With pulse oximetry, you measure the oxygen saturation of your blood. This is an indication of the functionality of your respiratory and cardiovascular systems. It represents the ability of your body to transfer oxygen from the air to your lungs, from your lungs to your arteries, and onwards to any organ. We do not measure blood, we measure tissue. This does not represent the functionality of your respiratory and cardiovascular systems but shows the efficiency of your (muscle-) tissue in extracting oxygen from the arteries.