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Why we need a lymphatic system
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.