When we breathe, the primary gas we exhale is carbon dioxide (CO2). This gas results from the metabolic processes in our cells during cellular respiration, where oxygen is used to break down glucose for energy. As glucose is metabolized, carbon dioxide is produced as a waste product and transferred back into the bloodstream. It is then carried to the lungs, where it is expelled when we exhale. The exhalation of carbon dioxide is an essential process for maintaining the body's acid-base balance and ensuring that our levels of oxygen remain optimal for cellular functions.