The gas necessary for human respiration is oxygen. Oxygen is essential for cellular respiration, a process through which cells convert glucose and other substances into energy. Humans and many other organisms inhale oxygen from the atmosphere, where it makes up about 21% of the air we breathe. Inside the lungs, oxygen diffuses into the bloodstream and binds to hemoglobin in red blood cells, which transport it throughout the body. A continuous supply of oxygen is critical for sustaining life, as it enables the production of adenosine triphosphate (ATP), the energy currency of our cells.