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.