carbon dioxide (CO2) - A colourless, odourless gas. Carbon dioxide is present in small amounts in the air and is an important by-product of metabolism in cells.
It is produced by the breakdown of substances such as carbohydrates and fats to produce energy, and is carried in the blood to the lungs and exhaled. Carbon dioxide helps to control the rate of respiration: when a person exercises, CO2 levels in the blood rise, causing the person to breathe more rapidly in order to expel carbon dioxide and to take in more oxygen.
When it is compressed and cooled to -75ºC, carbon dioxide becomes solid dry ice, which is used in cryosurgery.
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