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A healthy young man inhales to total lung capacity then exhales forcefully. The expiratory flow rate is maximum at total lung capacity and decreases linearly as expiration continues. Which of the following is the most likely cause of the observed decrease in expiratory airflow?

A- Airway compression
B- Decreased expiratory effort
C- Decreased intrpleural pressure
D- Doming of the diaphragm
E- Increased elastic recoil

Ex-USMLE Forums Staff
753 Posts
Answer is A

B) is wrong cause expiratory effort has an impact on the maximum flow rate you attain but it's not the reason why we have progressive decrease in flow rates toward the end of expiration (which happens in everyone no matter how hard or how feeble their efforts were).

C) is also wrong because as we exhale the intrapleural increases rather than decreases

D) there will be doming of diaphragm during expiration but it has nothing to do with decreased flow rates

E) the smaller the lung goes the less "elastic recoil" we have. Elastic recoil basically depends on how "stretched" your collagen fibers are.

So the correct answer is A
As the intrapleural pressure increases it will compress the airways and decreases the flow rates. It's called "dynamic compression of the airways". This mechanism is also responsible for the fact that we can only exhale 80% of our vital capacity and we cannot empty the whole lung because compression of the upper airways is more than the lower airway and it will reach a point where it closes off the system and no more exhalation is possible.
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