Breathing is the process of moving air into and out of the lungs to facilitate gas exchange with the internal environment, mostly to bring in oxygen and flush out carbon dioxide. The process of respiration, during which air is inhaled into the lungs through the mouth or nose due to muscle contraction and then exhaled due to muscle relaxation. Breathing, or "external respiration", brings air into the lungs where gas exchange takes place in the alveoli through diffusion. The body's circulatory system transports these gases to and from the cells, where "cellular respiration" takes place. The breathing of all vertebrates with lungs consists of repetitive cycles of inhalation and exhalation through a highly branched system of tubes or airways which lead from the nose to the alveoli.The number of respiratory cycles per minute is the breathing or respiratory rate, and is one of the four primary vital signs of life.
