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- [Teacher] As the food
enters the stomach,

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lot of times you think of the stomach

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as a storage facility.

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Well, the stomach is a storage facility,

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but it's only a temporary
storage facility.

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'Cause what ends up happening here is

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the stomach is storing it

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in order to regulate the delivery of that

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partially digested food
into the small intestines.

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What ends up happening is,

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when you eat, how long does it take you

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to eat a meal?

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Well let's say, eat dinner.

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Two hours?

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Yeah, I don't think maybe
we'll take two hours.

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An hour?

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Would be nice, but probly not.

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More realistically, most people

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probly eat in 30 minutes or less.

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Your body cannot digest
all that food that quickly.

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What ends up happening is,

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the stomach will store the food

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as it slowly gives a little bit at a time

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to the small intestines to process.

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But while it's storing it,

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it's also helping to start
the digestion of protein.

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It's gonna start breaking
those proteins down

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into smaller and smaller building blocks.

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Remember, the proteins
that can be broken down

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into the building blocks of amino acids.

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What do you find in the actual stomach?

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Well, we had this gastric juice, which

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one part of it is Hydrochloric acid.

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Now, the Hydrochloric acid in your stomach

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is some really, really strong acid.

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Its pH is around two.

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The acid itself is meant to break down

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the larger parts of food.

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It's not necessarily we're
making something new.

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Just take the big chunks and
break them to smaller chunks.

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By breaking them into smaller chunks,

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it allows a larger surface area.

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While that larger surface area

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means the enzymes that break down proteins

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can work more efficiently.

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Kind of think of it this way.

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If you're to take a large garbage bag,

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fill it with golf balls.

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Okay, fine, but the surface area on the

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outside of that garbage bag.

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But if you broke that bag open and

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let all the golf balls out,

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think about how many more surface areas

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there would be to work on.

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By having more surface area like that

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gives the enzymes a better ability

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to work more efficiently and faster.

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We're also gonna have intrinsic factor.

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Now, intrinsic factor's kind of helpful

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because it helps with the
absorption of Vitamin B12.

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Now it's not necessarily
absorbing B12 only in the stomach.

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It's looking primarily to
do this in the intestines.

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But intrinsic factor comes from

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the breakdown of proteins
in that gastric juice.

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We got mucus.

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There's mucus lining the entire stomach.

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Give or take it's almost an inch thick

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of mucus lining the stomach.

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Probly thinking, oh, that's just wrong.

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Well that mucus is what
prevents your Hydrochloric acid

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from eating away at your stomach lining.

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It's pretty important to make
sure that mucus is there.

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The mucus is gonna help buffer the acid

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and prevent it from reaching
the walls of the stomach.

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Then we have this Pepsinogen.

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Pepsinogen is inactive.

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It's not gonna do anything.

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But that's how it's
secreted into the stomach.

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Once it reaches the stomach,

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it comes in contact with that very acidic

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Hydrochloric acid,

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it converts to the enzyme, pepsin.

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Pepsin is the enzyme
that actually breaks down

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the proteins.

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Why would you want to bring

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Pepsinogen, which is inactive

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into the stomach and then
activate it in the stomach?

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Why not just bring in the
Pepsin straight from wherever?

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Well, one of the problems is

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if you just bring pepsin in,

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it'll start to break
down all the different

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ducts and tubing it travels through.

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Protein is a major structural
unit of your bodies.

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If you have pepsin, which
breaks down proteins,

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active elsewhere outside the stomach,

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you're really running a risk

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of actually causing the tubes carrying it

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to be broken down and damaged.

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This is showing an artist's rendition

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of the layers the stomach on the left,

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and then an actual, micrographic
image on the far right.

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What you're noticing

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is in the mucosa layer,

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which is the layer that
touches the digestive food in.

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You're gonna have these gastric pits.

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That's where the cells
are that make the acid

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that make the Pepsinogen
that make the mucus.

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Actually these tiny little
gastric pits that go down

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all over the entire
stomach, and just secrete

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this gastric juice.

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Mucus and acid and Pepsinogen

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to help break down and digest the proteins

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found in the stomach.

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Stomach contractions are
useful for mixing up the food.

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It's not to blend the food
with the gastric juices.

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So mix it with the Hydrochloric acid,

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mix it with hopefully the
active pepsin, by this point.

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And mix it with all the
rest of the components.

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But it's not just mixing
it up and blending it.

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It's meant to propel it forward.

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To move it through the
stomach, towards the opening

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to the small intestines.

