Figure 5. The mechanism underlying loss of hypoxic pulmonary vasoconstriction (HPV) during inhaled oxygen therapy. The mechanism depends on elevation of the PAO2 independent of any change in the PaO2. (A) Elimination of CO2 taken up from the alveolar capillaries and maintenance of the PAO2, depends on an adequate volume of air entering and leaving the alveoli per unit time (grey arrow). (B) Reduced regional alveolar ventilation results in reduced elimination of CO2 and a fall in the PAO2. The fall in PAO2 is sensed by the alveolar capillary endothelium (white circle) and regional arterioles are signalled to constrict. (C) Supplemental oxygen raises the PAO2 and HPV is lost. An increased volume of blood with high CO2 content reaches alveoli incapable of excreting the gas and the PaCO2 rises. (D) Elevated PaCO2 is sensed by brainstem respiratory centers, which increase global pulmonary ventilation. In a normal young individual, the excess CO2 is eliminated and the rise in PaCO2 is transient. DRG – dorsal respiratory group; VRG – ventral respiratory group.