Oxygen is the gas used for breathing in mammals. Mammals, including humans, have adapted respiratory systems that facilitate the intake of oxygen from the atmosphere. Through the process of inhalation, oxygen is absorbed into the lungs and then transferred into the bloodstream, where it binds to hemoglobin in red blood cells. This oxygen-rich blood is then circulated to different tissues and organs, supporting cellular respiration—a process that produces energy for bodily functions. The necessity for oxygen underscores the importance of a healthy respiratory system and highlights how vital this gas is for sustaining life in mammals.