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?
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The pulse oximeter emits light of certain wavelengths which passes through the skin and the blood in an artery. It measures how much light of each wavelength is transmitted, and because oxygenated blood and deoxygenated blood absorb different wavelengths can calculate the percentage of blood that is oxygenated.
One side of the clip has a light and the other has a sensor. When you clip it on your body (usually for adults it goes on a finger but toes work too) it shines both red visible light and infrared light through you finger (or toe) and that light hits the detector on the other side, after passing through your finger.
Hemoglobin is the protein in red blood cells that carries oxygen. Saturated hemoglobin (full of oxygen) and unsaturated hemoglobin (not full of oxygen) absorb different amounts of infrared and visible light, so by comparing the ratio of light that the sensor detects to the ratio that was emitted, it can tell how much of the hemoglobin in your blood is saturated (fully of oxygen) and how much isn’t.
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.
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