* The term compliance refers to change in volume divided by change in pressure. It is a measure of the degree of distensibility of the lungs. It has a nonlinear relationship with changing lung volume. Static compliance is determined from the ratio of the difference in lung volume at two different volume levels and the associated difference in intra-alveolar pressure. Dynamic compliance relates VT to intrathoracic pressures at the instants of zero airflow that occur at the end inspiratory and expiratory levels with each breath.
* The lung volume/pressure relationship forms characteristic counter-clockwise loops demonstrating hysteresis, i.e. the shape is different during inspiration and expiration. The term hysteresis denotes failure of a system to respond identically to the application and the withdrawal of a force. The presence of hysteresis denotes a proportionate energy loss for the system due to the resistive behaviour of the lungs.
* The width of the loop can be used as a quantitative measure of resistive behaviour. When divided by the rate of gas flow, it yields resistance.
* At low end-expiratory lung volumes hysteresis is mainly due to the opening and closing of airways. At high lung volumes, the primary source of hysteresis is surfactant, reducing the surface tension at the gas-liquid interface in the alveoli.
* Lung volume can be expressed as a proportion of the VC and pressure that the respiratory muscles must generate.
e The static pressure-volume curve of the lung is obtained by the simultaneous s
"S. measurement of airway opening pressure, lung volume and pleural pressure as
° estimated by a balloon in the lower one third of the oesophagus.
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