Decreased cardiac output will drop pulmonary pressures thus allowing vessels to collapse and increase pulmonary resistance which again maintains perfusion pressure. However, at some point too many vessels collapse. Alveoli which are not perfused because of this are dead space. Thus, physiologic dead space may increase as shock worsens and cardiac output decreases. Innervation of by both sympathetic and parasympathetic supply.
Systemic hypoxia does increase large vessel resistance by muscle constriction.
Alveolar hypoxia causes vasoconstriction. This is a very important response to maintain normoxia. If part of the lung is hypoventilated and thus decreases alveolar oxygen, then blood flow should be shunted away from this region to an area with normal ventilation and normal alveolar oxygen. Examples would include reducing blood flow in a collapsed lobe or in an area of pneumonia. The alveolar capillary membrane must be kept free of excess water to allow gas exchange.