- [Voiceover] Now, of course,
there are many nutrients in food,
including the imaginary cheeseburger
that I have just drawn here.
But in this video I want to follow the journey
of one nutrient.
We're gonna follow the journey of fat
and how the fat contained inside of this cheeseburger
is digested, transported, and stored
in this imaginary man that I've drawn here.
So let's say it's lunchtime, he's hungry,
he takes a big bite of the cheeseburger.
He chews it down in his mouth and it travels down
his esophagus which I've drawn in blue here,
into his stomach.
It's churned a little bit more,
mixed with all that acid in the stomach,
and then it enters the small intestine.
The small intestine is just many, many loops repeating.
And all of its surface area makes this an ideal place
for all of the nutrients in our food,
and of course fat is no exception to this,
to be absorbed.
And of course, anything that cannot be absorbed
will continue on in the colon,
which I've drawn here in blue,
and eventually you will have a trip to the bathroom.
But since we are interested in how fats
are digested and absorbed,
we want to focus our attention here to the small intestine.
So if you recall, the small intestine is lined
by specialized epithelial cells,
which is just a fancy way to say that there are
special cells that can absorb nutrients
that line the inside of this tube.
And so I'm drawing kind of a representative version
of one of those cells.
And just to kind of orient ourselves I will remind you
that this side is the lumen of the small intestine,
so the hole through which all the food is travelling.
And of course this is our cell.
Now just as for any other nutrient,
the small intestine contains an array of enzymes
to break down the fat molecules into smaller pieces
for the cells to absorb.
And as a class, these enzymes are called lipases
and some of them are secreted by the pancreas,
and others are found naturally along the border
of these cells.
And just to kind of indicate and remind ourselves
that these enzymes exist,
I'm gonna draw basically a Macman-like character
to represent these lipases that are ready to
essentially break down these fat molecules
into smaller pieces.
But notably, these lipase enzymes must function
in an aqueous environment.
Remember, most of our body functions in
a water-loving environment,
but this of course poses a problem because
fat molecules are very hydrophobic,
so remember, when we refer to a fat molecule,
we think of triacylglyerides,
which I'll abbreviate as TAG here.
And I won't draw out the full structure,
but the important things to recognize,
remember, are we have a glycerol backbone,
so three carbons here that I've shown
in this kind of line diagram at the corners here
and they're attached via oxygens to three acyl groups,
which remember are carbon double bond O
and then many, many carbon and hydrogens.
And of course some of these might have double bonds
or whatnot, but the key idea here is that because we have
many carbons and hydrogens,
we have a very hydrophobic molecule.
And for this reason all of these molecules
are not going to want to dissolve in the aqueous
environment of the small intestine.
In fact, they're probably gonna all essentially
group together and kind of want to bind with each other
instead of dissolving into the aqueous environment.
And so they'll kind of form these fatty droplets
that won't kind of want to be broken up very much.
To solve this problem you might recall that our
body secrets something called bile from the liver
once food enters the small intestine,
and bile is essentially a detergent for our food.
So what I mean by that is,
if you've ever had anything greasy on your hands,
you might realize that it's hard to just get
that grease off by running your hand under water.
But when you add soap,
it's much easier to get out.
And that's because soap has both a hydrophobic
as well as a very hydrophilic, or water-loving,
functional groups on its molecule.
And having both of these types of functional groups
allows it to essentially emulsify
or essentially solubilize these fatty molecules
a little bit better.
So what that does is it kind of breaks up
these fatty fat molecules in kind of smaller pieces,
increasing the surface area for which these
lipase enzymes can act upon these fat molecules.
Specifically, these lipase enzymes break down
these triacylglyceride molecules by cleaving
these molecules at these ester linkages
by adding a water of molecule across each of these spawns,
and so all together the end result is that we form
a free glycerol backbone,
and now this glycerol backbone has a hydroxyl group
because it's accepted a hydrogen from water,
as well as now instead of acyl groups,
we form what we call a carboxylic acid group
because we're adding the remaining hydroxyl group
from the water to these molecules.
And of course we form three of these,
and we call these specific carboxylic acid groups
when they come from a fat triacylglyceride,
we call these fatty acids,
appropriately since of course they're coming from
a fat molecule.
Now at this point these molecules are small enough
to be able to diffuse into the intestinal cell.
Now although we've officially absorbed these molecules,
we aren't finished,
because of course we know that we need to be able
to deliver these fatty acids to the tissues of our body,
most notably the adipose tissue where we can store fat
for later energy use.
So how do we do that?
Well, the first step once we get these molecules
into the small intestine is to turn them back
to triacylglycerides,
because you want to essentially pack them
in a compact unit so that we can send them off
to various tissues.
So once we reform these ester linkages
and reform these triacylglyceride molecules,
we then want to package them,
get them ready for transport.
And the way that our cells do this is by using
a carrier molecule called a lipoprotein.
And essentially what this is is,
the body essentially packages all of the triacylglycerides
along with any other hydrophobic substances
that the body absorbs, such as cholesterol,
into the core of a protein molecule,
which I'm indicating here by these purple circles.
And the benefit of this is that these proteins
have polar heads here.
That means that they interact with the aqueous
environment inside of the blood stream, for example,
but inside, they're relatively hydrophobic enough
to keep all of these hydrophobic molecules within them.
Notably, the specific name that we call this
particular lipoprotein that's produced
within the intestinal cell from absorbed fats,
we call this a chylomicron.
So once all of these triacylglycerides are packaged
into the chylomicron,
the next step is for this chylomicron to leave the cell.
But here's where things get a bit interesting.
So let me scroll down a bit to give us some room
at the bottom here.
Recall that all of these cells
are surrounded by capillaries,
so they have a source of oxygen and nutrients,
but they're also surrounded by these specialized
lymphatic capillaries called lacteals,
and I'll go ahead and draw one right next to it here
and label that.
So this here is called a lacteal,
and it's a lymphatic capillary.
And this right here is just our normal,
regular capillary.
Now notably these chylomicrons are kind of
big and bulky molecules,
and the capillaries have very tiny fenestrations
or essentially gaps to allow molecules
to be absorbed.
Now these tiny gaps are small enough to allow
capillaries to absorb proteins and carbohydrates,
but they're way too small to absorb chylomicrons,
and so the chylomicrons choose a different route.
They are instead taken up by these lacteals,
which have much larger pores relative to the capillary
and that allows these chylomicrons to travel within them.
So now that we know that these chylomicrons travel
through the lymphatic vessels by entering the lacteals,
you might be wondering, where do they go next?
And to answer this question we just need to
remind ourselves with the anatomy
of the lymphatic vessels are.
And the truth is we have lymphatic vessels
all over our body that drain many sites of our body.
Some of them in our legs, in our thorax,
some of them coming from our arms,
and even some draining down from our head, as well.
But the key point is that most of these
end up coalescing in a region that's near
our neck and our shoulder,
and at that point they form ducts
that empty into veins.
So I'm gonna just draw a vein next to each
of these two points here.
And most of our body drains the lymph into
this side, on the left side called the left thoracic duct.
But some of the lymph are also drained into
this right thoracic duct here, as well.
But the key point is is that eventually anything
that goes into these lymphatic vessels will drain
into veins in this part of your body.
And so with that brief overview of fat digestion
and absorption of the small intestine,
and then subsequent packaging into chylomicrons
and traveling through the lymphatic vessels
up to the veins in our body,
we'll go ahead and take a pause and pick up
and continue the journey of the chylomicron
in the blood vessels in our next video.