hypoxic vasoconstriction

Hypoxic vasoconstriction is a fundamental vascular response mechanism that occurs when oxygen tension falls below normal levels. In the pulmonary circulation, this response is particularly important - when alveolar oxygen levels decrease (PaO2 <60-70 mmHg), pulmonary arterioles constrict to divert blood flow away from poorly ventilated areas toward better-ventilated regions, thereby optimizing V/Q matching and maintaining arterial oxygenation. The mechanism involves oxygen-sensitive potassium channels in pulmonary vascular smooth muscle cells; hypoxia causes these channels to close, leading to membrane depolarization, calcium influx, and smooth muscle contraction. This response is opposite to systemic circulation, where hypoxia typically causes vasodilation. Chronic hypoxia (as seen at high altitude or in chronic lung disease) can lead to sustained pulmonary vasoconstriction, pulmonary hypertension, and eventually cor pulmonale. This mechanism is also relevant in pathologic states like acute respiratory distress syndrome (ARDS) and can be modulated by various factors including pH, temperature, and medications.