The gas necessary for respiration in humans is oxygen. This essential gas is a crucial component of the air we breathe and is vital for cellular respiration, a process by which cells produce energy. Inhaled oxygen travels to the lungs, where it moves into the bloodstream and is transported to cells throughout the body. Once in the cells, oxygen participates in biochemical reactions that break down glucose, resulting in the production of adenosine triphosphate (ATP), the energy currency of the cell. Without a sufficient supply of oxygen, cells cannot function properly, leading to fatigue, organ dysfunction, and ultimately life-threatening conditions.