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Welcome to the lab everyone.
Today's topic is going to center around a substance you produce on a day-to-day basis
and that substance would be feces or what some refer to as poop or other four-letter
words.
Now, you might be thinking why am I watching a video on this or why is this even important
for us to discuss?
Well, let me see if I can pick your interest with this question; if I asked you, what day
of the year do American produce the most poop or have the most bowel movements, what day
would you pick?
Well, I can tell you this, one of the busiest days for plumbers is the day after Thanksgiving.
What we refer to as Black Friday but what many plumbers refer to as Brown Friday for
obvious reasons and it makes sense you eat more you produce for.
So, in honor of us producing more poop and having more bowel movements around the holidays,
we're going to talk about all the mechanics, the muscles involved and the coordination
required for you to sit down on the toilet and have a successful bowel evacuation.
You will not be disappointed and frankly, I think you'll be quite relieved to understand
how this process works that we all participate in so that we no longer have to take pooping
for granted.
So, let's do this.
So, to help us understand how we poop or have a bowel movement, we need to talk about a
specific part of the bowel known as the large bowel or more frequently referred to as the
large intestine.
So, let's take a look at the cadaver dissection here on the tray.
This is a large intestine about how it would be oriented in the abdominal cavity.
If I just stepped away and essentially come up across down and then curve outward and
the large intestine has specific segments or specific names here.
FYI, this is the very end of the small intestine but the large intestine starts here with this
pouch known as the cecum then comes up as the ascending colon, the transverse colon,
the descending colon, the curved parts, the sigmoid colon, and then we have the straight
part of the rectum.
So, large intestine is from cecum all the way to the rectum here.
Now, fun little FYI quiz question here - there's something missing on this particular cecum
that should be there.
If you know the answer to that, go ahead and throw it in the comments and we'll pin the
answer at the top in a few days but coming back to this whole idea of what is the function
of the large intestine and how does it contribute to the formation of poop and bowel movements.
Let's talk a little bit about digestion and absorption.
The majority of digestion and absorption has taken place as foods pass through the stomach
through the small intestine which does the majority of the absorption and anything that
is left over at the end of the small intestine will be deposited into the large intestine
and those are things like indigestible fibers that we can absorb and what's left over is
going to contribute to our stool or our poop and at this point, it's more of a liquid form
and one of the functions of the large intestine as those substances are passing through is
it's going to absorb the last part of water and electrolytes.
Now, again the majority of the water was absorbed in the small intestine about 90%, but that
10% of absorption as that stool is passing through here is still significant amount to
help contribute to the body's water balance but as it's passing through and that water
is being sucked out of there, it's changing from more of a liquid state into more of a
semi solid to a solid state and that's an important function of the large intestine
but another important function, as we've already kind of implied here is propel the feces further
and further downstream and eventually to get evacuated and deposited into the toilet, hopefully.
So, let's talk a little bit more detail about this function of propelling or moving featal
contents further downstream within the large intestine.
If we are to go into the wall of the large intestine, we would see that there are two
layers of smooth muscle fibers.
Now, smooth muscle is under involuntary control, meaning the nervous system controls it automatically
in the background so you don't have to think about it.
That's different than the voluntary control say like your biceps or other skeletal muscles
that you can contract whenever you want.
But back to these two smooth muscle layers, the first layer we're going to talk about
is referred to as the circular muscle layer because it encircles the wall of the tube
and when that layer contracts, it constricts the diameter or has this sync reaction.
The other layer is oriented at 90 degrees to that circular layer there and runs parallel
with the length of the tube or longitudinally and so therefore, they call it the longitudinal
muscle layer and when it contracts, it shortens the length of the tube.
So, think about how cool and how important those two layers are.
We have one that can do this to the tube and one that can do this to the tube and when
the circular and longitudinal layers contract together, that can propel the feces further
downstream and we call that peristalsis.
Now, yes, peristalsis happens in other portions of the GI tract, but in a large intestine
are baseline peristaltic contractions or in other words how often we get those contractions
throughout a minute is about 3 to 12 contractions per minute and those baseline peristaltic
contractions are actually not enough or not significant enough to initiate a bowel movement
or the act of pooping.
That is going to be helped by another form of peristaltic that's unique to the large
intestine called mass peristalsis.
Mass peristalsis is awesome.
It happens three to four times per day and it happens around the times you eat or soon
after you eat and so, it starts in the transverse column and essentially, it's this wave-like
form of contractions.
