Eli5: how does the blood pressure cuff determine your blood pressure so accurately?
In: 200
Have you ever noticed that when you turn on a garden hose, it gurgles and murmurs initially, but then quiets down? The sound is due to the water flowing at different speeds and directions initially before it flows evenly. The “loud” flow is turbulent flow, the “quiet” flow is laminar flow.
When a blood pressure cuff is applied, it gets blown up until it fully stops blood flow in the vessels of your arm. A stethoscope put on the arm will hear nothing.
Then the pressure gets lowered until blood can flow again a little bit every time the heart squeezes. This flow is turbulent, so you can hear it with a stethoscope. The pressure at which that first sound happens is the first number in a blood pressure reading.
Further releasing the pressure gets you to a point when blood starts flowing even when the heart is not squeezing. This flow will NOT be turbulent, but laminar. Laminar flow is quiet. The pressure when the sounds stop is the second (lower) number of a blood pressure reading.
So a blood pressure of 120/80 means that at a pressure of 120 I heard turbulent flow starting and at 80 I stopped hearing turbulent flow and blood is now flowing quietly (laminar).
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