Control of the GI tract | Gastrointestinal system physiology | NCLEX-RN | Khan Academy

Control of the GI tract | Gastrointestinal system physiology | NCLEX-RN | Khan Academy

January 6, 2020 51 By Kody Olson


Voiceover: Other than
the brain, I’d say your intestinal tract, your digestive system, is one of the smartest organs in the body. Why is that? Well, the
digestive tract has its own nervous system, it’s got its own brain. In fact, we call it the
enteric nervous system, the enteric nervous system
because the GI tract is able to act on its own
without having to send neuronal information
or signals to the brain or the spinal cord to regulate its action. For example, if we’re in the presence of a really awesome meal,
something that’s good enough for any hero in a half shell and we consume this meal and
it goes through our mouth, to the esophagus, and it lands
in our stomach right here, and we’ve got our food right
there, what we initiate from this, thanks to our
enteric nervous system, is called the gastrocolic
reflex, the gastrocolic reflex. That involves the stomach, the
gastro part of this reflex, as well as the colon,
because what happens is that the presence of food in
your stomach, as signal one, tells the colon here,
“It’s time to make way “for food that’s coming down.” So your colon will actually,
as a response, take food that’s in here and push it further south. And it does so to make room
for the food that we’re eating. That’s the reason why after you eat you feel like going to the bathroom. It’s not because the
food you just ingested is about to come out, it’s because food that you ate awhile back has
come to the end of its journey, thanks to the gastrocolic reflex. So, neuronal control is one
mechanism our GI tract uses to control what we do
when food is present. The other component we’ll focus on, down here, is hormonal control. Now hormones, as you might
recall, are substances that are released by tissues in our body that then go through blood vessels, like this guy right here,
to go to some target organ, or target tissue to cause an effect. So, I’m going to talk about
a couple of hormones here that regulate our GI tract
when food is present. So the first hormone we’re going to talk about is called gastrin. Now gastrin is a hormone that’s released when we notice that there’s
food in our stomach. Gastrin is released out of the stomach to go to our bloodstream and
then come back out actually to the stomach to stimulate the secretion of digestive juices. So recall that when we
have food in our stomach, food in our stomach is
going to cause gastrin to be released from mucosal cells. The gastrin will then go on to cause secretion of a couple of things. One, you’re going to get stomach
acid, or hydrochloric acid, to be released from parietal
cells in the stomach, you’re going to get
pepsinogen to be released from chief cells in the stomach, and remember this is the
inactive form of pepsin that must be cleaved to become
active to digest protein. The other thing that gastrin does is that it increases stomach motility. Remember that one of the
functions of the stomach is to churn food that’s
present, it’s not just to release acid and pepsinogen
that’s going to digest food, it also physically breaks down the food, so we result in something
that’s called chyme, and I’ll write that right there, chyme is what the stomach
sends to the duodenum. Now gastrin release into our bloodstream is not unopposed, it’s
checked when the stomach acid reaches a PH of three. When this occurs, then we’re
going to have a decrease in gastrin release, so low PH
decreases our gastrin release. So, I’m going to say red is inhibition, green is the go ahead or the green light. So remember the next part
of our digestive process involves delivering chyme
to our small intestine and we’ll specify the first
part of our small intestine, and remember that’s called the duodenum, so chyme that’s delivered to our duodenum. Now this is going to cause the release of two other hormones. The first hormone I’m going to talk about is called secretin, so secretin, and I’ve sort of started
the color scheme here, but once we have chyme
delivered to our duodenum, secretin is released into the bloodstream and it goes two places. So, first it’s going to
go down to the pancreas, that I could have drawn
up here on our GI tract, it sits about right
there and the duodenum, but I’m just going to
write it out over here. So, secretin in the
pancreas is going to cause the release of bicarbonate-rich solution. So this bicarbonate-rich
solution is going to involve our pancreatic enzymes,
but the most important part that we want to focus on
here is the bicarbonate. And why is that? Well, the whole stimulus
for secretin release was the acidic chyme that was
delivered to our duodenum. So, of course we want to release a base, like bicarbonate, to neutralize that acid. The other place secretin is going to go from our bloodstream
is actually right back up into the stomach, this is
going to cause an inhibition of stomach motility and acid
release and pepsinogen release, the kind of things we
saw happen with gastrin. The other hormone that’s
going to be released because we have chyme in our duodenum, the acidic chyme in our duodenum, is called cholecystokinin,
and as you remember, this is a hormone that’s
related to our gall bladder. And just like secretin, our
cholecystokinin is released from our intestinal
mucosa, so I can draw it coming from, say here,
into our bloodstream, and it’s going to go two places as well. One, it’s going to go to the pancreas, to stimulate the release
of our pancreatic enzymes. So, release our pancreatic,
I’ll just write panc here, pancreatic enzymes, and this will help in our digestive process. One of the enzymes that is
released from the pancreas, that I’ll explain in a minute,
it will help clarify things, is lipase, remember lipase
is used to break down lipids. So, that’s what the LIP stands for, we’re breaking down lipids. The other thing
cholecystokinin is going to do, is go through the bloodstream and arrive at our gall bladder. It’s going to go to our gall bladder, and at our gall bladder,
cholecystokinin is going to cause the gall bladder to contract,
so contract our gall bladder. And what do you think happens when the gall bladder contracts? What’s the main function our gall bladder? Well, if you remember, the
gall bladder is holding bile, that was produced in the
liver, so when you squeeze the gall bladder, you’re
going to pump bile out of the gall bladder, into
the cystic duct, and down and out through the common
bile duct into the duodenum. And that’s going to help emulsify fat. And lastly, the other thing
that cholecystokinin does, is that it comes back here and decreases our stomach motility, we
want to slow down the release of our chyme from the
stomach because we need some time to process what
we already have in here. Now, is it plain old chyme that causes our cholecystokinin and our secretin to be released into the bloodstream? Or is it something more
specific in the chyme? Well, let me ask you: Why do you think
cholecystokinin was released? If we had to point to a specific nutrient that was in the chyme that
requires then our gall bladder to contract and release
bile and have our pancreas release an enzyme like lipase, what kind of nutrient do you think I’m suggesting here? Or, macromolecule? Well, if you said fat,
you’re absolutely right. It’s the fat in our
chyme that specifically causes cholecystokinin to be released. Okay, so what about our secretin? Well, again, this is up to you. Think about what’s happening
because of the secretin. The main thing here is that we’re having bicarbonate be released and
so bicarbonate’s a base, and we talked about how that’s important for neutralizing
our acidic chyme. So, it’s not really a macromolecule that causes secretin to be released, it’s hydrochloric acid, it’s
the acidity of our stomach acid that’s now delivered to our duodenum that causes the need for our
bicarbonate-rich solution from the pancreas, so great. I think we have a good
idea of how our GI hormones are helping us so far, at
least in our intestinal tract. Now, I know that I gave a little shout out to the pancreas here and
we separately talked about how insulin is released
when we have an increase in glucose levels in our bloodstream, to help us store that
glucose for later use. And then the opposite occurs when we have a decrease or low levels of glucose, and we cause glucagon to
be released to increase the amount of glucose then in our blood. Together, these hormones work
in a very beautiful manner, to make sure that we
have the correct hormone to respond to the right
stimulus, so we can get food that just arrived in our
stomach, processed all the way, to the very end, thanks to our hormones, and as we mentioned earlier as well, thanks to our enteric nervous system.