What triggers pulmonary vasoconstriction in COPD leading to pulmonary hypertension?

• A. Systemic hyperoxia
• B. Alkalosis
• C. Increased tidal volume
• D. Alveolar hypoxia

Answer: (D)

Alveolar hypoxia triggers this response. When parts of the lung are poorly ventilated in COPD, the oxygen level in those alveoli drops. The small pulmonary arteries nearby sense this low oxygen and constrict—a local protective mechanism called hypoxic pulmonary vasoconstriction. By narrowing these vessels, blood is diverted toward better-ventilated areas to improve gas exchange. But in COPD this hypoxic signal is chronic, so the pulmonary arteries remain constricted for a long time, increasing vascular resistance and eventually leading to pulmonary hypertension and right heart strain. Other factors listed don’t initiate this specific lung-wide reflex: systemic oxygen levels or pH changes don’t produce the same targeted pulmonary vasoconstriction, and increasing tidal volume typically improves oxygenation, which would lessen HPV rather than provoke it.