Compliance is the ability of the lungs and thorax to expand or it is the expansibility of lungs and thorax. It is defined as the change in volume per unit change in the pressure.

Significance of Determining Compliance

Determination of compliance is useful as it is the measure of stiffness of lungs. Stiffer the lungs, less is the compliance.


Compliance is expressed by two ways:

1. In relation to intraalveolar pressure

2. In relation to intrapleural pressure.

Compliance in Relation to Intra-alveolar Pressure

Compliance is the volume increase in lungs per unit increase in the intraalveolar


1.     Compliance of lungs and thorax together:

2.     130 mL/1 cm H2O pressure

2. Compliance of lungs alone:

220 mL/1 cm H2O pressure.

Compliance in Relation to Intrapleural Pressure

Compliance is the volume increase in lungs per unit decrease in the intrapleural pressure.

1. Compliance of lungs and thorax together:

100 mL/1 cm H2O pressure

2. Compliance of lungs alone:

200 mL/1 cm H2O pressure. Thus, if lungs could be removed from thorax, the expansibility (compliance) of lungs alone will be doubled. It is because of the absence of inertia and restriction exerted by the structures of thoracic cage, which interfere with expansion of lungs.

Specific Compliance

The term specific compliance is introduced to assess the stiffness of lung tissues more accurately. Specific compliance is the compliance per liter of lung volume. It is usually reported for expiration at functional residual capacity. It is the compliance divided by functional residual capacity.

Specific compliance Compliance of lungs of lungs = Functional residual capacity

Functional residual capacity is the volume of air present in lungs at the end of normal expiration.


Compliance is of two types:

1. Static compliance

2. Dynamic compliance.

1. Static Compliance

Static compliance is the compliance measured under static conditions, i.e. by measuring pressure and

volume when breathing does not take place. Static compliance is the pressure required to overcome

the elastic resistance of respiratory system for a given tidal volume under zero flow (static) condition.

2. Dynamic Compliance

Dynamic compliance is the compliance measured during dynamic conditions, i.e. during breathing.

Static Compliance Vs Dynamic Compliance

In healthy subjects, there is little difference between static and dynamic compliance. In patients with stiff

lungs, the dynamic compliance decreases while little change occurs in the static compliance.


Measurement of Static Compliance

To measure the static compliance, the subject is asked to inspire air periodically at regular steps from

a spirometer. In each step, a known volume of air is inspired. At the end of each step, intrapleural pressure is measured by means of an esophageal balloon. Then, the air is expired in steps until the volume returns to original preinspiratory level. Intrapleural pressure is measured at the end of each step.

Values of volume and pressure are plotted to obtain a curve, which is called pressure-volume curve. From this curve compliance can be calculated. This curve also shows the difference in inspiration and expiration.

Measurement of Dynamic Compliance

Dynamic compliance is measured during normal breathing. It is measured by determining the lung volume and esophageal pressure (intrapleural pressure) at the end of inspiration and expiration when the lungs are apparently stationary.

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