Both the systemic and the pulmonary circulations respond to local hypoxia in the appropriate manner, the former by vasodilating, thereby providing more oxygen, and the latter by constricting and rerouting blood flow to areas where more O2 is available. In either case, changes in local conductance affect total conductance, and through that variable, the perfusing pressure; as a result, the effects of local vasomotion should be reduced. In the systemic circulation, arterial pressure can be prevented from falling by two important mechanisms: vasoconstriction of other vascular beds, and an increase in cardiac output. There are no similar means for protecting pulmonary arterial pressure against a rise when vessels in hypoxic areas contract; the only defense is provided by passive expansion of the vascular bed. Thus, in the lung regional circulatory readjustments conflict with the need to maintain a reasonably low pulmonary arterial pressure and local regulation (and maintenance of arterial oxygenation) may be subordinate to prevention of pulmonary hypertension.