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# Flow and perfusion

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.