Final answer:
High altitude decreases PO₂, affecting pulmonary vascular resistance.
Step-by-step explanation:
High altitude leads to decreased PO₂, which in turn affects pulmonary vascular resistance. Lower partial pressure of oxygen at higher altitudes leads to a smaller difference in partial pressures between the alveoli and the blood, resulting in less oxygen crossing the respiratory membrane and fewer oxygen molecules being bound by hemoglobin.
However, the body adapts to high altitudes by releasing a greater proportion of oxygen molecules into the tissues and producing more BPG, which enhances the dissociation of oxygen from hemoglobin. These mechanisms help tissues obtain the necessary oxygen even at high altitudes.