Breathing is the process that moves air in and out of the lungs, or the diffusion of oxygen and carbon dioxide to and from the external environment into and out of the blood through other respiratory organs such as gills. For organisms with lungs, breathing is also called pulmonary ventilation, which consists of inhalation (breathing in) and exhalation (breathing out). Breathing is one part of physiological respiration required to sustain life. Aerobic organisms such as birds, mammals, and reptiles require oxygen at cellular level to release energy by metabolizing energy-rich molecules such as fatty acids and glucose. This is often referred to as cellular respiration. Breathing is only one of the processes that delivers oxygen to where it is needed in the body and removes excess carbon dio