You get these waves of contractions that are more forceful and can quick - and can quickly
propel feces down into the rectum.
So, moving that down quickly, the rectum is going to fill or stretch and in the wall of
the rectum, there are stretch receptors.
Those will send a signal to the brain and you'll think, "I might have to go to the bathroom
now" but it also initiates this thing called the defecation reflex.
So, back to this whole idea of this vast transit of feces being pushed into the rectum and
stretching, those stretch receptors will also create a reflex arc through the nervous system
back to the descending colon, the sigmoid, and the rectum and it'll really get those
longitudinal fibers to continue to contract to shorten the length of the tube and just
how my fingers are working there, you can kind of see how that would want to push the
fecal contents outward.
Now, there's one other thing that happens in this reflex and that is something that
you can't see here.
At the end of the rectum, it would be connected to the anus and remember that circular muscle
layer.
That circular muscle layer gets really robust around the anus and they call that muscle
the internal anal sphincter and during that whole reflex arc, that muscle will have to
relax and open up and that could potentially allow feces to move through.
But think about this, so far, we have only contracted smooth muscles to get this reflex
going and if this were the only thing to occur, you still wouldn't have a bowel movement because
luckily, we have to get some other skeletal muscles involved.
So, why would I say "luckily, we're going to have to get some other skeletal muscles
involved"?
Well, remember, skeletal muscles are under voluntary control where smooth muscle is under
involuntary control and if we didn't have these skeletal muscles that we're going to
talk about in a second, all those smooth muscle contractions we previously mentioned would
cause us to pass stool and poop randomly throughout the day after that mass peristalsis occur
and we wouldn't want that to happen.
So, I think we should all have a moment where we're grateful for our skeletal muscles like
our external anal sphincter and some of our pelvic floor muscles.
So, let's start with that external anal sphincter.
Remember down here, this is the rectum here and it would be attached to the anus and we
did mention an internal anal sphincter but that was a smooth muscle.
The external anal sphincter will wrap around the anus and have a sphincter action but again,
skeletal.
So, remember when we had that stretch reflex in the actual rectum and that sent a signal
to the brain of like, "oh, I need to go to the bathroom".
That's the moment where you decide if you're going to go to the toilet and relax the external
anal sphincter and some other pelvic floor muscles versus really constrict those down
and pinch them off and say, "I'm going to wait for a little while until it's a little
bit more convenient for me".
So, we've men the external anal sphincter.
You'll have to relax that in order for stool to pass through but there's also these pelvic
floor muscles that have a sphincter action.
Before I show you the dissection, I want to orient you on Geoffrey, the skeleton here.
We're going to show you a dissection where we've cut about through the midline of the
pelvis.
We're going to look at the inside of the pelvic cavity on the left side there to look at this
bowl-like form of pelvic floor muscles and you'll be looking at the left side here.
So, let me do some musical trays here and show you another dissection here and slide
that into view and like I said, here is that midline cut here and we're going to look into
the left side of the pelvis and I'm going to rotate here.
So, these muscles that we're going to talk about do have a sphincter action.
The first one we're going to mention is these fibers here that are referred to as the puborectalis.
Right next door, we're going to see some other fibers that are called the pubococcygeus muscle
and the third over here called the iliococcygeus.
Now, these muscles are going to come around and the anus would be about right where my
finger is coming here and these will circle around and again, create that sphincter action.
Now, they can also kind of create this puborectal sling especially the puborectalis muscle fibers
that I mentioned here.
Those muscle fibers really kind of sling around the anus and come attached to the right side
of the pelvis that we've removed there and kind of think of the sling that - as that
puborectalus is contracted, it kind of kinks the tube and doesn't allow for stool to pass
through very well, but when we relax that muscle, then it will almost straighten or
take the kink out of the tube and allow stool to pass through more freely.
Now, you anatomy geeks have maybe heard about a muscle called the levator ani muscle.
The levator ani muscle is actually a combination of those three that I mentioned, puborectalus,
pubococcygeus, and iliococcygeus and so now we're like, okay, we've Relax the external
anal sphincter, we've relaxed these other three muscles.
We've talked about four skeletal muscles and we have to relax for a stool to pass through
and to be honest, some of the stool may pass through just by the relaxation of those muscles
but I'm going to bring back the other cadaver dissection here.
Anything that's right at the end of the rectum and the anus may pass through with that.
