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Learn the difference between blood flow (Volume/time) and perfusion (Volume/time/amount of tissue). Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Video transcript
So here we are. We have our two lungs and the heart. I'm just going to quickly label stuff. We've got our right and left lung, and we've got our heart. And I want to make sure I label all of the four chambers of the heart. I've taken away a lot of the vessels. I just want to focus on a couple of things here-- mainly, the blue blood vessel coming off of the heart-- the one I've drawn in blue-- which I'm going to label here as the pulmonary artery. Remember, again, arteries go away from the lungs. So this is our pulmonary artery, even though it's got deoxygenated blood in it. A little counterintuitive, but I think you got it now. So this is our pulmonary artery, and it's going to the left and right lungs. And if we assume that there's, let's say, 5 liters of blood flowing through the heart per minute, that means that 5 liters are going to go through this vessel. And some of that is going to go to the right, and some is going to go to the left. Let's say I told you that 2 and 1/2 liters goes to the left lung per minute. Let's just assume that. Then you know that the other half of that 5 liters-- the rest of it, 2 and a 1/2 liters-- must also go to the right. Because whatever goes into this tube-- almost like a straw-- on one end has got to come out on the other end. So you just, basically, add up what's exiting. And it's got to equal what's entering. So here we have the idea of flow. And we've talked about flow in other videos, but basically, I just want to restate it. It's a volume over a period of time. And in this case, we're using liters over minutes. But really any kind of volume over time you could describe as blood flow. Now, let's say that a tragic event occurs, and I end up having a surgery to my lung. Let's say underneath this yellow line is my lower lobe and above it is my upper lobe. Let's say my lower lobe, it needs to be removed. It's a pretty drastic thing to have happen, but let's say this is what happens. What would change in terms of my blood flow? Well, the thing that is going to change is my resistance is going to change. Let's think about it. Before I had this surgery, I had a certain amount of resistance in this blood vessel and also some resistance in this blood vessel. And let's say it's about the same, just to kind of make things easy. Let's say the resistance was about the same. So again, I had a surgery. And before they removed the lower lobe-- just to make sure we are clear on what this surgery was-- so removed the lower lobe. So before the surgery-- I'll write "before" up here-- what was the resistance? Well, the resistance I was facing was-- remember, we have a branch here. So we have to add up the total resistance. You remember how to do this. Total resistance-- I'll call it R total-- equaled 1 divided by 1 over R-- because we said that's what the resistance is right there-- 1 over R plus 1 over R. And that second one is because of this guy. So we just kind of add it up. And I would say, OK. Well, that's equal to 1 over 2 divided by R. And I can flip the whole thing around. And I get R divided by 2 or 1/2 R. So this is my total resistance-- 1/2 R. It's a little counterintuitive-- the fact that you actually have half of the resistance just because you have a fork. The fork in the road-- meaning this fork right here-- offers you a chance to go one of two ways. And as a result, the resistance falls in half. So after my surgery, what was my resistance? Well, in my surgery, this all kind of went away. This is now all gone. Because my surgery removed the lower lobes, this is now gone. So what is my new R total? Well, if I had to calculate it again, I would say, OK, R total. In this case, it's actually really easy because it's just whatever's left. In this case, the total is going to be just R. So really, my resistance went from half R to R. And so my resistance really, by removing the lower lobe, it doubled. My resistance went much higher. So this is the first interesting point-- that by having a half a lobe removed, my resistance went way up. So on this side, my resistance after the surgery is much higher than it used to be. Now, remember this flow-- 5 liters a minute. Now, you still have that much blood coming in, but now there's extra resistance on the left side. So what's the blood going to do? Well, it's going to say, well, why would I go that way when I can go this way? So more of the blood's going to kind of go this way because there's more resistance on the left side. And so I can actually-- I don't know exactly what the amount of flow would be-- but I can kind of take a guess. And I would say, well, my guess is that the flow will be lower. So I'm actually going to redo these numbers. I'm going to give you new numbers. And let's say the new flows-- I'll write them in green-- are going to be 3 liters a minute