system
Given the following post, respond as if you're commenting on Reddit:
human
How do oxygen meters measure oxygen level externally through the thumb or index finger?

I was recently in the hospital for a respiratory illness and the nurse measured my oxygen level with a thing that clipped to my index finger. How does it work? Does it have to do with pulse?
gpt
Basically:

It passed a special light through your skin, and this light reflects off your inner fluids. Blood cells with oxygen will reflect slightly differently than blood cells without. As a result, the sensor in the clip can begin to decode the light, and determine how much blood has oxygen vs not.

Imagine shining a flashlight through a dusty room. You could count every dust particle in the room, or, assuming there's good air mixing (blood flow), measure just a cubic foot of air and extrapolate.

In this case, blood cells look different when they have oxygen vs when they don't. So they measure blood flowing through your finger and extrapolate.