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

Lymph & lymphatic system

Let's learn what lymph and lymphatic system are. Created by Mahesh Shenoy.

Want to join the conversation?

Video transcript

- [Instructor] When you go to the doctors for a routine checkup, they usually start by feeling your neck or sometimes behind of your ears. Why do they do that? What are they checking for? And you may have heard of this disease called elephantiasis, where the limbs of the body like, say, one of the legs starts swelling up. What causes this swelling? To answer these questions, we need to look at the lymphatic system of our body. So what is this lymph? Let's find out. To figure out what lymph is, let's start with something that we already know, the circulatory system. It consists of your heart, which doesn't look like this. I'm pretty sure you know about this. But since we are not focusing on the heart, let's just keep it this way. Anyways, this heart pumps this oxygen-rich blood through this artery all the way into the blood capillaries. This is where the cells of our body starts removing the oxygen from the blood and starts throwing carbon dioxide back into the blood. And now the blood is deoxygenated because it no longer has oxygen, it has a lot of carbon dioxide. And that's why it's shown in blue. It goes all the way up through this vein into the heart. And then from there, goes to purify in the lungs, and then it gets purified, comes back to the heart, and the cycle repeats. That's something we have seen before, right? But now let's talk about something that we haven't seen before. The first thing is remember that the blood inside the arteries, the vessels that take the blood away from the heart, so this one and this one. They are under pressure, right? Because as the heart squeezes, the blood is under pressure, which means the blood is pushing on the walls of these arteries. And another thing to focus on is that if you were to zoom into these capillaries, so if you were to look at one of those capillaries, then we see that the walls of the capillaries have holes in them. So now think about it. The blood over here is under pressure, that means the blood is pushing on the walls, and the capillary of that walls, and the wall of that capillary has holes in them. What's going to happen? Well, if you thought that due to the pressure the blood starts oozing out of these holes, then you are absolutely right. Now we will not call this blood because blood contains plasma, which contains proteins and then it has RBCs, and WBCs and everything, right? Now what oozes out is just the plasma. RBCs can't ooze out because the holes are too small for RBCs to go out. So RBCs don't ooze out. The plasma comes out with a little bit of proteins. So some small proteins, not big ones, but some small proteins also ooze out along with this plasma. Let me just clean this up a little bit. Okay, so this plasma that oozes out and then fills up this space between the cells, this is what we call the lymph. All right? And it's pronounced as lymph, with an F over there. So how is lymph different than blood? Well, lymph does not have RBC in it, right? And as it is our lymph, it's pretty much colorless. In fact, the word lymph comes from the Latin which means water like, water like. And why is it called water like? Because it is pretty much colorless. And this means wherever these capillaries are present, the plasma will ooze out to form the lymph. And capillaries are found all over our body. So you will find this lymph pretty much everywhere inside your body. Okay, so what's the big deal? Why should we care too much about this? Well, first of all, this means that every time blood goes through the capillaries, it will start losing some of its plasma. That's bad because that means the blood will start getting thicker and thicker because it starts losing plasma. And eventually, it becomes more and more difficult to circulate it because it's so thick. And that means our cells will not get any oxygen. And I'm pretty sure you know what would that mean. The cells would start dying. Secondly, as this lymph starts getting accumulated within our tissues, it starts pushing on the cells. And the tissues will start getting puffed up. And that causes swellings. And that means our organs will start swelling up. And that's one of the reasons why in elephantiasis our limbs swell up. This is because of the accumulation of the lymph over there. Now before we talk about how our bodies tackle this, I'll tell you what I should think about this. I should think that this is such a bad design. If we don't want the plasma to leak out, then why are there holes present in the capillaries? What a bad design, right? Well, guess what? The holes are necessary. Because, you see, cells aren't only exchanging gases like carbon dioxide and oxygen, which can easily pass through the walls but they're also exchanging other stuff like nutrients. For example, glucose. Also the cells need glucose. They get that from the blood itself. Then the cells produce some waste products, like some nitrogen as waste sometimes, and that needs to be put back into the blood. And for that, we need space. They can't go through the wall as they are big, big molecules. They can only go through the holes. And so capillaries need that holes to exchange the stuff. And so since holes are absolutely necessary, there's nothing we can do about this plasma leaking out. And so the big question is what does your body do about this? Well, thankfully, your body has a separate set of vessels, which are called lymph vessels, which clear out this lymph from your tissues. So just like how we have blood vessels and blood capillaries, these are called lymph vessels. So the thick ones are called lymph vessels and the thin ones are called lymph capillaries. So over here if I were to show you those lymph capillaries, this lymph fluid enters into these capillaries and it clears the lymph out from your tissues. Now one difference between blood capillaries and lymph capillaries you can immediately see is that blood capillaries connect two blood vessels, right? They're