What is meant by compliance of the lung and chest wall ?
The change in lung volume per unit change in airway pressure(ΔV/ΔP) is the strechability or the compliance of the lung and the chest wall.( The extent to which the lungs expand for each unit increase in transpulmonary pressure is called their compliance ) Reference Guyton
This is determined by the elastic forces of the lung
- The elastic force of the lung tissue itself
- The elastic force caused by the surface tension of the fluid that lines the inside walls of the alveoli and other lung space.
However compliance depends on the lung volume : an individual with only one lung has approximately half the ΔV for given ΔP.compliance decreased by pulmonary congestion and interstitial pulmonary fibrosis and is increased in emphysema .
Comment briefly on alveolar surface tension and the role of surfactant
An important factor affecting the compliance of the lung is the surface tension of the film of fluid lining the alveoli.It accounts for about 2/3 of the total elastic forces in the normal lungs .When water forms a surface with air , the water molecules on the surface of the water have an extra strong attraction for one another.As a result , the water surface is always attempting to contract in the fluid lining the alveoli ,the attempt to contract tries to force air out and collapse alveoli. This is referred to as surface tension
But the surface tension in the alveoli is much lower than the expected surface tension at the water - air interface of the same dimensions . This is due to the presence in the fluid lining the alveoli of surfactant , a lipid surface tension lowering agent . If the surface tension is not kept low when the alveoli become smaller during expiration , they collapse in accordance with the law of Laplace ( in spherical structure like alveoli the distension pressure equals to 2 times the tension divided by the radius ( P = 2T/R ): if T is not reduced as R is reduced , the tension overcome the distending pressure )
The surface tension is inversely proportionate to the concentration of the surfactant per unit area . As the radius of the alveoli increases during inspiration the surfactant molecules move further apart and surface tension increases whereas it decreases when they move closer together during expiration .
Surfactant also helps to prevent pulmonary oedema . if surfactant is not present the unopposed surface tension in thew alveoli would produce a 20 mmHg force favoring trasdation of fluid from the blood into alveoli .( in a normal lung , the hydrostatic pressure in the capillaries is less than the oncotic pressure so that threre is an inward directed pressure gradient of about 15mmHg , which keeps the alveoli free of fluid )
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