Pulmonary Blood Flow Regulation
- Physiological studies have demonstrated that the pulmonary vasculature displays highly unique responses to hypoxia. While the systemic arterioles typically dilate in response to low oxygen tension, pulmonary arterioles undergo vasoconstriction, resulting in reduced blood flow to regions of hypoxic lung. This phenomenon, termed "Hypoxic Vasoconstriction", is the primary process which actively regulates the blood flow distribution in pulmonary tissue. It should be pointed out that the pulmonary vasculature is also modulated by a variety of other physiological stimuli; however, it appears that hypoxic vasoconstriction represents the most important process.
- The precise mechanism of hypoxic vasoconstriction remains cryptic although several features of the response are worth mentioning. First, the vasoconstrictive response to hypoxia occurs in surgically isolated lung, indicating a mechanism operating intrinsically within pulmonary tissue. Furthermore, the response appears sense and respond to the partial pressure of oxygen in the alveoli rather than that in the pulmonary capillaries. Finally, vasoconstriction of the pulmonary arterioles becomes significant only after the partial pressure of alveolar oxygen declines below 70 mm Hg, representing a fall below 70% of the normal oxygen partial pressure. Further declines in alveolar partial pressure below 70 mm Hg result in an increasingly steep degree of vasoconstriction.
- Hypoxic vasoconstriction of pulmonary arterioles results in increased resistance to blood flow and thus reduced perfusion of the pulmonary capillaries which they supply. This response likely represents an evolutionary adaptation aimed at reducing perfusion to local areas of the lung which are encountering poor alveolar ventilation and thus display reduced levels of alveolar oxygen. By doing so, the lung can avoid generating significant right-left shunts by directing blood flow away from poorly-ventilated areas of the lung, as might occur during a lobar pneumonia or airway obstruction. However, hypoxic vasoconstriction can result in pathological rises in the entire pulmonary vascular resistance and thus result in pulmonary hypertension when the lung globally encounters hypoxia, as might occur in a scenario of high altitude.