Mechanism of Breathing

Ventilating the lungs maintains the diffusion gradient across the alveolar epithelium by refreshing the air inside the alveoli.Air, like other fluids, moves from a region of higher pressure to a region of lower pressure.The flow of air into the lungs requires that a pressure gradient between the atmosphere and the alveoli be established.This driving pressure gradient is accomplished by the contraction of the inspiratory muscles.When the air pressure in the lungs is lower than atmospheric pressure air rushes into the lungs down the pressure gradient. This is called inspiration. When the air pressure in the lungs is higher than atmospheric pressure air rushes out of the lungs down the pressure gradient. This is called expiration. The diaphragm is a sheet of muscle that separates the thorax from the abdomen. Two sets of intercostal muscles lie between the ribs. Quiet expiration is mainly passive; the internal intercostal muscles contract only in forced expiration. Inspiration occurs when the external intercostal muscles contract.