Mechanics of breathing

Airflow follows a pressure gradient. When the diaphragm and external intercostal muscles contract, the thoracic cavity expands, intrapleural pressure becomes more negative, the lungs are pulled open, and alveolar pressure falls below atmospheric pressure — so air flows in. This is Boyle's law in action: at constant temperature, increasing the volume of a container lowers the pressure inside it. Quiet expiration requires no muscular work. The muscles relax, the elastic tissue of the lung and chest wall recoils, alveolar pressure rises above atmospheric, and air flows out. Forced expiration recruits the abdominal muscles and internal intercostals.

Intrapleural pressure stays negative throughout the normal cycle, roughly −5 cm H₂O at rest and about −8 cm H₂O at the end of inspiration. That negative pressure exists because the lung's inward elastic recoil pulls away from the chest wall's outward recoil, and it is what keeps the lungs inflated. If air enters the pleural space — a pneumothorax — the negative pressure is lost and the lung collapses. Surfactant, produced by type II pneumocytes, lowers alveolar surface tension, which increases compliance, prevents small alveoli from collapsing into larger ones, and reduces the work of breathing.

Two properties characterize how easily the system moves. Compliance is the change in volume per unit change in pressure — how readily the lung stretches. It is increased in emphysema, where elastic tissue is destroyed, and decreased in fibrosis and in surfactant deficiency. Airway resistance determines how much driving pressure is needed for a given flow and rises sharply when airways narrow, as in asthma. Lung volumes and capacities — tidal volume, residual volume, functional residual capacity, vital capacity, and total lung capacity — quantify the results, with residual volume measurable only indirectly because it cannot be exhaled.

USMLE Step 1 tests respiratory mechanics in the physiology section. Expect questions on pressure-volume relationships, the direction of intrapleural pressure changes during the respiratory cycle, how compliance shifts in obstructive versus restrictive disease, and the role of surfactant.