The gas exchanged in respiration is carbon dioxide (CO2). During cellular respiration, cells break down glucose in the presence of oxygen to produce energy. This metabolic process generates carbon dioxide as a waste product, which is transported from the cells to the lungs through the bloodstream. In the lungs, carbon dioxide is expelled when we exhale, while oxygen, which is inhaled, is transported to the body's cells. This gas exchange is essential for maintaining the body's energy levels and overall metabolic functions, highlighting the critical role of respiration in sustaining life.