respired

Respired is the past tense and past participle of the verb respire, meaning to breathe. In a biological and physiological context, respired air refers to air that has passed through the respiratory tract and undergone gas exchange in the lungs. The term is commonly used to distinguish air that has been inhaled from air that has been exhaled after participating in respiration.

In respiration science, respiration is usually described in stages: external respiration, the exchange of gases between alveolar air and blood; transport of gases in the bloodstream; and internal respiration, the usage of oxygen and production of carbon dioxide by body tissues (cellular respiration). Respired air specifically denotes the air after external respiration, i.e., air that has left the lungs or is ready for measurement as expired gas. Compared with inspired (or ambient) air, respired air typically contains less oxygen and more carbon dioxide, and it is saturated with water vapor from the humidified respiratory tract. Typical values for human respired air are around 16% oxygen and 4–5% carbon dioxide, with notable humidity.

The term is also used in fields such as respirometry and metabolic studies, where the volume and composition of respired air are measured to estimate metabolic rate and gas exchange efficiency. While related terms include inspired (inhaled) air and expired (exhaled) air, respired air emphasizes the portion of air that has already participated in the respiratory process.