Final answer:
In healthy individuals, large airways contribute most to air resistance, but in diseases like emphysema, which is often caused by smoking, the small airways become the major contributor due to increased resistance caused by alveolar damage.
Step-by-step explanation:
In health, the major contributor to air resistance lies in the large airways, whereas in disease caused by smoking, the contributor lies in the small airways. The correct answer to the student's question is a) Large, small.
The size of the airway is the primary factor affecting resistance. In healthy individuals, most of the air resistance is encountered in the larger airways. When diseases like emphysema, commonly linked to smoking, damage the lungs, the resistance to airflow increases in the smaller airways.
This is due to the destruction of alveolar walls, which leads to a loss of lung elasticity and larger volumes of air being trapped in the lungs after exhalation.
Obstructive diseases such as emphysema and asthma lead to increased resistance in the small airways.
In emphysema, alveoli are destroyed, reducing the surface area for gas exchange and increasing lung compliance, trapping more air at the end of expiration. Asthma causes inflammation that obstructs the airways, resulting in increased resistance as well.