Final answer:
Increasing the airway radius decreases airflow resistance due to Poiseuille's law, which relates resistance to the fourth power of the radius.
Step-by-step explanation:
When the radius of the airway increases, the airflow resistance decreases. This phenomenon is well explained by Poiseuille's law, which states that resistance to airflow is inversely proportional to the fourth power of the radius. Therefore, if the airway radius is doubled, the resistance would decrease by a factor of 24, or 16 times. Hence, the correct statement is that resistance decreases because the airways allow for easier airflow when they are wider, not because the airways are more compliant, which is unrelated to the physical size of the airways.
Additionally, it is important to consider the presence of pulmonary surfactant which reduces the surface tension in the alveoli, facilitating easier lung expansion and reduced work during breathing. Factors like lung compliance and the presence of conditions causing swelling in the airways, such as an allergic reaction, can affect overall respiratory resistance, but they are separate considerations from the effect of airway radius on resistance.