system
Given the following post, respond as if you're commenting on Reddit:
human
How can SmartWatches measure the blood pressure?

And how accurate is it?
gpt
I do research on hemodynamics, which uses some of these same principles.

The smart watch estimates blood pressure based on something called pulse transit time, which is a calculation of how much time it takes between the heart squeezing and the wrist getting more blood flow.

The optical sensor can measure changes in oxygenated blood vs deoxygenated blood, and it can measure pulsatility of blood.

Ideally, you want an EKG to tell you when the heart contracts, and a device that measures the pulse. The longer the time between the two, the more relaxed your blood vessels are.

But smart watches don't have an EKG, and they don't know when your heart contracts. So it looks at the shape of the pulse curve, looking at kurtosis (how narrow the curve is) and skewness (how much the curve clumps to the right or left). A stiff blood vessel or a system with a lot of resistance will have a quick peak in pulsatility and a steeper slope.

These data are combined with estimates based on your age, sex, height, and weight.

The result is slightly better than a wild guess. The principles are correct, but the smart watch is uncalibrated, so it's not accurate. It could probably reliably detect when your blood pressure increases or decreases, but not give you an accurate number.

Ditto for its ability to detect oxygenation. The best devices rely on transillumination (like light going through your fingertip, rather than reflected illumination.

TLDR: smart watches are not accurate for blood pressure. It guesses how tight your blood vessels are based on the contour of pulse changes in blood flow, incorporating general demographic data.