Final answer:
Low airflow leads to low oxygen partial pressure, which causes the blood vessels to constrict and thereby decrease blood flow, optimizing oxygen delivery to tissues where it can be used more efficiently.
Step-by-step explanation:
When there is little airflow (shunt-like), the low partial pressure of O2 will act on smooth muscles to constrict blood vessels which will decrease blood flow. This response is an example of the body's local control mechanisms attempting to optimize oxygen delivery to the tissues. When an area of the lung is not well ventilated, and thus has a low partial pressure of oxygen, the local blood vessels constrict to redirect blood to better-ventilated areas where gas exchange can occur more efficiently. This process, known as hypoxic pulmonary vasoconstriction, helps to maintain an effective oxygenation of the blood despite local variations in lung function.