Oxygen is the chemical element essential for human respiration. It is a colorless, odorless gas that constitutes about 21% of Earth's atmosphere. Humans and other aerobic organisms rely on oxygen for cellular respiration, a process that converts glucose and oxygen into energy, carbon dioxide, and water. This energy is vital for the proper functioning of cells and organs. Humans inhale oxygen through the respiratory system, allowing it to reach the bloodstream via the alveoli in the lungs. The cardiovascular system then transports oxygen to cells throughout the body. Without adequate oxygen, cellular processes are impaired, leading to detrimental health effects.