If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Why we need a lymphatic system

Welcome to the lymphatic system! Find out why we need it, and how it interacts with our blood vessels. By Patrick van Nieuwenhuizen. Created by Patrick van Nieuwenhuizen.

Want to join the conversation?

  • leaf red style avatar for user Jack McClelland
    What exactly is the environment/system outside of the capillaries that the hormones and other cells leak out to?
    (16 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user Ijoni Lisha
      1.Cells don't leak out. The only cells able to move outside the capillary bed in the interstitium are white blood cells through a process called diapedesis.
      2. The space between cells also called interstitium contains interstitial fluid made of water(as a solvent), ions, aminoacids, neurotransmiters and other small substances, but not proteins.
      3.To pass from blood to interstitum, fluid must be filtered through :
      a)Glycocalyx; b)Endothelial cells; c)Basal Lamina
      So in a way Interstitial fluid is a filtrate of plasma.
      (41 votes)
  • blobby green style avatar for user Holly Bathen
    If Oxygen can diffuse through blood vessels at any point in the circulatory system, why does it predominantly happen in the capillaries? Why do the heme groups give the oxygens up at this point and not earlier or later?
    (16 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user yonatan porter
      First note the capillaries combined have a much higher surface area than the arteries, and also a much higher overall cross section area that results in a reduced speed in which the blood is traveling at the capillaries, hence allowing for more oxygen being released.

      Second, oxygen is predominately given of at the capillaries due to the fact that the capillaries contain a higher CO2 concentration that lowers the blood at the capillaries' PH and results in Hemoglobin changing its structure and lowering its affinity for oxygen- hence more oxygen is being released from Hemoglobin.
      (32 votes)
  • aqualine ultimate style avatar for user Guinea Gal
    So where exactly does the lymph flow back into the blood stream? At -ish Patrick said that lymph gets dumped back into the blood stream, but never actually said where. Does it get dumped into the vena cavas?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user James Li
      As I understand it, the lymph system receives through the blood stream by simple diffusion across capillaries. After traveling through the Lymphatic System (nodes and vessels), it returns by re-entering the circulatory system at a major vein known as the Subclavian Vein, which is paired and present on both the left and right sides of the body.
      (17 votes)
  • blobby green style avatar for user rifadulhossain
    Where there are no lymphatics in our body?
    (5 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user WJP
    How do the fluid, glucose, and protein escape out of the endothelial cells when it is encased by the basement membrane?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user Miki
    At around he said that the high pressure in the heart forces the fluid out of the blood vessels. What forces the fluid to flow back into the blood vessels?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • mr pants pink style avatar for user keith.zehnder
      The heart forces fluid out of the blood vessels AND into the blood vessels. The fluid does not flow "back in" once it is forced out though. Blood leaving the veins is oxygen poor and needs more oxygen, so the blood comes back to the heart (all veins go towards the heart) and then the blood is pumped out of the heart to the lungs to drop off carbon dioxide and pick up more oxygen. After the blood is pumped back (the trip to from the lungs to the heart is a very short trip), the oxygen rich blood gets pumped out of the heart into the arteries carrying oxygen all over the body. The heart pumps more and more blood into the arteries which carry oxygen all over, but the blood has to go somewhere once in is no longer oxygen rich. The veins are connected all over the body, and all the blood comes back from the body to the heart, being pushed by the blood behind it, right back into the heart to start the cycle again.
      (3 votes)
  • piceratops sapling style avatar for user chung hiu man
    why the pressure in the latter part of capillary goes down 'suddenly'?
    can sb explain this pls?
    (PS i haven't chosen physics as my elective subj so i can hardly understand by just a formula,pls explain in a more simple way if it is possible,love you^^)
    (2 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user Anna
      It is not sudden. Aortic pressure = 120 mmHg. As arteries branch pressure lowers since if it wasn't that way the arteries would rip and maybe even cause internal bleeding. It gets pretty low at the arterioles. It gradually lowers to 80 mmHg at the venules. Then as more veins join the pressure further lowers to 5 mmHg in the RA. The right ventricle doesn't raise the pressure all that much. However it is enough that the LA has noticeably more pressure than the RA. Then the left ventricular pressure is somewhere around Aortic pressure, maybe a bit lower and the whole cycle starts again.
      (5 votes)
  • aqualine ultimate style avatar for user Rudra
    I get that lymph vessels bring lymph back into blood vessels, but what function does lymph serve INSIDE the blood vessels?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • leaf yellow style avatar for user Taylor Logan
      Lymph is not present inside blood vessels. Blood plasma, containing nutrients and gasses, leaks out of the capillaries and into surrounding tissue. The fact that more leaks out of the capillary than leaks back into the capillary (as given by the Starling equation) means that there is an accumulation of this fluid in the interstitial space (the space between cells) and is then known as interstitial fluid. The purpose of lymph vessels is to give a place for this interstitial fluid to go, and to eventually be recycled back into the circulatory system. This fluid is only called lymph once it drains into the lymph vessels.

      So lymph is not found in blood vessels.
      (3 votes)
  • blobby green style avatar for user Maxi
    could anybody transcript what he says at - to -?. (... so let`s show that actually... so the concetration...) I'm translating it to spanish in order to create subtitles
    (3 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user khanm043
    why would so much content of the plasma leave the capillary in the first place, when it would just have to come back.
    (2 votes)
    Default Khan Academy avatar avatar for user

Video transcript

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.