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?
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I’ve got a pretty good Eli5 for this one.
hemoglobins are little molecules that carry oxygen around the blood to the various tissues in the body.
Think of these little hemoglobins as tiny cars (taxis really) each of these hemoglobin cars have 4 seas for oxygen molecules. When they get to their destination the cars empty out and the oxygens goes to work doing their job, but if they’re not needed at that stop, like a buss they’ll just continue onto the next work station and attempt to get off there. AT the end of the journey the hemoglobin taxi travels back to the lungs for more passengers.
Now there are millions of hemoglobin taxis in your body constantly at work. But they don’t always carry 4 oxygen passengers, sometimes less but never more.
The red light in the pulse oximeter looks into the windows of theses taxies and counts, on adverage, how many of the seats are full. If 95% of the seats are full you’re reading will be 95%, of only 80% of the seats are full you’re reading will be 80% and so on.
Now, it’s also important to know that the red light counts how many seats are full in the hemoglobin taxis, but doesn’t count how many taxies are on the road.
This important, because let’s say we have a patient who has had a massive blood loss or is hypovolemic, the light will still read that 90% or 95% of the seats in the taxis are full. Inevitably, the seats will start to empty really quickly and your O2 will drop, but not because you have fewer taxis, just that they’re working harder and can’t keep up (so always put O2 on a patient with a critically low hemoglobin).
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