呼吸在生理学上，是指氧从空气发送到生物组织，以及二氧化碳从生物组织发送回空气的过程。这与呼吸作用有所不同：呼吸作用是在细胞之内进行，将有机物氧化分解并产生能量；而呼吸则是在细胞之外进行。

In physiology, respiration is defined as the movement of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction.

The physiological definition of respiration should not be confused with the biochemical definition of respiration, which refers to cellular respiration: the metabolic process by which an organism obtains energy by reacting oxygen with glucose to give water, carbon dioxide and 38ATP (energy). Although physiologic respiration is necessary to sustain cellular respiration and thus life in animals, the processes are distinct: cellular respiration takes place in individual cells of the organism, while physiologic respiration concerns the bulk flow and transport of metabolites between the organism and the external environment.