The gas that humans exhale during respiration is carbon dioxide (CO2). This gas is a byproduct of cellular respiration, a process in which cells convert glucose and oxygen into energy. During respiration, oxygen is inhaled and transported to the body's cells, where it is utilized to extract energy from food. The metabolic process produces carbon dioxide, which is then transported back to the lungs and exhaled. Maintaining a balance of oxygen and carbon dioxide is essential for the body’s functioning and is regulated through the respiratory system.