In physiology, respiration is a process that facilitates the transport of oxygen from the outside environment to bodily tissues and the removal of carbon dioxide using a respiratory system.
The physiological definition of respiration differs from the biological definition of cellular respiration, which is a metabolic process by which an organism obtains energy (in the form of ATP and NADPH) by oxidizing nutrients and releasing waste products. 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 diffusion and transport of metabolites between the organism and the external environment.
Exchange of gases in the lung occurs by ventilation (commonly called breathing) and perfusion.