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Red blood cells

Oxygen uptake by hemoglobin in red blood cells. Created by Sal Khan.

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Video transcript

In the last video on the lungs or the gas exchange in our bodies or on the pulmonary system, we left off with the alveolar sacs. Let me draw one right here. So we have these alveolar sacs that I talked about and they're in these little clumps like this. Let me draw a couple of them just so you get the idea. And if you remember from the last video, these are kind of where air goes in through our trachea, then that splits up into our bronchi, and then those split into the bronchioles, and the bronchioles terminate at these alveoli. So that's the alveoli. These are these super-small sacs that we talked about in the last video on the pulmonary system. You might want to watch that video if none of this sounds familiar. And then of course we have our bronchiole that feeds into this, and then that might have branched off from another one that feeds into another set of alveolar sacs, but I don't want to get too focused on that. I covered that in the last video. In the last video, we saw that air, when we breathe in, when our diaphragm contracts and makes our lungs expand and fill up that space, air comes in. Air comes in and that air that comes in is going to be-- as we're breathing atmospheric air-- it's going to be 21% oxygen and it's going to be 78% nitrogen. And actually, in our atmosphere, carbon dioxide is actually almost a trace gas. It's less than 1%. So any time you breathe in on Earth, this is what you're going to get. And we said in the last video that you have these capillaries, these pulmonary capillaries that are running all along the side of these alveoli. So let me draw those pulmonary capillaries-- and so when they are de-oxygenated-- so they come here to be oxygenated. So when they're de-oxygenated, they might look a little purplish. And then they pick up the oxygen from inside the alveoli-- or the oxygen diffuses across the membrane of the alveoli, into these capillaries, into these super small tubes. And then once they do, that makes the blood red. I'm going to talk in a little bit about why it becomes red. So then it becomes red, and now that the blood is red, it has its oxygen. The whole point is to get the oxygen. It's ready to go back to the heart. So that's just one little part of it. And we learned in the last video that something that goes away from the heart-- so this is going away from the heart-- that is an artery. "A for Away" - Artery. And something that's going towards the heart is a vein. So this right here is a vein. Now one question-- and this actually came up in the last video. Someone asked-- which I think is a very good question-- is, gee, when we breathe in, most of the air is nitrogen. Only 21% is oxygen. What happens to all that nitrogen there? How come that doesn't go into our blood? And that's actually an excellent question. So to answer that, I think that actually helps explain what's going on here. Let's draw a little bit bigger. This is the inside of of an alveolus. This is its membrane right here, super thin, almost one cell thick. And then you have a capillary running right next to it. Let me do that in a neutral color. So you have a capillary that's maybe running right along the surface. And this is porous to gases like oxygen, and nitrogen, carbon dioxide. And what we have here-- let's say that this is-- so the heart is over here. So this is blood coming from the heart and then this is going to go back to the heart. Well, the heart's on both sides. So let me write it this way. From the heart and to the heart. And what you have here is-- when we're coming from the heart, this is de-oxygenated blood and it's actually going to have a high concentration of carbon dioxide. I already did nitrogen as green. Let me do carbon dioxide as orange. There's a lot of carbon dioxide and actually carbon dioxide actually gets diffused in the blood. It actually is carried in the plasma of the blood. It's not carried by red blood cells that we're going to talk about in a second. So that's a bunch of carbon dioxide here. And the concentration of carbon dioxide in the de-oxygenated blood is going to be higher than the concentration of carbon dioxide in the alveolus. so if this is porous to carbon dioxide, this membrane-- and it is, these carbon dioxide molecules are going to diffuse into the alveolus. Now on the other side of that-- we have oxygen here. We're breathing it in. The air is 21% oxygen so you're actually going to have a lot more oxygen than carbon dioxide. And this is de-oxygenated blood. We used all of the oxygen in our body and we'll talk more about that either at the end of this video or in a future video on how we use it or where