Pulmonay Compliance

Compliance

• Compliance is the ability of the lung & chest wall to expend or it is the expandability of the lung and chest wall.
• The compliance of a lung is also defined as the change in volume per unit change in the pressure of the lung.

Lung Compliance (C) = Change in Lung Volume (V) / Change in Transpulmonary Pressure (P)

Types of Compliance

1. Static compliance is the study of pressure and volume relationship of the lung when air is not flowing through the airways of the lung i.e. breathing is not taking place.

2. Dynamic Compliance is the study of the pressure & volume relationship of lung while airflow is continuing i.e. during breathing.

Q.  why during inspiration lungs are less compliant and why during expiration lung are more compliant

• This difference shows that when the lung is smaller, it is difficult to expand by increasing the pressure.
• At residual volume, lung size is small and alveoli are also smaller
• Alveoli are filled with fluid secreted by type 1 pneumocytes that have strong intermolecular forces & the lung is stiffened.
• When inspiration starts then intra-alveolar pressure is increased, which is initially required for the breakdown of intermolecular forces & stickiness of alveoli.
• As inspiration continues, the smaller alveoli become larger and the intermolecular force becomes weaker.
• When the lung reaches at End inspiratory volume or at total lung capacity then the lung is fully expended and intermolecular forces become less.
• During Expiration, the intra alveolar pressure is decreases and intermolecular force increasing 
• So, at specific pressure lung is less compliant during Inspiration & more compliant during Expiration.