*intro music*
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 somebody 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 fifty 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 pre-capillary sphincters regulate the blood flow into our capillaries.
And 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 our 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 ileocecal 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 of 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,
the 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 have 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 would probably be surprised to learn how much research that 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 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 and urethra.
So thanks to all the sphincters out there who help 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 our Subbable supporters
for keeping us going here at SciShow.
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Look down at the comments below, or on Facebook or Twitter
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[outro theme music] [NB: Support through Patreon]