The atmospheric pressure drops so the partial pressure of O2 drops proportionately. The hemoglobin must circulate faster to deliver the same quantity of oxygen to working muscles' mitochondria for cellular respiration. This increased basal circulation rate places a greater demand for oxygen to support it, which triggers in increase in red blood cell population so there is more hemoglobin to deliver O2. With more RBCs the circulation rate once more slows to the old basal rate.
