The gas we breathe in is oxygen. Oxygen is essential for human and animal life, as it is required for cellular respiration, a biochemical process that generates energy from food. When we inhale, oxygen enters our lungs, where it diffuses into our bloodstream and is transported to cells throughout the body. The importance of oxygen extends beyond humans to all aerobic organisms, which rely on it for survival. Although oxygen constitutes about 21% of Earth's atmosphere, its availability is crucial for maintaining life and supporting various biological processes.