Effects of Anoxia on Pulmonary Circulation: Reflex Pulmonary Vasoconstriction

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Abstract
The inhalation of 5 or 10% oxygen in anesthetized dogs causes increased pulmonary arterial pressure, increased pulmonary blood flow (measured directly by cannulalation of one lobar vein) but increased or decreased pulmonary vascular resistance. At least four opposing factors are responsible for the variable effects of anoxia on vascular resistance, namely: a) reflex pulmonary vasoconstriction from anoxic stimulation of chemoreceptors in the carotid and aortic bodies (demonstrable in innervated lobe that is perfused and ventilated independently of all other lobes); b) local pulmonary vasodilatation (brought about by reduction in oxygen content of the air ventilating or of the blood perfusing the lobe); c) passive reduction in pulmonary vascular resistance secondary to anoxic increase of pulmonary blood flow; and d) liberation of epinephrine capable of stimulating the heart and constricting the lung vessels. The reflex pulmonary vasoconstriction can be demonstrated only if the perivascular nerve fibers are preserved. This reflex response is mediated by the thoracic sympathetics and is activated by less severe anoxemia (reduction of arterial oxygen saturation by at least 10%) as compared to the other mechanisms (local and passive dilatation which require reduction of saturation by at least 30%).

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