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Chyme is the watery mixture

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of partially digested food and enzymes.

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This is what the stomach
is working to make.

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It's gonna take all those
enzymes and acid and mucus

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and water as well as all the food

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that's been chewed up.

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Mix it into this one kind of slushy mix

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and it's called Chyme.

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Then Chyme is slowly delivered

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down to the small intestines.

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Well, it requires, give
or take two to six hours

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for the stomach to completely empty.

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Well, why the variation?

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It depends on what the food is.

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A pure carbohydrate is gonna be toward

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the two hour side.

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Your body can break down
carbohydrates a lot faster.

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But towards that six hour side

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it's a very fatty, heavy food.

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Depending on what the meal actually is,

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your body can take longer,

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or be relatively quick at two hours

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to completely empty the stomach.

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Now the reason we're taking
hours and hours though

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is because the stomach can
only release a little bit

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of liquid at a time.

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Or a little bit of this Chyme at a time.

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It takes a long time

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for all this Chyme to slowly be released

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into the small intestines.

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The biggest and key thing to remember.

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During that entire two
to six hour time frame

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the stomach does not

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absorb one single nutrient.

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All the stomach is doing,

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storing, mixing, and then
propelling the food forward.

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It's meant to blend that food together

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with the enzymes, the mucus,

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form the Chyme, and allow the Chyme

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into the small intestines.

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The small intestines, well,

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one of its biggest functions is digestion.

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It's going to digest the Chyme

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it received from the stomach.

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But before it can do anything,

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it has to neutralize the
acid from the stomach.

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I could mention, your stomach contains

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Hydrochloric acid.

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Very acidic, very strong.

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But the small intestines?

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It likes to float right
around that neutral,

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the pH of seven.

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What ends up happening is

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the small intestines
are going to neutralize

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or buffer the acid as it's
released from the stomach.

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Beyond the neutralizaton?

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It's going to also add in
its own digestive enzymes.

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It'll add in all these pancreatic enzymes

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and these different
chemicals from the liver.

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Bile will be added in.

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Now it means we're able
to break down proteins,

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carbohydrates and lipids

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all into our building blocks.

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Proteins, they started
back up in the stomach.

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They're gonna be finished
down in the small intestine.

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They'll break down right into
those basic building blocks,

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those amino acids.

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Carbohydrates, they'll get
broken down smaller and smaller.

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Eventually they get broken down into

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monosaccharides, or more
commonly known as simple sugars.

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And the lipids also get broken down

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into their building blocks.

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The key is you have to break them down

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to the building blocks.

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It's these building blocks
that can be absorbed

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through the walls of the small intestine.

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Absorption, about 90% of food absorbed

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in the small intestines.

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So not everything, but the
vast, vast, vast majority

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is absorbed in the small intestines.

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So to move through these small intestines

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for all this digestion to occur,

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we have a total of
three different regions.

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We have the duodenum,
or called the duodenum,

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either pronunciation's fine.

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The majority of digestion occurs here.

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That's because the duodenum, it's directly

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after the stomach, and that's where a lot

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of these different digestive enzymes

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are being secreted into.

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Jejunum, that's gonna be the middle part

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of the small intestines.

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It's after the duodenum,
and it's after the

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majority of digestion has occurred.

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That means the jejunum is
primarily for absorption.

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Well the end is the ileum.

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The ileum will continue absorption

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right to the end of the small intestines.

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And all three areas of
the small intestines

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are gonna have a large surface area.

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That's because if you look at the

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right hand side diagram,

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you can see the top figure, how it has

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the label, "Folds."

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Well, these folds are going
to increase surface area

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by making the edges kind of rippled.

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But on each one of these folds

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we're gonna have villi.

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So you look down at the second image,

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letter B there.

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The villi are the small little protrusions

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or projections that are coming off.

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They're only meant to
increase surface area.

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From there, head on down to
C, the third picture down.

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You'll notice microvilli.

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Microvilli are even smaller.

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They're tiny, tiny little projections

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on the actual cell that lines

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the internal parts of the small intestine.

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These little projections,
those epithelial cells,

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the internal lining.

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We have total of three modifications.

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There are folds, then villi in the folds,

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then microvilli on the actual cells

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of the villi.

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A lot of little tiny pieces and parts here

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that are helping to increase
as much surface area

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as possible.

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Look down at the bottom right,
that's an actual micrgraph

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of what the human intestinal
villi actually look like.

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Those are the villi, not the microvilli,

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just the villi.