Let's talk about the cardiovascular system.
Here's a cartoon of it.
And we have our heart, which is the pump, which pumps blood
through the blood vessels, round and round through the body.
And the purpose of this whole system
is to get blood out here to the capillary beds.
So here's a capillary bed.
And that's where the blood vessels become very skinny.
And they divide.
And because they're so skinny, they
have a lot more surface area.
And that increased surface area allows the blood
to exchange its oxygen and nutrients with these cells
here that I'm drawing, which need those things.
So the blood gives its oxygen to these cells.
And by the time it gets through the capillary bed,
it's lost its own oxygen.
And so now it's blue.
And that blood goes back to the heart.
Now, I haven't drawn it, but here also the blood
goes through the lungs and gets new oxygen,
so that when it goes around again, it has oxygen.
So this is a closed loop, with blood going round
and round and round.
But it has a problem, which is that the heart needs
to pump quite strong for blood to go around fast enough
and feed all the cells in your body.
And what that means is that the pressure that the heart exerts
is very high.
So there's high pressure in the heart.
And there's high pressure throughout these blood vessels.
And what that actually does is it forces some fluid out
of your blood vessels, and especially
out of these capillaries here.
It forces some fluid out.
And that fluid is not exactly blood,
because, for example, the red blood
cells which are in your blood are too big to squeeze out.
But the sort of more liquidy part
of your blood, which is like the plasma, that does
get squeezed out.
And so that's what ends up here.
And that fluid mixes in with the cells
which are outside of the capillaries.
And it accumulates.
Now, this is a problem, because if we let this go on,
we would lose pretty much all of the fluid in the blood.
And we would get a buildup of fluid out here.
And that would be an unsustainable situation.
So that's where your lymphatic system comes in.
This is the problem that it addresses.
So your lymphatic system is actually
another plumbing system, which I'm going to try to draw here.
So it's another plumbing system.
It has these vessels which kind of
start in the middle of nowhere.
And they collect this fluid.
The fluid goes in.
They collect it.
And they bring it back into circulation.
So they bring it back into the blood.
And that way, it goes back in.
And the blood can continue to circulate.
And we now do have a sustainable situation.
So these vessels here we call lymphatic vessels.
And if you'll notice, they're a little bit
different from blood vessels because they
don't form a closed loop.
The lymph, as this stuff is called,
lymph-- so that's the fluid that gets forced out of your blood.
The lymph goes in here.
And it gets dumped out into the blood vessels there.
It doesn't go back around to the start.
So that's kind of the basic explanation
for why we need these lymphatic vessels.
But to give it a little bit more depth,
let's take one of these capillaries.
And we're going to zoom in.
And we're going to draw it a little bit rotated here.
Hope that doesn't throw you off.
These are the endothelial cells lining that capillary.
And we're going to study exactly how fluid gets forced out.
So as we move along this capillary,
the blood is going to become deoxygenated.
And so I'm drawing the changing colors.
Even though, in reality, it's not the endothelial cells that
are changing colors, but having these colors will help orient,
I think.
And so flowing through this capillary,
you have little red blood cells, which
I'll try to draw as little disks there.
You have little red blood cells.
And of course you have lots of water, a lot of water.
And finally, you also have a bunch of little proteins.
Some of the proteins are actually bigger than others,
so we'll draw them in varying sizes.
But all this stuff is present in the blood.
And because of the high pressure, it gets squeezed out.
So the first question is exactly where and how
does it get squeezed out?
And the answer is it gets through between the cells.
The cells are not quite held together
tightly enough to prevent stuff from getting out.
But they are held tightly enough to prevent the bigger things
from getting out.
So the red blood cells and the bigger proteins won't get out.
But what will get out is some of the fluid, some of the water,
and some of the smaller proteins, which
we'll draw like this.
So actually, by the time the blood gets down
to the latter part of the capillary,
it's lost some of these smaller proteins
and some of this fluid.
And so let's show that by actually erasing some of it.
And so the concentration of bigger proteins is higher.
And you could even say that the concentration of red blood
cells is a little higher.
So we have a higher concentration
of solutes in this part of the capillary
than we had over there.
And the reason why I'm mentioning this
is because there's actually a subtlety, a complication here,
which is that although lymph or what will soon
be lymph is being forced out of the capillary over here,
actually some fluid comes back into the blood
vessel over here.
And to understand that, you need to know
that the concentration of solutes
is higher here, which leads to a higher osmotic pressure pulling
fluid back in.
And to understand it, you also need
to know that the pressure, the regular hydrostatic pressure,
is lower in the latter part of the capillary
compared to the earlier part.
So the pressure goes down.
And the pressure was what was pushing the lymph out
of the blood vessel over here.
So now we have less pushing out, and more pulling back in.
The stuff that's pulling back in is the osmotic pressure
caused by the increased concentration.
So here, some of the fluid and some of the smaller proteins
might get pulled back in.
So now you might be confused because I
said that fluid is getting pushed out,
is leaking out of the blood vessels.
And here I'm saying that it's leaking out but then
coming back in.
But the fact is that more goes out than comes back in.
And so let's draw these arrows a little bigger.
So more goes out than comes back in.
And so net result of the blood traveling
through this capillary is that you end up
with lymph squeezed out.
So now you know a little more detail
about how and why we end up with lymph.
But you might be asking yourself,
why don't these cells just tighten up their connections
there to prevent stuff from getting out?
But the answer is, you have to remember
that we have other cells around here which want to be fed.
They want to get glucose and other such things
from the blood.
And so it's important that little glucose molecules
and small proteins like hormones can get out
into this space near these cells so
that it can interact with them.
So basically, if these endothelial cells
in the capillaries were tightly connected together,
then all these cells in your body
would kind of be sealed off.
And you might be wondering, well,
are these gaps also necessary so that oxygen
can get to the cells?
And actually, oxygen is different.
It's very small.
It's just two atoms, one molecule.
So oxygen can just go right through the cells.
It can just travel right through walls
and get where it needs to.
So hopefully this gives you a better idea
of how lymph is kind of formed from capillaries,
how a bunch is squeezed out and only some of it gets back in.
So finally, I just wanted to draw a pretty ugly human being,
and make the point that you pretty much have capillaries
all over your body.
They're feeding all your muscles, all
your different tissues.
And for that reason, you need lymph vessels
to take lymph away from pretty much all of your body.
So you have lymph vessels coming out of your legs,
coming out of your arms, and so on.
And in a sense, they have a similar structure
to blood vessels, which is that they start very small.
And many of them come together and join up
into a bigger lymph vessel.
And that lymph vessel joins with another, bigger lymph vessel,
and so on.
And so it's kind of like they start out as capillaries,
and then they merge together and get bigger and bigger.
But of course they have the big difference
from blood vessels, which is that they're not a closed loop.
Ultimately, when they've consolidated
into big lymphatic vessels, they just
dump their contents back into blood circulation.