What stimulates a healthy person to breathe?

The correct choice focuses on the role of carbon dioxide in the regulation of breathing. In a healthy individual, normal breathing is primarily stimulated by rising levels of carbon dioxide (CO2) in the blood. The brainstem, which houses the respiratory centers, responds to this increase by sending signals to increase the rate and depth of breathing. This mechanism is crucial because it helps maintain proper acid-base balance in the body, ensuring that CO2 levels do not rise excessively, which can lead to respiratory acidosis.

Other factors do play a role in respiration, but they are secondary to the influence of carbon dioxide. Low oxygen levels in the blood can stimulate breathing, but the body generally reacts to CO2 levels before oxygen levels become critically low in healthy individuals. While physical activity indeed increases the demand for oxygen and elevates CO2 production, it is not the direct stimulus as carbon dioxide levels are. External environmental factors like changes in temperature or altitude may influence respiration, but the primary driver in a healthy person remains the concentration of carbon dioxide detected by the chemoreceptors in the brainstem.