it goes in our body, but there's no oxygen here so the oxygen is going to be taken-- it's going to diffuse across this membrane because the concentration of oxygen is low. Now the question is-- so immediately you see that as the oxygen diffuses across this membrane, all of a sudden, this is oxygenated blood ready to go back to the heart. So this transition between artery and vein is a very subtle thing. Very clearly here, you say that, OK, this is going from the heart. This is our vein. This is going to the heart-- sorry. I always get confused. This is going away from the heart-- and I was looking for an A and I wrote from. This is away from the heart so this is an artery. And this is going to the heart so this is a vein. So you could make the division. You could say, OK, once it's oxygenated, maybe we're going back to the heart, but it's kind of an arbitrary-- sorry. I spelled artery wrong. These are my flaws. Spelling was never my strong suit. So it's hard to say where the artery ends and the vein begins. A good demarcation is when the carbon dioxide concentration goes low and that the oxygen concentration goes high. That's a good time, where we start from the pulmonary artery. Probably in the next video, I will a make a very-- you'll see why the pulmonary arteries are special, because pulmonary arteries coming away from the heart have no oxygen or very little oxygen and they have a lot of carbon dioxide. So pulmonary veins, which is-- it's arbitrary where the artery turns into a vein. Once it gets oxygenated, it's ready to go back to the heart. It's a pulmonary vein and it is oxygenated. So it has oxygenated-- and we could write de-oxygenated. Now the reason why I say it's special besides the fact that pulmonary arteries and veins go to and from the lungs, is that they're kind of the opposite. Because in the rest of the body when we're going away from the heart or we're talking about arteries, you're going to see that that's oxygenated blood, while when we're going away from the heart to the lungs, that's de-oxygenated blood. Similarly in the rest of the body, when we're going to the heart, where you're to see that that's de-oxygenated blood, but in the pulmonary vein, when we're going to the heart, it's oxygenated because the lungs are what take up the carbon dioxide and give us the oxygen. Now I still haven't answered that interesting question that rose on the message board on the last video. What happens to the 78% of nitrogen that's sitting here? There's just a ton of nitrogen over here, more than the oxygen, a lot more than the carbon dioxide. What happens to all of these nitrogen molecules? And the answer is, nitrogen can diffuse and does diffuse into the blood, but the blood's ability to take in nitrogen isn't that high. And you might say, well, why is oxygen special? Why can the blood take up oxygen so much easier than nitrogen? And that's where the red blood cells come into play. Let me write this down. I'll write it in red. Red blood cells, which are fascinating on a whole set of levels. What red blood cells-- these are these cells that are sitting in-- they're flowing through our circulatory system and they look kind of like lozenges, if I were to draw one. They're kind of like a flattened sphere with a little divot on either side of it-- a lot like a lozenge. So if I were to draw it from the side, it might look something like-- well, from the side, it would look like that and if you could see through it, there'd be a little divot on each side. If I were to draw it at an angle, it would look something like this. There'd be a little divot on that side and there'd be a similar divot on the other side. And red blood cells-- and I could do a whole set of videos just on red blood cells-- they contain hemoglobin. Maybe we'll do a whole video on hemoglobin. The hemoglobin are these small proteins that contain four hem groups. So inside of red blood cells, you have millions of hemoglobin proteins. And the hemoglobin proteins-- I'll just draw them as this-- they have these four heme groups. And heme groups, the main component is iron. And that's why iron is so important. If you don't have enough iron, you're going to have trouble processing oxygen in your blood and your hemoglobin won't be functional enough. But it has iron on it. It has four of these heme groups. And each of these heme groups can bond to oxygen molecules. They're very good binders of oxygen. And we're going to see in a little bit-- probably the next video-- how they release the oxygen, but this has tons, this has millions of heme groups in it and the oxygen diffuses across the membrane of the red blood cells and bonds to to the heme groups on your hemoglobin. So because the red blood cells have the hemoglobin inside of them, they're like these sponges for oxygen because hemoglobin is so good at