As exercise levels increase, which of the following physiological changes occurs in the respiratory system?
a. the respiratory muscles shrink.
b. the lungs expand in size.
c. the trachea widens.
d. the airways narrow.

The correct answer should be b. The lungs expand in size.

The thoracic cavity expands as well as lungs when breath is taken in. As activity increases, more CO2 is produced meaning you need more air, meaning that your lungs will become larger. This is just simply put of course.

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