Final answer:
Areas of the lung with little ventilation and low oxygen levels cause blood vessels in that area to vasoconstrict to redirect blood flow to better-ventilated regions.
Step-by-step explanation:
Areas of the lung that have little ventilation and subsequently low oxygen levels will cause the blood vessels of that area to vasoconstrict. This physiological response is due to the lungs attempting to redirect blood flow to areas with higher oxygen levels to maximize gas exchange. When ventilation decreases, the partial pressure of oxygen in the alveoli drops, and the pulmonary arterioles constrict. In contrast, if an area of the lung has high ventilation, pulmonary arterioles in that area will dilate to allow greater blood flow and facilitate gas exchange.