Compliance means stretchability. Both lungs and thoracic cage are elastic structures and can expand. Compliance is expressed as change in their volume per unit change in pressure across their walls.
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To determine how much compliant the lung is we need to know the pressure difference across its wall and lung volume change with that much pressure difference. For lung compliance we take difference between alveolar and pleural pressure. This difference is also known as transpulmonary pressure.
The pressure volume relationship of lung during inspiration and expiration is expressed using pressure volume curves. The relationship between pressure and volume change is not linear, rather slope is less or its kind of flat at the base and the top that is lung is less compliant at bery high and very low lung volumes. Second thing..the relationship is different during inspiration and expiration.
The phenomenon of lung behaving differently during inflation and deflation is known as hysteresis. But why is this occurring ? Why more pressures are required for inflating the lungs than deflating. This is due to two factors: elasticity and suraface tension. That means due to inherent make up of the lung tissue, this elastic tissue is resisting expansion but is promoting recoil. That means during inspiration there is an opposing force and during expiration there is a promoting force… so obviously pressure change required will be different during inspiration and expiration.
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Chapters:
00:00 Introduction
00:24 Compliance concept
01:26 Calculation of compliance
03:40 Lung compliance
06:20 Lung pressure-volume curve
08:08 Hysteresis
08:48 Reason for hysteresis
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