# Putting it all together: Pressure, flow, and resistance | NCLEX-RN | Khan Academy

On December 26, 2019 by Raul Dinwiddie

And I’m going to talk to you

about pressure, resistance, and flow. We’re going to try to make

sure you feel real comfortable with all three of these

things by the time we’re done. So we start with the heart, and

off of the heart is the aorta. This is the largest

artery in the body. And this is one

branch of the aorta. I didn’t draw a lot

of the other ones. This is the brachial artery. And the blood is

flowing from the aorta into the brachial artery. And let’s say that the blood

is trying to make its way out to a fingertip, for example. So on its way out there, it

makes its way to an arterial. And the blood continues

flowing, and it goes into the capillary

bed, and the vessels are too small to draw, so I

just kind of do that thing. And it then goes

into the other half of the capillary bed, where

now the blood is deoxygenated. So I’m going to

draw that as blue. That’s the part where now

the blood is without oxygen. And then it continues

to go and get collected into a venule, which sounds a

little bit like the arterial on the other side, right? And we’ve got a vein over here. And then finally, the

blood gets collected in a large vein

called the vena cava. And there are actually

two vena cavas, so this’ll be the

superior vena cava. There’s also an

inferior vena cava. And the blood flow through this

half is, as you would guess, continues to go around. And if I was to try to figure

out the pressures, the blood pressures, at different

points along the system, I’m going to choose

some points that I think would be

interesting ones to check. So it would be good

probably to check what the pressure is

right at the beginning. And then maybe at all

the branch points. So what the pressure is as

the blood goes from the aorta to the brachial artery. Maybe as it ends the brachial

artery and enters the arterial. Maybe the beginning and

the end of the capillaries. Also from the venue to a vein,

and also, wrapping it up, what the pressure is at the end. Now, these numbers,

or these pressures, can be represented

as numbers, right? Like what is the

millimeters of mercury that the blood is

exerting on the wall at that particular

point in the system? And earlier, we

talked about systolic versus diastolic

pressure, and there we wanted to use two

numbers, because that’s kind of the range, the upper

and the lower range of pressure. But now I’m going to

do it with one number. And the reason I’m using

one number instead of two, is that this is the

average pressure over time. So the average pressure over

time, for me– keep in mind my blood pressure

is pretty normal. It’s somewhere around

120 over 80 in my arm. So the average pressure in

the aorta kind of coming out would be somewhere around 95,

and in the artery in the arm, probably somewhere around 90. Again, that’s what you would

expect– somewhere between 80 and 120. So 90 is the

average, because it’s going to be not exactly

100, because remember, it’s spending more time in diastole

and relaxation than in systole. So it’s going to be closer

to 80 for that reason. And then if you check the

pressure over here by this x, it’d probably be something

like, let’s say 80. And then as you

cross the arterial, the pressure falls dramatically. So it’s somewhere closer to 30. And then here it’s about 20. Here it’s about 15. Let’s say 10 over here. And then at the very end, it’s

going to be close to a 5 or so. So here. Let me just write that again. 10 and 5. And the units here are

millimeters of mercury. So I should just write that. Pressure in

millimeters of mercury. That’s the units that

we’re talking about. So the pressure falls

dramatically, right? So from 95 all the way

to five, and the heart is a pump, so it’s going to

instill a lot of pressure in that blood again and

pump it around and around. And that’s what keeps the

blood flowing in one direction. So now let me ask

you a question. Let’s see if we can

figure this out. Let’s see if we can figure

out what the resistance is in all of the vessels

in our body combined. So we talked about

resistance before, but now I want to

pose this question. See if we can figure it out. So what is total

body resistance? And that’s really

the key question I want to try to

figure out with you. We know that there is some

relationship between radius and resistance, and we

talked about vessels and tubes and things like that. But let’s really figure this out

and make this a little bit more intuitive for us. So to do that, let’s

start with an equation. And this equation

is really going to walk us through this puzzle. So we’ve got pressure, P,

equals Q times R. Really easy to remember, because

the letters follow each other in the alphabet. And here actually, instead

of P, let me put delta P, which is really

change in pressure. So this is change in pressure. And a little doodle

that I always keep in my mind to remember

what the heck that means is if you have a little tube,

the pressure at the beginning– let me say start;

