A man runs up a fl ight of stairs. His respiratory rate rose from 18 breaths per minute (resting) to 36 bpm (at the top of stairs). The increase was probably a result of:

B: Muscular activity would have increased the rate of oxygen consumption and carbon dioxide production by cells. A higher breathing rate is required to take in the needed oxygen and to excrete the produced carbon dioxide. It is increased CO 2 rather than decreased O 2 that stimulates respiratory rate.