The heart pumps blood
throughout the body, and it pumps so powerfully
that it actually squeezes some fluid out of
the capillaries. And that fluid becomes lymph. And it's the lymphatic system
that gathers all that fluid and brings it back
into circulation. But this idea might bother you. You might have an
objection, which is these lymph vessels don't
have a heart of their own. I didn't draw one, and it's
because they don't have one. So how is it that
they're able to pump this fluid in one direction? And moreover, how
is it that they're able to pump it back into this
high-pressure system, given that they don't have a pump? That must be quite difficult. There are two answers
to this question really, and the first is
a little simpler. So let's start there. The first answer is essentially
in the location of reentry. So the reason why lymph
is able to get back into the high-pressure
system is because lymph is quite intelligent
about where it tries to reenter the
high-pressure system. So I'm drawing a star
here because that's where the lymph reenters. I didn't draw that by accident. It reenters at the very end of
the venous circulation system, and the pressure
there is much lower than the pressure
in the arteries and even the pressure
in the capillaries. The numbers are actually
pretty striking. The pressure here in
the early arteries can be about 120
millimeters of mercury. That's kind of like your
average blood pressure. But over here, just before
it enters the heart, the pressure is more like
five millimeters of mercury. So the fact that the lymph fluid
enters circulation at the end here makes it much easier. It has to fight
against much less. If the lymph vessels ended
up trying to drain here, let's say into the aorta
or into a big artery, probably none of
this would work. So that makes the
job easier, but it doesn't explain how lymph
moves sort of one-directionally into the blood vessels. What is actually
forcing the lymph? What is getting it to
move in this direction? Why wouldn't it just come back
as much as it would go forward? And the reason why that's not
the case is because of valves. So valves are structures
within the lymph vessels, which prevent fluid from going back. So let's draw a
lymph vessel here. And a valve might look
something like this. And what this valve accomplishes
is that if fluid gets pushed that way, it'll part
the two leaves of the valve, and it will get through. But let's say that fluid
tried to come back. When it tries to
come back, it'll push the leaves of the
valve back together, and it will create an obstacle. Now this is a
really smart system, and actually, it's not only
used in your lymph vessels. You might know that it's also
used actually in the heart. You have four big
valves in the heart that operate on this exact principle. And it's also used in
veins because the pressure to push fluid through veins
is sometimes not sufficient. And so having a mechanism
to prevent backward flow is very helpful. But still you might
not be satisfied because you might
say, well, what causes the lymph to
move at all here? Why wouldn't it just
sit where it is? And there are actually two
ways that the motion begins. One is that you actually have
a little bit of smooth muscle attached to your lymphatic
vessels, and that can contract. And just by
squeezing, it'll cause motion of the fluid forwards. So that's the first mechanism
by which the motion starts. And the second is
skeletal muscle. Skeletal muscle just refers to
basically all the big muscles in your body that are
under voluntary control. Think about your leg muscles,
your arm muscles, and so on. And the reason why
this does something is that just throughout the
course of your average day, you use your muscles,
you move around, you have a certain amount
of jostling in your body. And so it's
inevitable that you're going to squeeze certain parts
of your body at some times. And when you squeeze a
part of the body that has a lymph vessel like this,
it's going to start the motion. It's going to squeeze it just
like the smooth muscle did, and you'll get fluid moving
ahead in the direction that the valves allow them to. And so in addition
to the valves, which we can draw in to our diagram
here, in addition to those, there's an interesting
contraption where the lymph first enters
the lymphatic vessels. So I drew it here
is an open tube, but in reality it's closed. At least, it looks closed,
but it's porous, of course, and that's what allows
fluid to get in. Let me just draw
this a little bigger. But basically, this is where
the lymph vessel starts. And the walls of
this lymph vessel are kind of like valves
in and of themselves in that they allow
fluid to come in, but when the pressure
inside rises, it prevents fluid
from going out. So the lymph cannot go back out. So finally, let's just
look at a human body to get a sense of where
everything that we're doing fits in. So here is another ugly human. Let's quickly give
him some arms, a head. And now we can
actually show where the lymph reenters circulation. So let's say that
that's the heart. Well, you probably know that
one of the huge veins leading into the heart is the
superior vena cava. And right near the
superior vena cava, you have what are called
the subclavian veins. And you have one on the
right and one on the left. They're called subclavian
veins because they pass just under the clavicle, and it's
actually right into these that the lymph is reentered. So I'm drawing that in green so
that it corresponds with that. It's at this point that
we have very low pressure, and it's there that all
the lymph in the body goes. So that includes lymph from down
in your legs and from your arms and from your neck and so on. And it's pretty remarkable
if you think about it that all the lymph in your body
traveled up this entire path and got dumped back
into circulation there, and all of it was
without an active pump. It was all done
through this method of-- a little bit of
squeezing with smooth muscle-- but mostly, just the ingenuity
of these one-way valves that take advantage of the fact that
you're always moving around to sort of squirt the lymph
back up all the way up your body to essentially your neck.