What is blood pressure? Find out what a systolic and diastolic blood pressure mean. Rishi is a pediatric infectious disease physician and works at Khan Academy.
What is blood pressure?
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- So recently, I went into my doctor's office,
- and I was told that my blood pressure
- was 115 over 75.
- So I thought we would talk about exactly what this means
- and trying to figure out
- how to think about blood pressure in general,
- using these numbers and this experience
- as kind of a launching point.
- So, the way I think about blood pressure is,
- I always imagine a kind of tube,
- and I imagine blood going through that tube.
- And this tube is like a blood vessel.
- So, there is blood trying to get its way
- from one side to the other.
- And on its way, the kind of neat thing that it's doing
- is, as it flows, it's pushing out.
- So it's forcing against these walls.
- And, specifically what I mean by that,
- there's cells and there's plasma,
- and all that stuff is pushing out against the walls
- of the blood vessel.
- So you've got a force,
- and that force is being exerted
- on the surface area of the blood vessel.
- So it's force over a surface area,
- and anytime you see a force over an area,
- you know that equals a pressure.
- And in this case it's a blood pressure,
- because it's the blood that's doing that work.
- So, this is how I think of blood pressure,
- specifically those little blue arrows.
- And the two questions that pop into my mind
- any time I'm thinking about blood pressure,
- are where is the blood pressure being taken,
- and when are you taking it?
- So, let's start with the first question: where?
- And by that I mean, where in the circulatory system?
- So you've got the heart.
- And it's my Valentine's Day heart.
- And you got the aorta coming of off the heart,
- and it's got lots of branches,
- but I'm going to just draw one branch,
- which is the artery leading off to my arms.
- This is called the brachial artery.
- Going off to my arm.
- And usually, let's say 90% of the time,
- maybe even more,
- the blood pressure that you're getting recorded,
- or the number that is being told to you,
- is being checked at this point.
- I marked it with a little x
- because that's kind of the upper arm.
- So that's usually where they're checking the blood pressure.
- And again, they're checking the force
- that the blood is putting on the vessel walls.
- These little blue arrows.
- So that answers the 'where' question.
- And certainly you can imagine,
- if I'd check blood pressure at some other spot,
- let's say over here, or over here,
- you know, you might get a different blood pressure reading
- than if I check it at the yellow x.
- So really, we're just talking about that one spot.
- Now the other question is, when are you checking it?
- So for this, let me show you a little table,
- or a figure of it.
- So imagine that over time,
- time is this way,
- you have different recordings for blood pressure.
- So this will be blood pressure.
- And blood pressure is usually measured
- in millimeters of mercury.
- So I'll write mm of mercury here.
- And let's go from 200
- all the way down to 0.
- And I'll use my numbers,
- that I got from the doctor the other day,
- to illustrate what I'm trying to say.
- So, right at that x, that yellow x,
- at the bottom, let's say, of my reading, I got 75.
- So let's start at 75, which is about here.
- As the heart is pumping,
- the pressure starts building up.
- And it gets up to about 115.
- So really, what is happening is,
- blood is making its way from the heart,
- as it squeezes, towards that x.
- And as it gets there, the force
- that it's putting on the walls is going up, up, up.
- And it goes from 75 all the way to 115.
- So that all happens during the squeezing part
- of the heart cycle.
- We call that systole.
- So this is all happening during systole,
- which is when the heart is squeezing down.
- And we know the heart is a pump.
- Now from that point forward,
- the heart begins to relax.
- So at this point, at 115, the heart is now relaxing slowly.
- And as it relaxes, the pressure begins to fall.
- And it continues to fall all the way up here.
- And the pressure gets down to about 75 again.
- And this is diastole.
- So this is when the heart is actually
- taking a break from squeezing, and is now refilling.
- And we call that diastole.
- So I'll call that refilling.
- And so during systole, you spend about 1/3 of the time.
- And in diastole you spend about 2/3 of the time.
- And I'm talking about 1 heartbeat here.
- So this would be 1 heartbeat.
- So it's not 50-50.
- And the easy way to remember this is
- in a given day, I spend about 8 hours working,
- and about 16 hours relaxing, or not working.
- So I think of the heart doing its squeezing, its work,
- for 8 out of 24, or about a third.
- About the same as me. About a third of the time,
- the heart is squeezing.
- So that's how the heart is spending its time
- in systole and diastole.
- And the pressure is ranging between 115 and 75.
- So really, it never just 115 of just 75.
- It's somewhere between the two.
- For example, if I check right here in time,
- it might be 85, or here it might be 102, or here it might be 87.
- So it's somewhere between 115 and 75.
- So this number then, this number I wrote up here,
- becomes a range.
- This is the range of blood pressure.
- So when I talk about 'when', it really depends on
- where in the cycle you're at.
- And you're somewhere in that range.
- And when I talk about the 'where', you know we're talking
- about a reading that's taken in the upper arm.
- So those are the two thoughts that should cross your mind
- every time you hear a blood pressure recording.
- And of course, the top number, 115,
- is the systolic blood pressure.
- And 75 is the diastolic blood pressure.
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