Eli5: How the finger light thing can tell how much oxygen is in your blood?

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As the the title suggests, how is it that a little light they put on our finger when at the doctors/hospital/etc.. is able to tell us our blood oxygen levels?

In: 334

In blood, the oxygen is carried by some molecules.
It just so happens when the oxygen binds to those molecules they change color.
So they shine a light, read the color and that figures out what proportion of those molecules have changed to that color.

My moment to shine no pun intended! I’m an RN and we use these lights a lot. The principle is pretty simple. See at end for edit:

If you shine a light through your finger, some of the light is absorbed by a protein called hemoglobin. hemoglobin carries oxygen. There is a tiny camera on the opposite side of the light that measures how much of the light was “removed”. A really simple version of this device would just say “yes, you have hemoglobin in there” by seeing that drop off in the light, the “signature” of hemoglobin. So , the really cool part is this: Hemoglobin changes color depending on how much oxygen is stuck to it. If your hemoglobin is 100% packed with oxygen it’s a nice cherry red color and the sensor can see this. As the oxygen in your blood drops, the color of the hemoglobin changes to a darker red, almost purply red when it’s really low. This color tells the sensor that your oxygen level is dropping. In fact the sensors can be thrown off by certain chemicals that alter the color of your blood.

Edit: figured i should clarify there are two light sources in a pulse oximeter. The color change does depend on how fat your finger is! just like if you look down the length of a a long tube of colored water it looks darker than when you look through the side. a computer tries to correct for this by comparing how much light was lost from both light sources.

Interesting side note – some SpO2 sensors can’t tell the difference between oxygen bound to hemoglobin and carbon monoxide that’s bound to hemoglobin. So, it’s possible to have CO poisoning, but still have a 90’s-100% SpO2 reading.

Since this has already been answered. I wanted to point out, some smartphones phones can do this now too. I’m using Galaxy S9 and it has this ability. Just found out when I got Covid a few months ago. Compared it to a hospital “finger light thing” and it was accurate.