Question 8a

Let us assume that airways are subjected to the same smooth muscle stimulation.

Your answer was one of the following:

• Airway narrowing will be more pronounced in large airways due to their greater muscle mass.
• Airway narrowing will be more pronounced in small airways due to their relatively larger muscle mass.
• Airway narrowing will be most pronounced in large airways as their smooth muscles can shorten more.

None of the above answers is correct. Whatever the muscle mass, the airway is not only muscle. Large intrathoracic airways have a significant cartilage component in the airway wall, and the cartilage can limit the extent to which the airway circumference can be shortened with airway smooth muscle contraction. This difference should result in the smaller airways having a relatively greater degree of airway narrowing compared to the larger cartilagenous airways. Smooth muscle in the trachea is only present in the posterior membranous sheath (30% of circumference [1]); in peripheral airways smooth muscle is arranged in a helical fashion, completely encircling airway lumen. There maximal smooth muscle shortening results in a greater narrowing of the airway circumferende in a bronchiole [2] than in the trachea.

1. Von Hayek. The Human Lung. New York, Hafner Publishing Company, 1960.
2. Moreno RH, Hogg JC, Pare PD. Mechanics of airway narrowing. Am Rev Respir Dis 1986;133:1117-1180.