open-ended. But as you can see, the lymph capillaries, they are closed at one end. Can you see that? They're closed. And so I was really curious when I was studying this. How does the lymph just enter into this lymph capillary? It's pretty awesome. Let me show you how. Let me get rid of this leg. And let me zoom in to one of those capillaries. The cells of this lymph capillaries, as you can see, have little bit of overlapping structures. That's very important. So now when they are surrounded by these lymph fluid in the tissue, it's the same lymph fluid over here, the fluid starts pressing on the walls. So it starts pressing on the walls. And now look carefully what happens when the fluid starts pressing on the walls. These walls open up. Look at the junction carefully. The walls open up like this. Cool, right? Let me show you one more time. The walls open up like this. And now the fluid enters the capillary. And now if you're wondering why doesn't the fluid just flow back out through the same opening, then once the fluid gets inside, it'll start pushing from inside, right? And as it pushes from inside, look at what happens to these walls, they will start closing now. Look carefully. They will now close. So it's like a one way door. It only allows the fluid in but doesn't allow it to go out. That's how these capillaries clear out the lymph from your tissues. So now the next question would be where do these vessels take that fluid eventually? Where do they put it? Can you guess this? Can you guess where they should put it back? Well, since lymph is basically plasma leaking out from the blood, it makes sense to put it back into the bloodstream, right? And so these lymph vessels eventually connect to one of the blood vessels to recycle it back into our circulatory system. But a question for you would be do you think we should connect these lymph vessels to an artery? Or a vein? Can you pause the video and think about which one would you connect it to? All right, let's see. If we were to connect this to an artery, then we would have hard time trying to force that lymph into the circulatory system, because the arteries, inside the artery the blood is under a very heavy pressure, right? In fact, the pressure of the blood would actually force the lymph out, back out of these capillaries. It might even shatter the walls of the lymph capillaries. This is gonna be very bad. So we want to reenter it somewhere where the pressure is very low. And that is somewhere in the veins. And so the lymph vessels eventually connect to a vein, somewhere close to the heart, that's where the pressure is the lowest, and then it reenters into our circulatory system. And this is how the lymph vessels ensure that we get back the lost blood from the capillaries. And so let's write this down. One of the most important jobs of the lymph vessels is to clear that lymph out. So I'm just going to write clear lymph. So it takes the lymphs from the tissues and puts it back into the veins. And so we are done, right? Because your blood is regaining the lost plasma from the capillaries, right? Well, not yet. Not quite. And the reason is what if your vessels, lymph vessels, start picking up something unwanted. Like, say, for example, your tissues are infected by some pathogens like some bacteria. Then when the lymph enters into these vessels, even the bacteria might start entering over there. And then these bacteria might enter into your bloodstream. You don't want any bacteria in your circulatory system. That's gonna be pretty bad. So what to do? Don't worry, your lymph vessels are awesome. They have security points all over them. These are called lymph nodes. Let me just write that down. Lymph nodes. And they have a lot of WBCs over there. WBCs are like police. So when the lymph enters into these nodes, if there are any unwanted stuff like bacteria, the WBCs are going to kill it. This will make sure that no unwanted stuff enters your bloodstream. It's pretty cool, right? And so you see these lymph vessels also help in killing unwanted stuff from your body. So they also help in immunity. So they help in immunity. That's pretty cool. And guess what? If any of your lymph vessels pick up a lot of bacteria, then a lot of the WBCs will gather in that lymph node and that node will now swell. Which means swollen nodes are an early indication of infections. And that's what the doctors are checking for when they are feeling your neck. It turns out there are lot of these nodes near your neck and your ears. And so they touch that and see if any of those are swollen. If they're swollen, good chance there's an infection somewhere. Now before we wind up, another important job that the lymph vessels have is to transport fat into your bloodstream. So I'm just going to write transport fat, for short, into your bloodstream. And what I mean by this is imagine you have your small intestine somewhere over here. All your food gets digested and then they come over the small intestine. And most of the nutrients can directly enter into the capillaries through these holes. And that's how the nutrients enter your bloodstream. And then they are circulated to all the cells. But the digested fats, which come out of your intestine, are just too big to enter into these capillaries. So they can't directly enter the bloodstream. So they are again picked up by the lymph vessels or the lymph capillaries, then they go through the lymphatic system and then they reach the bloodstream. And so that's how the fat enters your bloodstream, through the lymphatic system. So what did we learn in this video? We talked about the awesome lymphatic system of our body, which consists of the lymph, the lymph nodes, and the lymph vessels. The lymph is basically the plasma that leaks out of your capillaries due to blood pressure. This lymph is picked up by these lymph vessels, which recycle it back into our circulatory system through the veins. And the lymph nodes act like security to make sure that no pathogens enter our bloodstream. And finally, the lymph vessels found near the small intestine help in transporting these digested fat into our bloodstream as well.