Here is a great explanation: https://www.youtube.com/watch?v=nO4xgcIaPeA https://www.youtube.com/watch?v=nO4xgcIaPeA There are three main causes of hypoxemia (

Here is a great explanation: https://www.youtube.com/watch?v=nO4xgcIaPeA https://www.youtube.com/watch?v=nO4xgcIaPeA There are three main causes of hypoxemia (low blood o2): 1) "V/Q" (volume/flowrate), where uneven flow of blood carries unoxegenated blood past alveoli faster than it can oxygenate, 2) "Shunt" where blood flows through the lungs without being able to exchange gasses at all 3) "Diffusion Abnormality" - usually seen in COPD, this is where the membrane between alveoli and the capillaries becomes thicker or different. CO2 can diffuse through a thicker membrane more easily than oxygen, so that can explain some of the findings other posters are discussing. Nothing peer reviewed on this yet, though we have techniques for analyzing exactly whats happening in the lungs by injecting inert gasses to the body and measuring how they leave.