The gas essential for human respiration is oxygen. Humans inhale oxygen from the air, which is then transported by the bloodstream to cells throughout the body. This process is crucial for the production of ATP (adenosine triphosphate) during cellular respiration, enabling our cells to perform essential functions and sustain life. Oxygen makes up about 21% of the Earth's atmosphere, and without it, aerobic organisms, including humans, would not be able to survive.