This is simply the percentage of oxygen saturation in the blood or “tissue perfusion” (actually the measure of oxygenated hemoglobin molecules). It remains fairly constant until a person reaches high altitude. At sea level it’s 95 to 100%. At Everest base camp (17,600 feet) it is between 70 and 80%. Once at high altitude, the Sa02 drops, but then flattens out due to increased breathing and heart rate. There is quite a bit of variation between individuals for this measure.