Question

what does the brain monitor to control the rate of breathing during normal activities?
○ the level of carbon dioxide in the air
○ the level of carbon dioxide in the blood
○ the level of carbon dioxide in the lungs
○ the level of carbon dioxide in the cells

Explanation:

To determine what the brain monitors to control breathing rate, we recall respiratory physiology: The brain (specifically the medulla oblongata) monitors blood gas levels, especially carbon dioxide ($\text{CO}_2$) in the blood. When $\text{CO}_2$ in blood rises (forming carbonic acid, lowering pH), chemoreceptors in the blood vessels and brain signal the respiratory center to increase breathing rate to expel excess $\text{CO}_2$. The air’s $\text{CO}_2$ level is not monitored for normal breathing control. Lungs and cells do not provide the primary feedback for this regulation—blood levels of $\text{CO}_2$ (and resulting pH changes) are the key signal.

Answer:

B. the level of carbon dioxide in the blood