Breathing is the process that moves air in and out of the lungs, to allow the diffusion of oxygen and carbon dioxide to and from the external environment into and out of the blood. “Breathing” sometimes also refers to the equivalent process using other respiratory organs such as gills in fish and spiracles in certain arthropods. 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 (all animals, most plants and many micro-organisms) 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 dioxide. After breathing, the next process in this chain of events is the transport of these gases throughout the body by the circulatory system, and then their uptake or release from the respiring cells. Breathing fulfills another vital function: that of regulating the pH of the extracellular fluids of the body. It is, in fact, this homeostatic function which determines the rate and depth of breathing.