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What is actually in lymph

Learn about the differences between lymph and blood. Find out what is actually in lymph, and how it might be different across your body. By Patrick van Nieuwenhuizen. . Created by Patrick van Nieuwenhuizen.

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Video transcript

So we've talked a lot about the lymphatic system. And in this video, I wanted to talk more about lymph, which is the fluid that's in the lymphatic system. So we know that lymph comes from blood vessels originally. It's the fluid that's kind of squeezed out of blood vessels. And so here let's say this is a capillary. I'm drawing the endothelial cells around the capillary. So lymph is what comes out of here, what's squeezed out through the little holes between the capillaries. And just so we have our terminology straight, really, lymph is only called lymph once it gets into the lymphatic vessels. So once this fluid get into here, now we can actually call it lymph. But before it does, it's more called extracellular fluid, because it's outside the cells that are out here. But really, the composition is the same. And so this is just a semantics thing. So in this video, let's try to figure out exactly what lymph is made of. And so to do that, let's start with blood, because lymph originally comes from there. So in blood, we obviously have red blood cells. So let's draw a couple of those in here. These are red blood cells. And in reality, we should draw more than a couple, because we know that blood is actually consisting of about 40 or maybe even 45% red blood cells, 45% by volume. And that's in the average person. If you're anemic, it'll be less. So blood has about 40%. We're drawing much less than that here. But it's kind of hard to draw them all packed in, so let's leave it like that. And so other than that, of course we have water, which we'll draw white. This is water. And we also have proteins. We have proteins. And we'll draw those of varying sizes, because proteins really do vary by size. And we have many different kinds of proteins in the blood that do many different kinds of things. And that's all that we'll draw here. Even though there are also glucose molecules and fats and other things in the blood, we can ignore those for now. So what gets out here? What manages to squeeze through? Well, we know that the red blood cells don't manage to squeeze through. So certainly, by the time we get to extracellular fluid, we'll have 0% red blood cells. And so lymph will have no red blood cells. But other than that, let me ask you a question. Do you think that lymph has more protein or less protein than blood? And let's just say by concentration. So it's actually a tough question, because if all the protein came out freely of these vessels, then lymph would actually have a higher concentration of protein than blood, because blood had these red blood cells that were taking up space. And so the protein was, so to speak, diluted by this huge amount of red blood cells. And out here, it won't be diluted by red blood cells. But actually, there's less protein out here in the lymph. And that's because water gets out so much more easily from these capillaries than protein that you end up with fairly dilute fluid here. So proteins have a kind of tough time getting out. And certainly some of them do get out. And so we'll draw those here. But it's much less than are in blood. And so we end up with about a half to a third the amount of protein in lymph compared to blood. So lymph has about a half to a third versus blood. Let's draw that like that. Another thing we can ask ourselves is what kinds of proteins are going to be in this lymph? Since we said that there were so many different kinds of proteins in the blood, which ones are going to get into the lymph? And actually, it's going to be the smaller ones, generally. The smaller ones are going to have an easier time getting through these holes. And so we can say, for example, that the ratio of a small protein like albumin, over a large protein like an immunoglobulin-- and I'm just going to write Ig. These are antibodies. And they're generally bigger than albumin. So the ratio of albumin to immunoglobulin is going to be bigger in lymph than it will be in blood. And the reason for that is what we just said, that smaller proteins have an easier time getting through than large proteins. So even if you have less albumin overall in lymph compared to blood, you'll have a greater albumin over immunoglobulin ratio. And let's actually pause here a moment before we move on from talking about protein in the lymph, because it's actually very important that there's less protein in the lymph than there is in blood. Because if you watched one of the earlier videos, you remember that we said that actually the way that fluid gets squeezed out of the capillaries is not so straightforward. Because what happens is, first, some of it gets squeezed out. But then some of it gets taken back up. And the reason why some of it gets taken back up is because, as you travel down the capillary, the hydrostatic pressure that pushes this fluid out goes down. So there's less pressure pushing it out. But there's still some pressure pushing it out. So why is some of this fluid going back into the blood vessel? And the reason is that there's a higher concentration of things like proteins in here. And that higher concentration makes the blood more-- and this is going to be a scary word-- but more osmotically active. And so there's a higher osmotic pressure pulling fluid back in, exactly because you have more protein in the blood than you do out here in the lymph. And so it's very important that you do have more protein in the blood. And now I'd like to ask you a question, which is, how much lymph do you think is produced per day? So the amount of lymph that's produced is the amount that's squeezed out of these blood vessels minus the amount that's taken back up. And so how much do think that adds up to in an entire day? So how much lymph do we produce? And let me give you a moment to guess. So how many liters? So come up with a number, the number of liters of lymph that we produce per day. And the answer is 3. So we make about 3 liters of lymph per day. And that's all filtered out through these capillaries. That's a word people like to use, filtered. And actually, it turns out that the amount that actually comes out first is about 20 liters. And I'm going to draw it small because these are really small details that you don't need to know. But about 20 liters is squeezed out. And then about 17 liters comes back in. And so that leaves a difference of about 3 liters that comes out per day. So this is per day. The final thing that we should really mention is that lymph has a different composition depending on where you are in the body. So let's quickly look at a human body so we can talk about how lymph might differ from place to place. So here is a human. And the one thing we talked about was how, in the small intestine over here, your lymphatic vessels, which there are called lacteals, are taking up a lot of fats, a lot of chylomicrons. And so as you might guess, the composition of lymph coming from there is going to be much higher in those chylomicrons, which are collections of fats. So there you're going to have more fats. Whereas, when you go to the liver-- and this is not something necessarily that you know or should know, but it's interesting fact. The liver produces a ton of proteins that end up going into your blood. And one of the ways that they get them into the blood is to put them into lymph. And so your lymph coming from the liver has a much higher concentration of protein than your lymph coming from elsewhere. And it actually can get to as much as 10 times as much. You can have 10 times as much protein in the lymph coming from the liver compared to other places. And so this number here that we gave, one half to a third, is really just an average of all the lymph all over your body, because the protein content can vary widely from place to place. As we know, eventually all the lymph vessels, including the lacteals coming from your small intestine, they all join up. And they come up your body. And also the ones coming from the liver will merge and join and come up and travel up through the body. And eventually they'll be put back into circulation. And that will be here in the veins up here, which I'll draw in blue, even though it's getting a little confusing there. I'm drawing them in blue because we generally draw veins in blue. And those merge and go to the heart. So all these different lymph fluids with different compositions come together and go back into circulation.