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How lymphatic vessels move fluid

Find out how the body's mysterious second circulatory system works, and how it can move fluid even when it has no heart of its own. The lymphatic system cleverly returns fluid to the circulatory system without a pump. It reenters at the low-pressure end of the venous system, uses one-way valves to prevent backflow, and relies on body movement and smooth muscle contractions for propulsion. This ingenious system ensures efficient fluid circulation.
By Patrick van Nieuwenhuizen.
Created by Patrick van Nieuwenhuizen.

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  • male robot hal style avatar for user BCShaw
    How do people with very limited mobility, or perhaps none at all, generate enough pressure to drive the lymphatic fluids back into the blood stream?
    (47 votes)
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    • male robot hal style avatar for user Wudaifu
      Good question. When people are NOT able to return all the lymph back to their blood stream, some of it may stay in the interstitial fluid outside of the circulatory system and cause swelling in those areas. This swelling is called EDEMA and commonly occurs in the feet, legs, or other parts of the body where gravity makes it most difficult for the lymph to return to the blood stream. In addition to the folks that you mentioned who may have limited mobility, other folks who are at risk of developing edema include anyone who sits or stand a lot, pregnant women (where the enlarged uterus makes interferes with blood & lymph flow), as well as individuals with other medical problems such as heart, liver of kidney disease. Hope this helps. Good luck.
      (79 votes)
  • male robot johnny style avatar for user Raj
    Does taller people heart need to pump more or apply more pressure? I can guess from the videos that the average BP right at the aorta is ~ 100mmHg. Is this higher in taller people. I ask because i am thinking about the distance blood needs to travel from their legs from inferior vena cava into the heart. (apologies if this sounds stupid. I am from finance and got pretty much interested in human biology when i was looking at at sal's video on TSH)
    (7 votes)
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    • old spice man green style avatar for user mazzapants140894
      I couldn't find any information on the correlation between height and elevated blood pressure.
      My guess or at least what makes sense in my mind (highly likely to be incorrect) is that the size of the heart scales in relation to the size and/or mass of the individual. A pathological example of this would be cardiomegaly ( an enlarged heart) which occurs for many reasons, one being obesity.
      In a non-pathological state ie: a tall individual, perhaps the heart is slightly bigger to accommodate for the larger work load. Thus, the heart has a higher contractile force and the individuals blood pressure does not need to be elevated.
      (2 votes)
  • male robot hal style avatar for user Luke
    How long does it take for the lymph to reenter blood flow after it enters the lymph vessel?
    (6 votes)
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  • blobby green style avatar for user Josephine  Mustapha
    how do interstitial fluid and lymph differ from blood plasma?
    (1 vote)
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    • piceratops tree style avatar for user Maximus Peto
      Hello Josephine,

      Those three fluid components are all different - interstitial fluid, lymph, and blood plasma. I'll describe each, so you understand how they're different from one another:

      (1) "Interstitial fluid" is the fluid surrounding cells. It is a common misconception that the fluid surrounding cells is blood - this is not the case. Cells are actually surrounded by a thin layer of liquid, and this liquid is called "interstitial fluid".

      (2) "Lymph" is a fluid in a different compartment than blood or interstitial fluid. Lymph is in the "lymphatic system", and is an important part of the immune system (perhaps you've heard of "lymph nodes", which can swell with fluid and immune cells when you have an infection).

      (3) "Blood plasma" is the fluid-part of blood, after you remove all the cells, such as red and white blood cells. It contains salts, proteins, and water, but no cells.

      I hope this helps!
      (7 votes)
  • purple pi purple style avatar for user Judith Jones
    Do people with very limited mobility suffer health problems because their lymph system is not working as well as it would if they could move?
    (4 votes)
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  • old spice man green style avatar for user BAM'99
    Do people with low blood pressure have less lymph surrounding the capillary beds?
    (3 votes)
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    • piceratops tree style avatar for user TheSqueegeeMeister
      Not necessarily, if the low blood pressure is a result of vasodilation then the lymph formation will usually be increased. This is also the case in patients with right side heart failure, as the right side of the heart is unable to pump blood out to the lungs blood gets backed up in the venous system leading to edema and excess fluid/lymph in the peripheral tissue. If the person is otherwise normal then yes lymph formation is a direct result of blood, osmotic and hydrostatic pressures.
      (2 votes)
  • hopper cool style avatar for user bee anchor
    what makes the interstitial fluid go into the lymphatic vessels?
    (3 votes)
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  • blobby green style avatar for user Wynette Allen
    What happens when a collection of lymph is swelling in places such as axillary, the perineal area, and back? Is it safe to squeeze these areas of collection? I guess they are called "cysts." How does this interrupt the flow of the lymph system?
    (3 votes)
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  • leaf green style avatar for user dinesh.c.upreti
    Does the lymph use osmosis or diffusion to get into the tube.
    (2 votes)
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  • male robot hal style avatar for user Camile
    Why does the lymphatic system do not reenter in the arteries? Indeed, the differential pression would be much higher and this will promote a better lymphatic return?
    (2 votes)
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Video transcript

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.