The primary gas emitted by animals during respiration is carbon dioxide (CO2). When animals, including humans, breathe, they take in oxygen (O2) and use it to metabolize food for energy. This metabolic process allows cells to generate energy while producing carbon dioxide as a by-product. The CO2 is then transported through the bloodstream to the lungs, where it is expelled from the body when exhaling. This respiratory cycle is essential for maintaining energy balance and supporting life in aerobic organisms.