system
Given the following post, respond as if you're commenting on Reddit:
human
ELi5 : How do modern smartwatches calculate steps, blood pressure and oxygen level in blood ?

What are the technology behind this ?
gpt
Oxygen level is done via the color of your blood. That is called  *Reflectance pulse oximetry.*

The watch shines a light on your skin and the color reflection lets it measure blood flow and oxygenation.

Steps are calculated by the accelerometer, which basically measures how fast and where the watch is going. There is a little math and analysis going on, but basically one swing forward and back is one step, because we swing our arms synchronous with our legs.