Medical Minutes | Why do we measure blood pressure using mercury (mmHg)?
Dr. Matt and Dr. Mike's Medical Podcast
Dr. Mike Todorovic
4.8 • 675 Ratings
🗓️ 18 November 2025
⏱️ 7 minutes
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Transcript
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| 0:00.0 | So I had a medical student asked me the other day, why do we measure our blood pressure |
| 0:05.1 | in millimeters of mercury? It's a really good question. You've probably had your blood pressure |
| 0:09.6 | taken and you've been given a value that says something like 120 over 80. Those two values |
| 0:16.0 | are measured in millimeters of mercury. And it's strange because we don't often use mercury. I mean, it's toxic. It's |
| 0:22.1 | not something we really want to handle. So the reason why we use it is a throwback a couple hundred |
| 0:27.5 | years ago in the 19th century. So the 1800s by a French physician called Jean-Lernard-Marie |
| 0:33.0 | Persuay. I'm sure I'm mispronouncing that name. But if you know Persuay's law, you know that it has |
| 0:40.1 | something to do with the diameter of a vessel and the pressure that goes through it. |
| 0:45.0 | Now, to understand all this, we need to go back to those two values, the 120 over 80. |
| 0:50.9 | So the 120, that top value given on a blood pressure reading, that's your systolic |
| 0:56.6 | value. Now, systolic is referring to contraction. So when your heart contracts, let's say we know |
| 1:04.3 | our heart's made up of atria, two chambers at the top, and ventricles, two chambers down the |
| 1:09.2 | bottom. When those ventricles contract, they push blood out of the heart. Now, the left ventricles, two chambers down the bottom. When those ventricles contract, they push |
| 1:11.7 | blood out of the heart. Now, the left ventricle pushes out to the rest of the body via the aorta. |
| 1:18.5 | This is the one we're looking at at the moment. So this is what we call a systemic circulation. |
| 1:23.8 | It's going through all the tissues of the body. So that left ventricle contracts, it squeezes. |
| 1:30.4 | And as it squeezes, it squeezes blood out into that aorta, and there's going to be a degree |
| 1:35.3 | of pressure that that blood is under. |
| 1:37.9 | So that blood is going to exert a force on the blood vessels, such as the aorta and its |
| 1:43.6 | branches. |
| 1:44.0 | The maximum force that it places on the blood vessels such as the a order and its branches. The maximum force that it places |
| 1:47.0 | on these vessels is 120 millimeters of mercury, right? That's the maximum force. So that's |
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