and 2 liters a minute. They still have to add up to 5, of course. That's not changed. But you have more blood going to the right lung. So here let me introduce another word. So we've talked about flow, but now let me talk about perfusion. And sometimes people actually think they're the same thing. They sometimes will use them kind of synonymously. But really, perfusion is volume over time. And so, so far you're thinking, well, it is about the same. But actually, it's all divided by amount of tissue. And when I say amount, I could do either be talking about a volume of tissue or a weight of tissue. So amount of tissue. Just to kind of make this a little bit more concrete, I'm going to assume that I'm going to use 100 grams here. And that's often used. Not always. Sometimes you'll see other units. But I'm going to use 100 grams here. So let's now think about this entire scenario with the new numbers-- 2 liters a minute and 3 liters a minute-- in terms of perfusion. What would that mean? Well, let's say I weigh out my two lungs. And here I only have an upper lobe on my left side left. So let's say that weighs half a kilogram. And let's say, on the right side, I've got 1 kilogram. Let's say this is 1 kilogram. These are the weights of my two sides. And to figure out perfusion, then all you really are doing is taking the flow-- because remember, this whole chunk, this whole part right here is just flow-- and dividing it by the amount of tissue. So I could figure out perfusion pretty easily. I could say, OK. Well, on the right side-- let's do right side first-- I've got 3 liters a minute. I'm going to write that as 3,000 milliliters, just to make it a little easier to see. 3,000 milliliters per minute divided by-- I said 1 kilo, which is the same as 1,000 grams. So what does that turn out to be? If I'm going to use 100 grams as my denominator, I could say, well, that's-- let's see, 0s cancel. So I've got 300 milliliters per minute per 100 grams of lung tissue. And so this is for the right side. And I could do the same thing for the left side. I could say, well, what would it be for the left side? It would be-- I've got 2,000 milliliters. We said 2 liters. And of course, the 2 and 3 I was just kind of estimating. But we'd have to actually measure to see what the actual flow is. But here I've got 500 grams. And so that works out to 400 milliliters per minute per 100 grams. So what I wanted to show you is an interesting thing, which is that you can actually have, on the one side-- if I said which side, the right or the left, after my surgery, which side has more blood flow? Well, then, this side has more blood flow. The right side has more flow. But if I said which one has more perfusion, well, it turns out that actually that left upper lobe is actually getting more perfusion. So just because one side has more flow doesn't necessarily mean that it has more perfusion. Oftentimes that is the case because you can see how closely flow and perfusion are related. But it just depends on exactly what the weight is for the tissue. Kind of a classic example of this I'm going to write out over here that you might hear people talk about sometimes is-- if you say this side is high and this side is low-- let's do flow and perfusion-- they'll say, well, if you have flow and you're trying to talk about different organs, one of the organs with the highest flow in the body would actually be your liver. This is, let's say, your liver. This is your liver. And then, with a little bit less blood flow would be your kidneys. This would be your kidneys, let's say. I'll write K for kidney. Or actually, I guess I'll spell it out. I have enough space. And then, something that has almost no flow relative to the other two would be bones. And actually, compared to this, if you were to now talk about perfusion, it would actually looks slightly different. So for perfusion-- using these same three organs-- if I was to kind of rank them based on which one gets the most perfusion or blood perfusion, the kidney actually does the best. So here you have to take a certain amount of tissue. And it's got to be the same amount. So I'm just imagining if I took a little chunk of kidney tissue. And if I did the exact same thing and I took a little chunk of liver tissue. And this is kind of the way to think about it is that, if you want to balance things out, you've got to take the exact same amount of tissue. In this case, it would be 100 grams, let's say. Maybe these boxes are 100 grams of tissue. It would be something like this. And this would be the bone. So the liver ends up not doing as well. It gets a little bit less perfusion in terms of 100 grams. The kidney does a little bit better when it comes to perfusion. And the bones-- the sad, little bones-- they actually don't get much blood flow. And even if you do it by 100 grams of tissue, they actually don't get much perfusion either. So this is kind of another way to think about it, and you might hear these examples. So I wanted to give them to you here.