Breathing is the process that moves air in and out of the lungs. It allows the diffusion of oxygen and carbon dioxide to and from the external environment into and out of the blood.
“Breathing” sometimes refers to the equivalent process that utilizes other respiratory organs such as gills in fish and spiracles in certain arthropods. For organisms with lungs, breathing is also called pulmonary ventilation. It 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 microorganisms) require oxygen at the cellular level to release energy. It is done 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 deliver 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: regulating the pH of the body’s extracellular fluids. It is, in fact, this homeostatic function that determines the rate and depth of breathing. The medical term for normal relaxed breathing is eupnea.
At the end of each exhalation, the adult human lungs still contain 2.5 – 3.0 liters of air, termed the functional residual capacity (FRC). Breathing replaces only about 15% of this volume of gas with moistened ambient air for each breath. This ensures that the composition of the FRC changes very little during the breathing cycle and remains significantly different from the composition of the ambient air. The partial pressures of the blood gases flowing through the alveolar capillaries equilibrate with the partial pressures of the gases in the FRC.