If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:5:27

Learn how a stethoscope can help determine blood pressure

Video transcript

So this is a picture of Doctor Korotkoff. Doctor Korotkoff is a very famous Russian doctor. He lived about 100 years ago, and something he came up with, something very clever, still affects all of us, or most of us today. So he lived in the early 1900s. And around 1905, he started thinking. He was thinking about blood vessels. And he was trying to figure out how to use his stethoscope-- let's draw the skin here over the blood vessel-- how to use his stethoscope to listen and figure out the blood pressure. So he was thinking, how can I get about to do that? So, first thing he decided to do was inflate a cuff, like a blood pressure cuff. And it would put a lot of pressure on the outside of the skin on that blood vessel. So he thought, well, let's, for example, let's do something like 200 millimeters of mercury. And for this example, let's just assume that he's working with me. Let's assume that he's working with my blood vessel. And my blood pressure is about 115 systolic over 75 diastolic. So, if you put 200 millimeters of pressure on the outside, what's going to happen? The first thing is that that vessel is definitely not going to stay open, right? It's going to collapse down. So it's going to do this. As it gets all that pressure in the blood, is going to turn right around and say, well, we can't go through this way. So if he listens with his stethoscope here, if he's listening, he would really hear silence here. He would hear nothing at all. It would be very quiet. So let me put "silence" because there's no blood flowing at this point because the vessels collapsed. OK. So Doctor Korotkoff thought about this and said, OK, well, what if I go ahead and do something slightly different? Let's open this up a little bit, because what he's going to do next is instead of putting 200 millimeters of pressure as he did the first time, he's going to put exactly 115, exactly the systolic pressure in my blood vessels. And he's still going to listen with his stethoscope. So here's his stethoscope. As before he's going to listen. And now the pressure on the inside of the blood vessel actually does reach 115 for just a moment. So it gets there for just a moment. And a little spurt of blood gets through. So I'll say a little "pulse" or spurt of blood makes it through. And here's the interesting bit. As that blood is going through, because that vessel just barely opens up, it actually has to squeeze its way through. And it creates a lot of turbulence. Meaning that, that blood is not flowing smoothly. It's actually bouncing off of the walls creating lots of movement. And that actually translates into sound waves because all the little air molecules get shaken up. And you can actually hear that. So when you hear that pulse, when you get a pulse, you actually hear some sound. So for the first time you actually get a little bit of noise. I'll put that down too, "noise." And the reason for this noise is that you have turbulent flow, which is actually really, really important. And it's interesting that he observed that or figured that out. So when you go from silence S, to noise N, Doctor Korotkoff realized that that's the moment when you've actually figured out someone's systolic blood pressure. Really kind of a neat way to deduce this, right? Systolic blood pressure. And so then he said, OK, what if I take it one step further? What if I relax that pressure on the outside even more? So instead of 115, now let's say the pressure is, I'm going to say 75. 75 millimeters of mercury. And he is, of course, using his stethoscope to listen. He's still listening. And he realizes that now blood is actually going to flow really smoothly, very smooth flow through this vessel. And we actually call that laminar flow. And then you get laminar flow as opposed to the turbulent flow you had before. And now that you have laminar flow you don't have all that bouncing off of the walls that I was talking about earlier. And so because of that, it's actually quite quiet. It's very silent again. So again, you go back to silence, but for a very different reason. The first time there was no blood flowing, that was when there was 200 millimeters of mercury. But now there's 75 millimeters of mercury, which is a lot less. And you still get silence for a different reason. Now there is blood flowing, but it's flowing so smoothly-- called laminar flow-- that you really don't hear anything with your stethoscope. And he realized that now when you go from a noisy situation back to a silent situation, now you figured out someone's diastolic blood pressure. So every single time we go to the physician's office or see the nurse practitioner to get our blood pressure taken, we owe it to Doctor Korotkoff and his observations about 100 years ago.