of the lymphatic system.

The first was to bring the fluid that was squeezed out

of the capillaries back into the blood,

and the second was to help out the immune system.

But there's one more.

There's a third.

And that's what we're going to talk about in this video.

So let's look at a piece of the small intestine.

Now, obviously the small intestine carries bits of food

that your body wants to absorb and use for energy.

So one of the most important elements of that food

is little glucose molecules, which

are absorbed and are used as the primary source of energy

for the body.

So actually, since it'll be helpful for us,

let's at least make this two dimensional.

So let's look at some cells here of the small intestine.

So these are cells lining the small intestine.

And the way that glucose is actually absorbed by your body

is that it's pumped into these cells

here, in the cells lining the intestine.

And then it's pumped out of them.

And then once it's here in this space,

it's able to diffuse into capillaries which are nearby.

So it'll diffuse into the capillaries,

and once it's there can go and spread out over the whole body

and feed various cells.

But glucose is not the only thing

which needs to be absorbed in your small intestine,

and actually another compound is fat.

So fatty acids which are floating around

in your small intestine need to be absorbed as well.

And the structure of these things, to remind you,

looks something like this.

You've got this carboxylic acid group

attached to a long chain of carbons.

Now, like glucose, these guys are taken up

by these cells in the intestine.

But when they're put out by those cells,

they no longer look like this.

By the time they're put out, they're

packaged into these actually rather large spherical objects

which have a weird name.

It's chylomicron.

So they're packaged into these spherical things called

chylomicrons, and these chylomicrons

are useful for holding a lot of these fats together.

But they have a problem, which is

that chylomicrons, as you might guess,

are too big to diffuse into the capillaries.

So they can't do that.

But you don't want to have chylomicrons just

accumulating here in your intestine.

You want to spread them out to the body where they're needed,

and so that's where our good friend the lymphatic system

comes in.

So we have these lymphatic vessels nearby

that are much easier to enter than the capillaries,

and the chylomicrons able to diffuse into these vessels

and be transported through the lymphatic system

and eventually emptied into the blood circulation

so that they can spread throughout the body.

Because this is such an important processes,

these lymphatic vessels in your small intestine

are given a special name.

And that name is lacteal.

So the lymphatic vessels in your small intestine

are known as lacteals, and those are

the ones that take up your fats in the form of chylomicrons.

So here we have our third important function

of the lymphatic system, which is to transport things which

didn't originally come from the blood,

to transport them into the blood.

And we can sort of generalize it a little bit

and say that maybe a little further away

from the small intestine somewhere else you

have some cells that are hanging out.

And actually, there are some other things

which your body might want to put into lymphatic circulation.

So you may have some cells here or elsewhere

in the body that are producing maybe some proteins

like hormones or maybe some waste products.

And in some cases those proteins might

have trouble getting into the capillaries,

and these waste products also might have trouble.

But you want these things to get into the blood

because, for example, for hormones, you

want them to be able to travel throughout the body

and have their effects on the tissues

that they're supposed to effect.

And for waste products, you want them

to be able to get into the blood and eventually

get to the liver where they might be broken down

or to the kidney where they can be excreted.

So in those cases, you have this secondary system

of getting these things into blood circulation

through the lymphatic vessels.

So these are proteins and wastes.

And the reason that we're talking

about these at the same time as chylomicrons

is because these are all things that don't actually

come from the blood, but we want to put them in the blood.

And we can't do it directly, so we put them

into lymphatic circulation first.

So, now that we've learned about the third purpose

of the lymphatic system, it might be a good time

to review all three purposes at once.

So why don't you pause the video and see

if you can remind yourself of all three.

Otherwise, I'll go ahead and describe them here.

So the first purpose of the lymphatic system--

let's draw another lymphatic vessel here.

The first purpose was to collect all the fluid which

was forced out of the capillaries

by the high pressure that's present in the blood,

to gather all that fluid and bring it back

into blood circulation via the lymphatic vessels

so that you don't have too much fluid accumulating here

in the tissue.

So that's the first purpose.

This is just fluid with maybe some small plasma proteins,

but not with any red blood cells or anything like that.

The second purpose had to do mostly with infection.

So if you have some bacteria infecting

some tissue in your body, the lymphatic system basically

offers a way to bring those bacteria

to the attention of your immune system.

And they do that by basically taking them up and bringing

them to the nearest lymph node.

So that's a lymph node, and that is where your B cells and T

cells are hanging out.

So that's the second purpose right there,

the second purpose of your lymphatic system.

And the third purpose, of course,

is what we discussed today.

It's transporting other things, such as chylomicrons

or other proteins or wastes, transporting them

into the blood when they have trouble simply getting

into capillaries the normal way.