Seems a bit daft to post on this subject, but I struggled to find anything out on the web about what the numbers actually represent. There's plenty of stuff telling you what values are high, normal etc. but nothing about what is actually happening during the course of a measurement. But I found some documentation eventually, looks like it is aimed at clinicians, which I'll try to explain.
You wrap the cuff around your upper arm. At that point, you're over your brachial artery, so a direct route to the heart. The documentation I looked at made a big deal of this, so this is important.
The cuff inflates sufficiently high that it stops blood flowing. If you were listening with a stethoscope, you'd hear nothing.
You gradually let the pressure out of the cuff. Your heart pumps with sufficient pressure that, sooner or later, it overcomes the pressure in the cuff, and the blood starts flowing again. Again, if you had a stethoscope, you'd hear the heart beating. When blood first starts flowing, this is the systolic blood pressure - the pressure when the heart beats.(
You keep releasing pressure from the cuff. You can still hear the heart beating. Again, sooner or later, you stop hearing anything. At that point, this is the diastolic blood pressure. Basically the pressure in the cuff is sufficiently low that your heartbeat can't be heard. It's the pressure when your heart is resting. In my simple world, I think of the heart as a machine which is either on (pumping) or off (resting). In this scenario, at any rate.
I mean, a stethoscope is just one way of detecting these signals. I would imagine an electronic machine would detect these points by "feeling" when the pulse starts and stops (a momentary slight increase in pressure, say, as the heart pulses). I think my next task is to find this out.
I've tried to explain this briefly and in layman's terms. If you feel i could do better, please leave a comment, or there's a link at the bottom of the page which you can use to contact me.
BEFORE YOU START: Please note that although I currently volunteer for both the Stroke Association and Age UK, the views expressed in this blog are strictly my own. I am not a spokesperson for either (or, indeed, for any) organisation, and I accept complete responsibility for the views expressed herein. As indicated by the domain name, I am based in the UK and the blog therefore has a UK bias - I've tried to use the Glossary to explain any ambiguous terms, but if you think there is anything I've missed, please message me.