13 Fundamental Facts About the Anus

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.