How do we get oxygen readings from external sources?

Question

I am currently in the hospital with my daughter and they are having to periodically check her oxygen levels. To do so they are putting a sensor on her foot. How do these readings work that they can tell something like oxygen levels using that method?

In: 3

Answer 1

to answer why the foot since others answered the how part. The extremities like the fingers and toes have the longest distance and fewest blood vessels to transport oxygen to from the lungs. If not enough oxygen is getting out there the fingers or toes could start to die from the inside.