Lung Mechanics

Lung mechanics describes the physical properties of the lungs and chest wall that govern the effort required to breathe. Key concepts include intrapleural pressure, elastic recoil, compliance, surfactant, and airway resistance.

Intrapleural pressure is the pressure within the pleural space between the visceral and parietal pleurae. At rest and end-expiration it is approximately −0.5 kPa (negative relative to atmosphere), holding the lung partially inflated and preventing collapse. Transpulmonary pressure is the difference between alveolar pressure and intrapleural pressure; it is the distending pressure across the lung wall and must be positive for the lung to remain inflated.

Elastic recoil refers to the tendency of the lung to return to a smaller volume after being stretched. It arises from two sources: the elastic fibres within the lung parenchyma, and the surface tension at the air-liquid interface lining the alveoli. Surface tension, if unchecked, would cause alveoli to collapse (atelectasis) and would make the work of re-inflation enormous.

Pulmonary surfactant, secreted by type II pneumocytes, dramatically reduces alveolar surface tension. Surfactant is a complex of phospholipids (primarily dipalmitoylphosphatidylcholine, DPPC) and proteins that adsorbs to the air-liquid interface and reduces surface tension, particularly as alveolar radius decreases. By reducing surface tension more in smaller alveoli, surfactant stabilises alveoli of different sizes and prevents the collapse of smaller alveoli into larger ones (as predicted by the Law of Laplace: pressure = 2T/r). Surfactant deficiency in preterm neonates causes neonatal respiratory distress syndrome.

Lung compliance is the change in lung volume per unit change in transpulmonary pressure (ΔV/ΔP). High compliance (emphysema) means the lung is easily distended but loses elastic recoil, trapping air. Low compliance (fibrosis, pulmonary oedema, ARDS) means the lung is stiff, requiring greater effort to inflate.

Airway resistance is governed by Poiseuille's law: resistance is proportional to the length of the airway and inversely proportional to the fourth power of the radius. Small reductions in airway radius cause dramatic increases in resistance. Dynamic airway compression occurs during forced expiration when the pleural pressure rises above airway pressure in small, unsupported airways, causing them to collapse and limit expiratory flow — the mechanism underlying air trapping in obstructive lung disease.