PO2 on carotid bodies. PCO2 on carotid bodies. Alveolar pressure is less than atmospheric pressure during a normal inspiration. Alveolar pressure is greater than atmospheric pressure during an expiration. Alveolar pressure is the sum of pleural pressure and elastic recoil of the lung.
Alveolar pressure equals atmospheric pressure at end expiration of a normal breath.
Traveling to 15,000 ft. above sea level. Acute blood loss from trauma. Your PaO2 would fall. Oxygen carrying capacity would fall.