Compliance of lungs, Respiratory Work, and Pneumothorax.

Mechanism of breathing
Muscle contraction and relaxation function to bring air in and out of the respiratory system.

During normal breathing, contraction of the diaphragm and external intercostals increases the volume of the thoracic cavity, which decreases the intrathoracic and intrapleural pressures and works in inspiration. Relaxation has the opposite effect in expiration.

During exercise and respiratory distress, other muscles are also used – scalene and sternocleidomastoid (inspiration), internal intercostal and abdominal (expiration).

Lung compliance
Compliance is a description of the ability of the lungs and chest wall to distend. It inherently decreases with increasing stiffness, but also depends on physical circumstances:

C = V/P

For Compliance C (mL/mm Hg), Volume V (mL), Pressure P (mm Hg)

Plotting V vs P can give the slope (a measure of compliance).

Inspiration and expiration follow different pathways on this plot, since it takes work to overcome surface tension during inspiration but not during expiration.

Respiratory work includes the work needed to overcome


 * Elastic forces in lungs
 * Viscosity of surfactant
 * Airway resistance

Pneumothorax
If air is introduced into the intrapleural space, the pressure there will increase to the level of the surroundings. The lungs will then collapse, and the chest wall will go outward.