- [Teacher] As the food enters the stomach,
lot of times you think of the stomach
as a storage facility.
Well, the stomach is a storage facility,
but it's only a temporary storage facility.
'Cause what ends up happening here is
the stomach is storing it
in order to regulate the delivery of that
partially digested food into the small intestines.
What ends up happening is,
when you eat, how long does it take you
to eat a meal?
Well let's say, eat dinner.
Two hours?
Yeah, I don't think maybe we'll take two hours.
An hour?
Would be nice, but probly not.
More realistically, most people
probly eat in 30 minutes or less.
Your body cannot digest all that food that quickly.
What ends up happening is,
the stomach will store the food
as it slowly gives a little bit at a time
to the small intestines to process.
But while it's storing it,
it's also helping to start the digestion of protein.
It's gonna start breaking those proteins down
into smaller and smaller building blocks.
Remember, the proteins that can be broken down
into the building blocks of amino acids.
What do you find in the actual stomach?
Well, we had this gastric juice, which
one part of it is Hydrochloric acid.
Now, the Hydrochloric acid in your stomach
is some really, really strong acid.
Its pH is around two.
The acid itself is meant to break down
the larger parts of food.
It's not necessarily we're making something new.
Just take the big chunks and break them to smaller chunks.
By breaking them into smaller chunks,
it allows a larger surface area.
While that larger surface area
means the enzymes that break down proteins
can work more efficiently.
Kind of think of it this way.
If you're to take a large garbage bag,
fill it with golf balls.
Okay, fine, but the surface area on the
outside of that garbage bag.
But if you broke that bag open and
let all the golf balls out,
think about how many more surface areas
there would be to work on.
By having more surface area like that
gives the enzymes a better ability
to work more efficiently and faster.
We're also gonna have intrinsic factor.
Now, intrinsic factor's kind of helpful
because it helps with the absorption of Vitamin B12.
Now it's not necessarily absorbing B12 only in the stomach.
It's looking primarily to do this in the intestines.
But intrinsic factor comes from
the breakdown of proteins in that gastric juice.
We got mucus.
There's mucus lining the entire stomach.
Give or take it's almost an inch thick
of mucus lining the stomach.
Probly thinking, oh, that's just wrong.
Well that mucus is what prevents your Hydrochloric acid
from eating away at your stomach lining.
It's pretty important to make sure that mucus is there.
The mucus is gonna help buffer the acid
and prevent it from reaching the walls of the stomach.
Then we have this Pepsinogen.
Pepsinogen is inactive.
It's not gonna do anything.
But that's how it's secreted into the stomach.
Once it reaches the stomach,
it comes in contact with that very acidic
Hydrochloric acid,
it converts to the enzyme, pepsin.
Pepsin is the enzyme that actually breaks down
the proteins.
Why would you want to bring
Pepsinogen, which is inactive
into the stomach and then activate it in the stomach?
Why not just bring in the Pepsin straight from wherever?
Well, one of the problems is
if you just bring pepsin in,
it'll start to break down all the different
ducts and tubing it travels through.
Protein is a major structural unit of your bodies.
If you have pepsin, which breaks down proteins,
active elsewhere outside the stomach,
you're really running a risk
of actually causing the tubes carrying it
to be broken down and damaged.
This is showing an artist's rendition
of the layers the stomach on the left,
and then an actual, micrographic image on the far right.
What you're noticing
is in the mucosa layer,
which is the layer that touches the digestive food in.
You're gonna have these gastric pits.
That's where the cells are that make the acid
that make the Pepsinogen that make the mucus.
Actually these tiny little gastric pits that go down
all over the entire stomach, and just secrete
this gastric juice.
Mucus and acid and Pepsinogen
to help break down and digest the proteins
found in the stomach.
Stomach contractions are useful for mixing up the food.
It's not to blend the food with the gastric juices.
So mix it with the Hydrochloric acid,
mix it with hopefully the active pepsin, by this point.
And mix it with all the rest of the components.
But it's not just mixing it up and blending it.
It's meant to propel it forward.
To move it through the stomach, towards the opening
to the small intestines.