However, in a lot of cases, we're still going to need even more skeletal muscles to help
us propel the feces outside of the body, especially if the stool is harder and larger and that's
where we're going to get some of our body wall muscles involved.
So, your body wall muscles are also referred to as your trunk or your core muscles and
the first trunk muscle that I want to mention that helps with pooping or having a bowel
movement is this muscle here called the diaphragm.
As you can see, the diaphragm is this dome-shaped muscle that's draped over the liver.
And yes, the diaphragm is your breathing muscle but this dome-shaped diaphragm, when it contracts,
it has this downward force or pulling force downward and that downward force is going
to increase the pressure in the abdominal cavity and here's the sigmoid colon right
here, so that increased pressure can help propel feces out of the body.
Now, some other muscles get involved as well and let's take a look at this other dissection.
Here is an anterior view of the abdomen.
So, here's the belly button.
We kept it there for a frame of reference, not to be creepy and we'll also look at these
muscles on the right side.
So, we're looking in right here like it would be on me and one of the first muscles we'll
mention is this muscle called the external oblique.
Then, we also have the internal oblique right here.
What's kind of cool as if you look closely, you can see the fiber orientation is at 90°
compared with the external oblique to the internal oblique and then we also have the
transversus abdominis, that third muscle down.
Now, we can't forget about our six-pack muscle here and we haven't exposed all the blocks
of the six-pack muscle because we've left some of that connective tissue on there but
this six-pack muscle is actually called the rectus abdominis.
So, the obliques, the transversus and the rectus abdominis are also going to help by
increasing the pressure, kind of compressing and contracting to help increase this pressure
of the abdominal cavity like the diaphragm was doing and therefore again helping to propel
the feces outside of the body during defecating and having a bowel movement.
But, kind of just some things to consider how forcefully you're going to have to contract
these muscles will depend on how hard the stool is and how big is.
Some of you won't have to contract very forcefully, others were really going to bear down on the
toilet and for those of you who are really contracting these muscles forcefully, we would
recommend drinking more fluid and increasing your fiber.
So, all these muscles have contracted, the smooth muscles, these skeletal muscles and
if all this occurs as it's supposed to, which normally it does happen as it's supposed to,
you can complete a wonderful bowel movement.
And two last things we need to mention - one, what about babies or infants and two, are
there ever any dysfunctional moments that adults participate in when it comes to this
defecation or this pooping or bowel movement process?
Let's start with the infants.
Infants actually don't have the ability to voluntarily control their external anal sphincter
and those other pelvic floor muscles that acted as sphincter.
So, bowel movements in infant is essentially mostly just reflexive and relies on that smooth
muscle.
Now, luckily, the stool in the early ages of life is relatively soft and passes through
relatively easily with just the smooth muscle contractions and as most parents know as they
start introducing new foods to the child or the baby, the stool changes its consistency,
it can get a little bit harder but luckily the baby starting to develop the ability to
contract those abdominal muscles and will eventually continue to gain more and more
control of the sphincter action during potty training and things of that nature.
But what about the adults?
Have adults ever had dysfunctional moments?
Now, I want to bring you back to this whole idea of when we are moving the stool through
the digestive tract and we have that stretch detected.
Now, the large intestine has stretch receptors throughout the wall of the tube and the stool
or the feces is not the only thing that can put stretch on the wall of the tube.
Gas can also do that.
All of us have had that moment where the gas really builds up in there and it's uncomfortable
and luckily, usually it's pretty straight forward to get rid of it and we relax the
external anal sphincter and those other sphincter muscles and the gas passes through and we
get some relief and the stretch gets taken off the wall of the tube and most of the time,
we have a pretty good idea if it's gas or stool.
There are those moments where usually around times where we've had diarrhea or more of
a loose stool where that area gets a little bit more gray where we're not totally sure
if it's gas or stool and unfortunately we've all had those moments where we might have
made a mistake and thought it was gas and it wasn't and there's some of you that are
extremely confident in that decision.
You're like, "Is it gas?
Is it stool?"
Yeah, it's totally gas and you release the external anal sphincter and it was not gas.
It was the stool.
In that case, those of you who are a little overly confident, we like to refer to you
as overly confident flatulators.
And speaking of confidence, Geoffrey and I think that if you want to continue to build
confidence, not only with your bowels but also with developing your scientific mind,
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Thanks for watching and we'll see you in the next video.