- [Lecturer] I'm sure everyone's heard
about the digestive system and about nutrition.
So first off, we have the digestive system
broken into kind of two main areas.
We can talk about it as the GI tract,
or the gastrointestinal tract.
In essence, it's one, long, hollow tube.
Running all the way from mouth,
all the way down through the anus.
The space inside is called a lumen.
The lumen is where all the food will go as you digest it.
It'll travel through this space
as it goes through the entire GI tract.
You have things like your mouth, your pharynx,
esophagus, stomach, small intestines, large intestines,
rectum, anus, all these make up one
very long continuous tube.
But that's not it.
You have other organs, other structures
that are going to help out.
The salivary glands are gonna help
by secreting obviously, saliva.
The liver, gall bladder, pancreas,
will all help by secreting different enzymes
and different molecules.
They'll aid in the digestive process
as you go through the GI tract.
So let's take a look at what we have here.
First off, mouth.
Now I'm willing to be you can probably
figure out what the mouth does.
Yeah sure, it chews the food.
But what's the tongue doing?
It moves the food around.
But more importantly, it's not just moving it
in a random order, it's moving it so the food is positioned
in between the teeth as you chew.
But there's a trick here.
The trick is to get the tongue
out of the way for you to chew.
Well, once you've chewed up your food enough,
so hopefully it's not a solid mass,
you can swallow.
Once you swallow, you head down into the pharynx.
The pharynx is the passage at the back,
or the posterior side of the mouth.
It allows the food to head down towards the stomach,
but the pharynx also allows the air
you breathe through your nose
to travel down towards your lungs.
That means the pharynx is going to be
used for both digestive and the respiratory systems.
Esophagus.
Once the food passes through the pharynx,
it heads down through the esophagus.
The esophagus is a long, muscular tube
that runs all the way down into your stomach.
Now the stomach is kind of a holding ground,
kind of a storage area.
The foods you eat will be momentarily held in the stomach.
Won't be kept here for days,
just kept here for a couple dozen minutes.
Might be pushing an hour or two.
Depends on what you ate and how much you ate.
But in the stomach, you start to mix up the food.
You're gonna mix it with different enzymes.
You're gonna mix it in with different chemicals.
It's meant to start the digestive process.
Now that physical mixing where the stomach's actually
churning and mixing it up,
that's kind of like a mechanical.
You're squeezing, so mechanical digestion.
While the enzymes that are being secreted
are gonna help to start with a chemical digestion.
You actually start to break down
the physical protein molecules.
But every so often a little bit of this mixture
is heading to the small intestines.
Now the small intestines is going to help
with digesting more protein.
It'll start the digestion of fat.
It'll do some more digestion of carbohydrates.
Most of the water and nutrients are absorbed
in the small intestines.
You can see how long that small intestines
is curling and curving around for.
So we have this slow progression
heading down through the small intestines
that allow for more chemical digestion because
there's going to be more digestive enzymes being secreted.
The enzymes here can come from the
walls of the small intestine.
They can come from the liver or
the gall bladder, the pancreas.
So all these different organs, these helping
or accessory organs also can secrete enzymes
and other chemicals into the small intestine.
Now once they finish in the small intestine
it's time for the large intestine.
You may have heard the of the large intestine
by name of the colon.
The colon is the same as the large intestine.
Now the large intestine is meant to be a storage area.
It stores the waste material.
Whatever is waste, is what was not
absorbed by the small intestines.
It could be undigested food products
they didn't need to use.
It could be plant components, things like fiber
that you simply cannot digest.
So large intestines is going to store this waste,
while absorbing as much of the water
and nutrients that are left over.
So that stored waste eventually
heads down to the sigmoid colon to be stored as feces.
The feces must exit through the rectum.
So the rectum is simply a passageway
to get this waste, to get these feces, out of the body.
So from mouth all the way down to rectum and anus,
that is the complete pathway that food
can take through your body.
