Which gas is primarily responsible for stimulating breathing in healthy individuals?

The gas primarily responsible for stimulating breathing in healthy individuals is carbon dioxide. The respiratory system's drive to breathe is primarily regulated by the levels of carbon dioxide in the blood rather than the levels of oxygen. When carbon dioxide levels rise, it leads to an increase in acidity of the blood (lower pH), stimulating the central chemoreceptors located in the medulla oblongata of the brain. This response prompts an increase in the rate and depth of breathing to expel more carbon dioxide and bring in oxygen.

In contrast, while oxygen is crucial for cellular metabolism, it only significantly influences the breathing drive when levels drop critically low, as detected by peripheral chemoreceptors in the carotid and aortic bodies. Nitrogen is an inert gas and does not play a role in stimulating respiration, and carbon monoxide, while it can be harmful by binding to hemoglobin and preventing oxygen from being carried, does not actively stimulate the respiratory drive in healthy individuals. Thus, the role of carbon dioxide as the primary regulator makes it vital in the body's respiratory control system.