Respiratory System MCAT Practice Question
A 28-year-old healthy woman completes a maximal cardiopulmonary exercise test. During peak exercise, her oxygen consumption increases from a resting value of 250 mL/min to 2500 mL/min. Her heart rate and minute ventilation increase appropriately. A respiratory physiologist measures her airway resistance at rest and during peak exercise. Which of the following best explains the physiologic mechanism by which airway resistance decreases during exercise?
Answer choices
- AActivation of parasympathetic nervous system increases acetylcholine release, causing bronchial smooth muscle contraction and increased airway diameter
- BSympathetic stimulation and circulating catecholamines cause bronchiolar smooth muscle relaxation, increasing airway radius and dramatically reducing resistance according to the relationship R ∝ 1/r⁴Correct answer
- CIncreased minute ventilation generates positive pressure that mechanically dilates airways and reduces the work of breathing proportionally to ventilation rate
- DDecreased intrathoracic pressure during increased respiratory effort creates a suction effect that passively stretches airway smooth muscle, though this effect is offset by increased airflow velocity
- EAirway resistance remains essentially unchanged during exercise because smooth muscle tone does not significantly contribute to airway caliber in healthy individuals
- FTurbulent flow patterns at high ventilation rates create dynamic airway compression that paradoxically reduces measured resistance despite increased breathing effort
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.