Final answer:
The most likely cause for normal alveolar PO2 but low arterial PO2 is shunting of blood. This condition results from the bypassing of non-oxygenated blood from the right to the left side of the heart, leading to insufficient oxygenation in the arterial blood.
Step-by-step explanation:
If alveolar PO2 is normal but arterial PO2 is low, the most likely cause is b) Shunting of blood. Shunting refers to the bypassing of blood from the right side of the heart to the left without participating in gas exchange in the lungs, which could happen due to a physiological defect or pathology. Despite normal alveolar oxygen pressure, the oxygen doesn't make it into the blood, resulting in lower arterial PO2.
Concerning the additional information provided, when ventilation is not sufficient, the body responds by constricting the capillary to redirect blood flow to well-ventilated alveoli. This mechanism ensures that poorly ventilated alveoli do not receive as much blood flow, conserving the body's small venous reserve of oxygen and promoting efficient gas exchange in functional alveoli.