ELI5 How can a blood oximeter read your level of oxygen in the blood without requesting a drop of blood?

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The trick is they shine two colors of infared light in your skin, and then look at the reflection. The two colors reflect very differently off oxygenated and unoxygenated blood, and the ratio between the brightness can be used to figure out the oxygen concentration in your blood.


Fun fact. They don’t actually read how much oxygen is in the blood. They read how much *stuff* is bound to the red blood cells. They don’t know the difference between oxygen and carbon monoxide for example. Carbon monoxide poisonings show up with a high spo2 (the reading you get from a pulse odometer) because the CO has come in and replaced all the O2. It also doesn’t tell how many red blood cells are floating around. You may have lost a significant amount of blood but your red blood cells are still saturated with oxygen… just you have 1/2 the red blood cells you started with. As a result, you still have a high spo2 but you’re not perfusing well (the cells aren’t getting oxygen and getting rid of co2 / waste).


Eli5 version “Ya know when you shine a light through your thumb in the dark and you can see the red through the other side? well they just do that with 2 specific lights and read how dark or light the other side is, this tells you the amount of oxygen with some clever math”


It shines two lights through your finger, actually; one ultraviolet, and one infrared or just red. UV light passes through your blood fairly easily, regardless of its oxygen level. But the less oxygen in your blood, the more it obstructs the red light. So you shine both of them at the same strength at your finger, and put sensors on the other side. If the sensors pick up just as much red light as UV light, your blood is fine. But if they only pick up half as much red light as UV light, then it knows your blood doesn’t have as much oxygen. It’s pretty much just figuring out the right fractions at that point.


Light absorption – everything absorbs light, oxidized iron specially. Think of it as having multiple layers of transparent red film. The more layers you add, the more light is absorbed. Oximeters have a reading of the light that passes and equates that to the amount of blood that carries oxygen, which absorbs light and only let’s red pass through.