The gas that we primarily breathe in is oxygen. In Earth's atmosphere, oxygen makes up about 21% of the air we breathe. It is essential for human life and is required for cellular respiration, a process that converts food into energy within our cells. During respiration, we inhale oxygen, which travels to the lungs and is then transported to the bloodstream, where it is delivered to cells throughout the body. As we exhale, we release carbon dioxide, a byproduct of the energy-making process. Thus, oxygen is vital for sustaining life and maintaining the functions of living organisms.