The primary gas expelled when humans breathe out is carbon dioxide (CO2). During the process of respiration, cells convert glucose and oxygen into energy, producing CO2 as a byproduct. This gas is then transported via the bloodstream to the lungs, where it is expelled when we exhale. The exchange of gases—taking in oxygen and releasing carbon dioxide—is vital for maintaining the body's acid-base balance and overall metabolic function. This respiratory process is essential for sustaining life, as it enables oxygen to reach cells for energy production while removing metabolic waste in the form of CO2.