How lymphatic vessels move fluid

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.