So right from mouth through anus.
But we have some accessory organs.
The salivary glands.
You can see there are three lines here
because there are three different major salivary glands.
Now saliva's meant to mix with the foods you eat.
Helps to moisten the food, but it also helps to maintain pH.
So if you're eating something very acidic,
it'll partially help to neutralize it.
If something's a little basic,
it'll help to neutralize it.
Tries to keep the natural pH in a normal range.
You're gonna have the enzyme amylase present in the saliva.
Amylase is an enzyme that can digest starch.
Well maybe you ever had to try this experiment
back in you know middle school, high school,
some other college class, where you had
to chew on a cracker for 10 to 15 minutes, the same cracker.
You're probably thinking oh, that's just disgusting.
What was interesting, the cracker will not taste
like a cracker after that 15 minutes, because
while you're chewing on it you're constantly
mixing in these enzymes, this amylase.
The amylase will break down the starch
into simple sugar.
That cracker should become very sweet
by the end of that 15 minutes.
But it's not just the amylase that's here as an enzyme.
We also have lysosomes which contain plenty of enzymes
meant to destroy, or nothing else, inhibits
bacterial progression.
The liver.
Now liver sits up just below the diaphragm,
so think about the top of your abdominal cavity.
This will produce bile.
Now, bile is going to help with the breakdown of lipids.
But what's interesting, is that bile
is not actually secreted from the liver.
The bile is sent down to the gall bladder.
The gall bladder can concentrate and then secrete.
But that's not the only thing the liver does.
The liver does quite a few other functions.
Quite a few other steps and processes.
From cleaning out some particles that shouldn't
be there in the blood to detoxifying to helping out
with a ton of other processes from digestion.
Even stores nutrients.
So here's that gall bladder I mentioned a second ago.
The gall bladder stores and concentrates the bile.
The gall bladder does not make the bile.
The liver makes it.
Pancreas, more digestive enzymes.
These digestive enzymes are heading
right down into the small intestines.
But it's not just enzymes, it's also bicarbonate.
Bicarbonate is gonna be a buffer.
The buffers help keep the pH neutral.
Well your stomach is very acidic.
So having bicarbonates in the small intestines,
which is right up to the stomach,
you can help neutralize some of that stomach acid.
And the appendix.
Even though it's attached to the large intestines,
you'd think it'd be part of the digestive system.
Well unfortunately they don't know of
any digestive functions for the appendix.
The only thing they've been able to figure out
is the appendix actually has some immune function to it.
So appendix is part of the digestive system, structurally,
but doesn't really do anything for digestion.
Now if we start looking at the actual walls of the GI tract,
doesn't matter if we're talking about esophagus,
stomach, small intestines, large intestines.
Any of the actual GI tract has a total of four layers.
You have mucosa which is the innermost layer.
It's a mucus membrane.
By being a mucus membrane, it allows
the particles of food to slide by with less friction.
The mucus helps to coat the walls
and ease the passage of the chewed up food.
Some mucosa, well it's just behind that.
It's just a little bit further away
from the mucosa in the lumen.
It's mostly connective tissue, blood vessels,
lymph vessels, nerves.
It's meant to help with the controlling factor of mucosa.
The muscularis layer.
The muscularis layer has either two
or three layers of smooth muscle.
Pretty much it's almost always two.
The three layers is only found
in the stomach, if you're curious.
But this muscularis layer is responsible for movement.
It's what helps to push and propel the digesting food
through the entire GI tract.
So its main function is motility.
Then the outermost layer is the serosa.
The serosa's kind of meant to keep everything together,
provide a little bit of protection on the outside,
help anchor it in place.
So if we go from the most internal
to the most external, the most internal is mucosa.
Then moving out we go submucosa,
muscularis, and then serosa.
But every so often throughout the walls
of the GI tract, you get these things called sphincters.
Sphincters are going to be a muscular ring.
They're meant to create stop points.
