A colourless, odourless gas that makes up 21 per cent of the Earth’s atmosphere. Oxygen is essential for almost all forms of life, including humans, because it is necessary for the metabolic “burning” of foods to produce energy – a process that takes place in body cells and is known as aerobic metabolism. Oxygen is absorbed through the lungs and into the blood, where it binds to the haemoglobin in red blood cells. As oxygen-rich blood circulates around the body, the oxygen is released from the red blood cells into the body tissues. Additional supplies of oxygen are used to treat conditions such as severe bronchitis or hypoxia. High-pressure oxygen (see hyperbaric oxygen treatment) is sometimes used to treat decompression sickness or carbon monoxide poisoning. (See also ozone.)


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