The gas that humans inhale is oxygen (O₂). Oxygen is an essential component of the air we breathe, comprising about 21% of the Earth's atmosphere. When inhaled, oxygen is transported through the bloodstream to various cells in the body, where it plays a crucial role in the process of cellular respiration. This process converts glucose and oxygen into energy (ATP), producing carbon dioxide and water as byproducts. Oxygen is vital for the survival of aerobic organisms, including humans, and is critical for sustaining life on Earth.