Either preventing particles from going forward,
or preventing things from going backwards.
For example, you have a sphincter at the start
and end of the stomach.
It's meant to keep the contents within the stomach.
So sphincters can keep things within,
or prevent backflow.
Because the sphincter that's at the top of the stomach,
right at the bottom of the esophagus,
it's meant to prevent backflow,
prevent the stomach acid from going
up into your esophagus.
If you've ever had heartburn,
you'll know how unpleasant that is.
Here we're looking at a histological view
kinda artist's rendition.
You can see the four different layers.
The lumen is labeled.
That's where all the food particles will pass through.
You have the mucosa, which is that pinkish layer,
the most internal.
Then the submucosa.
You can see how it's supporting, it's around mucosa.
Then this case they're showing two layers of smooth muscle.
Two layers of the muscularis.
The internal layer you can see
is almost forming rings around it.
That internal layer, the one touching the submucosa,
is what helps squeeze the tube
propelling the particles forward.
And the most external, the serosa
is covering and anchoring the entire tube.
So how do things move?
How do we accomplish anything?
We have first off a mechanical process and movement.
Chewing, mixing.
You're mechanically grinding down the food
to break it into smaller pieces.
It's still the same food,
just smaller pieces of that food.
Secretions.
Well various fluids can be secreted.
We have enzymes, we have hormones, bile, acid,
alkali, mucus.
There's tons of different things.
Everything has its own purpose.
Obviously enzymes, that are digestive enzymes
are meant to break down.
Hormones help regulate the process.
Bile from the liver and then stored in the gall bladder
was mentioned to help with lipid, or fat breakdown.
Acid, alkali, mucus.
All helps with the movement because it creates
a more slippery environment.
Digestion itself.
With digestion, you're now breaking
down the food in the smallest absorbable units.
You have what's called a chemical
and a mechanical breakdown.
Mechanical breakdown, or mechanical digestion
is known as when you're chewing,
when you're grinding the food particles.
So it's very similar number one.
Chewing is mechanical digestion.
But then chemical digestion is when you take
the actual molecule and you're
breaking the bonds of the molecule.
You might be making it into a smaller particle
to be easier for absorption,.
Now the whole point is absorption.
You must absorb the nutrients, absorb
the particles that have been digested
through the most inert layer called mucosa.
These absorbed particles go into
the blood or the lymph vessels.
This way they can be transported
throughout the entire body.
And finally elimination.
Whatever is not utilized, whatever is not absorbed,
must be removed from the body.
If it's not removed, the body would
turn into a toxic waste dump
with all this undigested, non-necessary material.
So simply passes on through, and once it's done
there's no more digestion.
It will eliminate this material from the body.
For the way all this food is moving,
so obviously you can think of okay
in the mouth it's meant to swallow.
Well, all this movement is aided by smooth muscle movement.
Once you've swallowed, you have what's called peristalsis
and segmentation.
Peristalsis is helping to propel the food forward.
What will end up happening here is
the tube that it's in, so the esophagus,
small intestine, large intestine,
it'll squeeze at one point
and then it'll just run down the tube.
Imagine if you're squeezing a tube of toothpaste.
You start the one end and you kind of pinch it and squeeze
all the way down to get it all empty.
That's the idea of peristalsis.
While segmentation is meant to mix the food.
It'll move the food forward, move the food back,
move the food forward, kind of is shuffling
back and forth, moving it,
to mix it as it goes down through the tubing.
So here you can see peristalsis.
The esophagus, you see how it's pinched
just above the bulge of food?
That pinching just squeezes
all the way down the esophagus until you reach the stomach.
Segmentation is showing the bottom.
You notice how there are two boluses of food there.
But then what happens is the muscles
are going to pinch those boluses apart,
creating four smaller boluses.
Those four now get pinched in half
themselves into smaller parts.
And you can see how the parts get squeezed
towards each other, to cause them to mix with each other.
So segmentation.