physiology

Last reviewed 01/2018

• cyclical movement of ambient air into and out of the lungs, and the distribution and mixing of the inspired air within the lungs; adults require about six litres per minute
• achieved via the contraction and relaxation of the respiratory muscles together with the elastic recoil of the lungs.
• alveolar ventilation, Va - fraction of inspired gases that reaches perfused alveoli (about 2/3 during normal quiet breathing). The rest of the inspired air is known as physiological dead space, Vd (about 150mls).
• ventilation is generally measured in litres/minute and equals the sum of Va and Vd.
• static volumes (e.g. vital capacity, residual volume) are dependent on the compliance (or distensibility) of the lung. This in turn is affected by the elastic recoil due to the connective tissue elements in the parenchyma and the surfactant that lines the alveolar surface. Thus lung compliance is reduced in conditions such as pulmonary fibrosis and respiratory distress syndrome, and increased in emphysema.