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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. Created by Rishi Desai.

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Video transcript

So recently I went into my doctor's office, and I was told that my blood pressure was 115/75. So I thought we would talk about exactly what this means and try 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 kind of a tube and I imagine blood going through that tube. And this tube is like a blood vessel. So there's blood trying to get its way from one side to the other. And on its way, the 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 is there are cells and there's plasma, and all that stuff is pushing out against the walls of the blood vessels. 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 any time you see 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 as those little blue arrows. And the two questions that kind of pop into my mind anytime 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 this is my Valentine's Day heart-- and you've got the aorta coming off of 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, I'd say 90% of the time, maybe even more, the blood pressure that you're getting recorded, or the number that's 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. So these little blue arrows. So that answers the where question. And certainly, you can imagine if I checked blood pressure let's say at some other spot, let's say over here or over here, you might get a different blood pressure reading than if I checked it at the yellow x. So really were 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, rather. 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 millimeters 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 yellow x, at the bottom, let's say of my reading I've 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 as it goes up, 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 out 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 one heartbeat here. So this would be one heartbeat. So it's not 50-50. And the easy way to remember this is in a given day, 24 hour day, I spend about 8 hours working and about 16 hours relaxing, or not working. And so I think of the heart doing its squeezing, its work, for eight out of 24 would be about 1/3. Kind of the same as me. About 1/3 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's never just 115 or just 75. It's somewhere between the two. And for example, if I checked right here in time, it might be let's say 85, or here it might be 102, or here it might be 87. So it's somewhere between 115 and 75. So this number I wrote up here becomes the range. This is the range of blood pressure. So when I talk about when, it really depends on when in the cycle you're at and you're somewhere in that range. And when I talk about the where, we know that 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 again, is the systolic blood pressure. And 75 is the diastolic blood pressure.