

Your intestines take up a whole lot of square footage inside you. "The surface area of the intestines, if laid out flat, would cover two tennis courts," Colby Zaph, a professor of immunology in the department of biochemistry and molecular biology at Melbourne's Monash University, tells Mental Floss. The small intestine alone is about 20 feet long, and the large intestine about 5 feet long.

4. ... and they're pretty athletic.

The process of moving food through your intestines requires a wave-like pattern of muscular action, known as peristalsis, which you can see in action during surgery in this YouTube video.

5. Your intestines can fold like a telescope—but that's not something you want to happen.

Intussusception is the name of a condition where a part of your intestine folds in on itself, usually between the lower part of the small intestine and the beginning of the large intestine. It often presents as severe intestinal pain and requires immediate medical attention. It's very rare, and in children may be related to a viral infection. In adults, it's more commonly a symptom of an abnormal growth or polyp.

6. Intestines are very discriminating.

"The intestines have to discriminate between good things—food, water, vitamins, good bacteria—and bad things, such as infectious organisms like viruses, parasites and bad bacteria," Zaph says. Researchers don't entirely know how the intestines do this. Zaph says that while your intestines are designed to keep dangerous bacteria contained, infectious microbes can sometimes penetrate your immune system through your intestines.

7. The small intestine is covered in "fingers" ...

The lining of the small intestine is blanketed in tiny finger-like protrusions known as villi. These villi are then covered in even tinier protrusions called microvilli, which help capture food particles to absorb nutrients, and move food on to the large intestine.

8. ... And you can't live without it.

Your small intestine "is the sole point of food and water absorption," Zaph says. Without it, "you'd have to be fed through the blood."

9. The intestines house your microbiome. 

The microbiome is made up of all kinds of microorganisms, including bacteria, viruses, fungi, and protozoans, "and probably used to include worm parasites too," says Zaph. So in a way, he adds, "we are constantly infected with something, but it [can be] helpful, not harmful."

10. Intestines are sensitive to change.

Zaph says that many factors change the composition of the microbiome, including antibiotics, foods we eat, stress, and infections. But in general, most people's microbiomes return to a stable state after these events. "The microbiome composition is different between people and affected by diseases. But we still don't know whether the different microbiomes cause disease, or are a result in the development of disease," he says.

11. Transferring bacteria from one gut to another can transfer disease—or maybe cure it.

"Studies in mice show that transplanting microbes from obese mice can transfer obesity to thin mice," Zaph says. But transplanting microbes from healthy people into sick people can be a powerful treatment for some intestinal infections, like that of the bacteria Clostridium difficile, he adds. Research is pouring out on how the microbiome affects various diseases, including multiple sclerosis, Parkinson's, and even autism.

12. The microbes in your intestines might influence how you respond to medical treatments.

Some people don't respond to cancer drugs as effectively as others, Zaph says. "One reason is that different microbiomes can metabolize the drugs differently." This has huge ramifications for chemotherapy and new cancer treatments called checkpoint inhibitors. As scientists learn more about how different bacteria metabolize drugs, they could possibly improve how effective existing cancer treatments are.

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