taking in oxygen. So the red blood cells are able to essentially suck up all of the oxygen out of the plasma. The plasma we can view as just the general fluid of the blood, not including the red blood cells. So the red blood cell here isn't so red. And the reason-- and this is the key point-- the reason why it's not so red-- maybe we had a red blood cell over here-- let me make it clear. Carbon dioxide for the most part is traveling within the plasma. It gets absorbed into the actual fluid and I'll talk about it in a future video. It's actually in a slightly different form. It's as carbonic acid and that's actually a key point for how the plasma knows where to dump the oxygen, but I'll talk about that in a future video. But over here, this red blood cell has a bunch of hemoglobin proteins in it, but those hemoglobin proteins have dumped their oxygen. And it actually turns out it's the hemoglobin-- so with oxygen, hemoglobin looks red. It reflects red light. When it doesn't have oxygen, hemoglobin does not look red. It looks kind of purplish, bluish, darkish-- something. And that's why in most of your body, your veins that have de-oxygenated red blood cells look kind of bluish. And the reason why it changes color is that when the oxygen bonds to the hem sites on the hemoglobin, it actually changes the entire confirmation, the entire structure of the protein. We've see that multiple times. The whole protein folds in such a way that all of a sudden, instead of purplish or dark light being reflected, now red light is reflected. And that's why red blood cells will become red once they take the oxygen. But I'm going on a tangent. The whole point here is saying, why we taking up so much more oxygen than nitrogen, given that there's less oxygen in the atmosphere than nitrogen? And the key is these red blood cells. These red blood cells have these millions of hemoglobin proteins inside of them and they take them up and they sop up all of the oxygen out of the plasma. Actually, they sop about 98.5% of the oxygen. So these red blood cells are just traveling and they're going to go back to the heart. They are what make our blood red. So you have this thing, hemoglobin, that's sitting in red blood cells. It's sopping up all the oxygen. So it keeps the oxygen concentration and the actual plasma low. You have nothing like that for nitrogen. There is no cell that's sopping up the nitrogen. Nitrogen does not bond to hemoglobin. So that's why oxygen is taken up so much better than nitrogen. It's a very interesting question because if you just think about how much nitrogen is, it's kind of a very natural idea. Now I want to focus a little bit on the red blood cell itself because it's fascinating. In the video on the structure of the cell, I start off saying, all cells have a membrane and they all have DNA. Now, the fascinating thing about a red blood cell-- I already said it has millions of hemoglobin molecules or proteins inside of it. The fascinating thing about a red blood cell-- it has no nucleus. And no DNA. This is mind boggling when I first found out. I was like, well, why is it a cell? Is it really even a living thing? And it turns out when it's growing, it does have a nucleus. All cells need a nucleus with DNA in order to generate the proteins that build it up, in order to exist and structurally make itself the way it needs to be made, but the whole point of a red blood cell is to contain as much hemoglobin as possible. And so you can imagine, this is actually a favorable evolutionary trait, that as red blood cells are ready to go into business, you've built the whole structure, they actually get rid of their nucleus. They actually push their nucleus out of the cell and the whole reason why that's beneficial is, that's more space for hemoglobin. Because the more hemoglobin you have, the more oxygen you can take up. And I can do a ton of videos on hemoglobin and all of that-- and actually, I'm going to do a lot more on the circulatory system so don't worry about that, but I want to go over one other really interesting thing about hemoglobin. We already talked about red blood cells. I think it's fascinating that they actually don't have a nucleus in their mature form. They actually have very short lives. They live maybe 80, 120 days so they're not these long lived cells-- so it's almost a philosophical question. Are are they still alive once they've lost their DNA or are they just vessels for oxygen that aren't really alive because they aren't regenerating and producing their own DNA? So actually, instead of going into the hemoglobin discussion right now, I'll leave you there in this video. I realize I've been making 20-minute videos where my goal is really to make ten-minute ones. So I'll leave you here and in the next video, we'll talk more about hemoglobin and the circulatory system.