S is for start– and the pressure at the end can

be subtracted from one another. And that gives you PS

minus PE equals delta P. The change in pressure

is really the change from one part of tube

the end of the tube. And that’s the first

part of the equation. So next we’ve got

Q. So what is Q? This is flow, and more

specifically it’s blood flow. And this can be thought of

in terms of a volume of blood over time. So let’s say minutes. So how much volume–

how many liters of blood are flowing in a minute? Or two minutes? Or whatever number of

minutes you decide? And that’s kind of a hard

thing to figure out actually. But what we can figure

out is that Q, the flow, will equal the stroke

volume, and I’ll tell you what this is

just after I write it. The stroke volume

times the heart rate. So what that means

is that basically, if you can know how much

blood is in each heartbeat– so if you know the

volume per heartbeat– and if you know how many

beats there are per minute, then you can actually figure out

the volume per minute, right? Because the beats would

just cancel each other out. And it just turns

out, it happens to be, that I’m about 70 kilos. That’s me. I’m 70 kilos. And for a 70 kilogram

person, the stroke volume is about 70 milliliters. So for a 70 kilo person, you

can expect about 70 milliliters per beat. And as I write this,

let’s say my heart rate is about 70

beats per minute. I feel pretty calm, and

so it’s not too fast. So the beats cancel

as we said, and I’m left with 70 milliliters

times 70 per minute. So that’s about 4,900

milliliters per minute. Or if I was to simplify,

that’s a 5, let’s say about. So the flow is about

5 liters per minute. So I figured out the blood

flow, and that was simply because I happen

to know my weight, and my weight tells

me the stroke volume. And I know that there’s

a change in pressure. We’ve got to figure

that out soon. And lastly, this last thing

over here is resistance. And know I’ve said it before. I just want to point

out to you again, the resistance is going to

be proportional to 1 over R to the fourth. And so just remember that

this is an important issue. R is radius. And that’s the

radius of the vessel. So let’s figure

out this equation. Let’s figure out the

variables in this equation and how it’s going to help

us solve the question I asked you– what is the

total body resistance? OK. So if I have to figure out

total body resistance– let me clear out the board–

I’ve got, let’s say, the heart. I like to do the heart in red. And it’s pumping

blood at my aorta. So blood is going

out of the aorta. And then it’s going

and branching here. And then it’s going

to branch some more. And then it’s going

to branch some more. And you can see

where this is going. It’s going to keep branching. And eventually every branch kind

of collects on the venous side. All the blood is kind of

filtering back in slowly into venules and veins. And finally into a vena cava. And I should really draw

this going like that. The blood is going to go

back into the vena cava. So that’s my system. And I got to figure out what the

total body resistance is here. So if I have a system

drawn out for myself, and I happen to know

that here I said 95. And here I said

the pressure was 5. Then delta P equals 95

minus 5, which is 90. And I know that there are 5

liters of blood flowing through per minute. And that was my

Q. So I could say 90 equals 5 liters per minute. Actually let me take

a step back from that. Instead of 90, let

me write the units. 90 millimeters of mercury

equals 5 liters per minute. That was my flow. That’s my Q. And I’ve

got delta P here. And my resistance

is the unknown. So I’ll just leave that as R. So let’s just solve

for R. So I’ll move my flow to the other side. So R equals– I’ll put it here–

90 divided by 5, which is 18. And the units are

a little funky, but I’ll just write

them out anyway. Millimeters of mercury times

minutes divided by meters. So this is the answer

to my question– what is the total

body resistance? Well, we know what

the pressures are at the beginning and

end of our system. And we know that the flow has to

be around 5 liters per minute, because that’s based on my

weight and my heart rate. Therefore, the resistance

must be 18 millimeters of mercury times

minutes over liters. Whatever that set of

units means to you. It’s kind of an abstract thing. But basically, I want

to demonstrate to you that this powerful

equation helps us solve for what

would otherwise become a tricky

problem to figure out.

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