The brain coordinates the circulatory and respiratory systems of the human body. The control of breathing, for example, involves neural pathways among the structures represented in the figure above. One important stimulus in the control of breathing is an increase in blood $CO_2$ concentration, which is detected as a decrease in blood pH. Which of the following best describes the physiological response to an overall increase in cellular respiration in the body?
A In response to depleted blood $CO_2$ levels, the pH sensors send signals directly to the rib muscles, resulting in an increase in the rate of $CO_2$ uptake by the lungs and a decrease in $CO_2$ utilization by the brain.
B In response to low blood pH, the pH sensors send a signal to the brain, which then sends a signal to the diaphragm, resulting in an increased rate of breathing to help eliminate excess blood $CO_2$.
C In response to high blood pH, the pH sensors send a signal directly to the lungs, resulting in a slower rate of breathing, and the lungs send a signal back to the heart once $CO_2$ availability has been restored.
