Editorial

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Abstract
Knowledge of the mechanism by which pulmonary hypertension occurs and is maintained is still rudimentary. Studies on animals in which many of the factors affecting pressure and flow in the pulmonary vascular bed can be accurately controlled have established clearly that the pulmonary vessels are capable of active changes in caliber. Recent experiments have demonstrated that the normal pulmonary vessels in man need not behave as a passive viscoelastic system; tone is present in these vessels. Smooth muscle tone has also been demonstrated in every abnormality in which pulmonary hypertension occurs, but the manner in which this tone is maintained is unknown.