The process by which air passes into and out of the lungs to allow the blood to take up oxygen and dispose of carbon dioxide. Breathing is controlled by the respiratory centre in the brainstem. When air is inhaled, the diaphragm contracts and flattens. The intercostal muscles (muscles between the ribs) contract and pull the ribcage upwards and outwards. The resulting increase in chest volume causes the lungs to expand, and the reduced pressure draws air into the lungs. When air is exhaled, the chest muscles and diaphragm relax, causing the ribcage to sink and the lungs to contract, squeezing air out. In normal, quiet breathing, only about a 10th of the air in the lungs passes out to be replaced by the same amount of fresh air (tidal volume). This new air mixes with the stale air (residual volume) already held in the lungs. The normal breathing rate for an adult at rest is 13–17 breaths per minute. (See also respiration.)


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