Chyme is the watery mixture
of partially digested food and enzymes.
This is what the stomach is working to make.
It's gonna take all those enzymes and acid and mucus
and water as well as all the food
that's been chewed up.
Mix it into this one kind of slushy mix
and it's called Chyme.
Then Chyme is slowly delivered
down to the small intestines.
Well, it requires, give or take two to six hours
for the stomach to completely empty.
Well, why the variation?
It depends on what the food is.
A pure carbohydrate is gonna be toward
the two hour side.
Your body can break down carbohydrates a lot faster.
But towards that six hour side
it's a very fatty, heavy food.
Depending on what the meal actually is,
your body can take longer,
or be relatively quick at two hours
to completely empty the stomach.
Now the reason we're taking hours and hours though
is because the stomach can only release a little bit
of liquid at a time.
Or a little bit of this Chyme at a time.
It takes a long time
for all this Chyme to slowly be released
into the small intestines.
The biggest and key thing to remember.
During that entire two to six hour time frame
the stomach does not
absorb one single nutrient.
All the stomach is doing,
storing, mixing, and then propelling the food forward.
It's meant to blend that food together
with the enzymes, the mucus,
form the Chyme, and allow the Chyme
into the small intestines.
The small intestines, well,
one of its biggest functions is digestion.
It's going to digest the Chyme
it received from the stomach.
But before it can do anything,
it has to neutralize the acid from the stomach.
I could mention, your stomach contains
Hydrochloric acid.
Very acidic, very strong.
But the small intestines?
It likes to float right around that neutral,
the pH of seven.
What ends up happening is
the small intestines are going to neutralize
or buffer the acid as it's released from the stomach.
Beyond the neutralizaton?
It's going to also add in its own digestive enzymes.
It'll add in all these pancreatic enzymes
and these different chemicals from the liver.
Bile will be added in.
Now it means we're able to break down proteins,
carbohydrates and lipids
all into our building blocks.
Proteins, they started back up in the stomach.
They're gonna be finished down in the small intestine.
They'll break down right into those basic building blocks,
those amino acids.
Carbohydrates, they'll get broken down smaller and smaller.
Eventually they get broken down into
monosaccharides, or more commonly known as simple sugars.
And the lipids also get broken down
into their building blocks.
The key is you have to break them down
to the building blocks.
It's these building blocks that can be absorbed
through the walls of the small intestine.
Absorption, about 90% of food absorbed
in the small intestines.
So not everything, but the vast, vast, vast majority
is absorbed in the small intestines.
So to move through these small intestines
for all this digestion to occur,
we have a total of three different regions.
We have the duodenum, or called the duodenum,
either pronunciation's fine.
The majority of digestion occurs here.
That's because the duodenum, it's directly
after the stomach, and that's where a lot
of these different digestive enzymes
are being secreted into.
Jejunum, that's gonna be the middle part
of the small intestines.
It's after the duodenum, and it's after the
majority of digestion has occurred.
That means the jejunum is primarily for absorption.
Well the end is the ileum.
The ileum will continue absorption
right to the end of the small intestines.
And all three areas of the small intestines
are gonna have a large surface area.
That's because if you look at the
right hand side diagram,
you can see the top figure, how it has
the label, "Folds."
Well, these folds are going to increase surface area
by making the edges kind of rippled.
But on each one of these folds
we're gonna have villi.
So you look down at the second image,
letter B there.
The villi are the small little protrusions
or projections that are coming off.
They're only meant to increase surface area.
From there, head on down to C, the third picture down.
You'll notice microvilli.
Microvilli are even smaller.
They're tiny, tiny little projections
on the actual cell that lines
the internal parts of the small intestine.
These little projections, those epithelial cells,
the internal lining.
We have total of three modifications.
There are folds, then villi in the folds,
then microvilli on the actual cells
of the villi.
A lot of little tiny pieces and parts here
that are helping to increase as much surface area
as possible.
Look down at the bottom right, that's an actual micrgraph
of what the human intestinal villi actually look like.
Those are the villi, not the microvilli,
just the villi.