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Sphincters -- they're not just for butts! Hank explains the fascinating truth about these magic rings of muscle, where they appear in the human body and the pretty fantastic functions they perform in the animal kingdom.


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SOURCES - blowhole - artificial sphincter
Let's do a little word association. I say bird, you say nest. I say fire, you say smoke. I say sphincter, you say butt, right? That's probably why calling someone a sphincter is basically saying they're a butt. But not all sphincters control the flow of poop! We've got lots of other non-excrement related sphincters throughout our bodies. About 50 different types of sphincters, actually! Yes, they help us not poop our pants or wet ourselves, but they do so much more than that. They're some of the most important unsung heroes of our anatomy.

Broadly speaking, a sphincter is a ring-shaped muscle that flexes to open or close a biological opening. They help regulate the entrance, circulation, and exit of fluids, solids, gases, and bile throughout our digestive tracts and help move blood around our circulatory systems.

Our many sphincters vary in size and function. We've got papillary ones in our eyeballs that contract around the pupil, and our many tiny precapillary sphincters regulate the blood flow into our capillaries. The cardiac or lower esophageal sphincter keeps acidic gastric juices from splashing back up and burning your esophagus, usually. If it doesn't do its job, you may get acid reflux, and if you've got a belly full of corn dogs and are getting tossed around the Tilt-a-Whirl, don't be surprised if that little sphincter experiences a moment of weakness.

Certain sphincters are governed by autonomic nerves, which means that they function involuntarily. Others are controlled by the somatic nerves and function voluntarily, like we make them do the things they do.

For example, the involuntary ileo-cecal sphincter seals off the small intestine until the food within it is digested fully and ready to pass into the large intestine. And the rectum, as you probably know, ends with the voluntary external anal sphincter, the one that ideally gives you control over your bowel movements.

Of course, other animals have their share of similar sphincters, and some are lucky enough to get extra special ones. For example, mama marsupials like lady koalas carry their ridiculously adorable babies in secret pouches. Unlike the kangaroo's iconic upward opening pouch, a koala's pouch is backward opening, sort of upside down. These pouches have powerful sphincters at their openings that act like drawstrings to keep those fuzzy babies from falling on their furry little heads.

Likewise, you might've heard the cocktail party trivia that ducks and geese don't have anal sphincters, which would explain why poop just seems to slide out of them all the time. It also might explain why you're going to the wrong cocktail parties. You'd probably be surprised how much research has gone into this, but I can say that studies of duck anatomy done in the 1980s concluded that they do have sphincters in their butts. The difference is just that they might not have voluntary use of them.

Don't get me started on blowholes. Whether the orifices that allow mammals to breathe are true rings of muscle or some other feature has been a contentious idea. Sphincters are controversial.

And important. You can imagine how a faulty sphincter can wreak havoc on your health. One of the most common and embarrassing places for a voluntary sphincter to weaken is in the bladder and bowels, places you really don't want to lose control over.

This weakening can come from nerve damage, complications of surgery, aging, or external stresses as simple as a good hard sneeze, or you know, when something really funny happens and suddenly problems. But don't lose hope if you've got a sad sphincter. Often, those muscles can be worked back into shape with special exercises or drugs. And if things are super serious, you can have an artificial sphincter surgically inserted into your body to help maintain control of your anus or urethra.

So, thanks to all of the sphincters out there for helping keep our pants clean, our blood flowing, our food down, and baby koalas everywhere safe from brain damage. And thank you for watching and thanks to all of our Subbable supporters for keeping us going here at SciShow. If you have any questions or comments or ideas, we're down in the comments below or on Facebook or Twitter, and if you wanna keep getting smarter with us here at SciShow, you can go to and subscribe.