Pulmonary ventilation (mechanics of breathing)

Air flows from high to low pressure. Because P × V is constant for a gas at fixed temperature (Boyle's law), enlarging the thorax lowers alveolar pressure below atmospheric and air enters; reducing thoracic volume raises alveolar pressure and air leaves. Four pressures matter: atmospheric (reference, 0), alveolar (intrapulmonary), intrapleural (≈ −5 cmH2O at rest) and transpulmonary (alveolar − intrapleural), which distends the lungs.

Quiet inspiration is active: the diaphragm descends and the external intercostals (with the parasternal internal intercostals and scalenes) raise the ribs — pump-handle motion of the upper ribs increases the anteroposterior diameter and bucket-handle motion of the lower ribs increases the transverse diameter. Intrapleural pressure falls from ≈ −5 to ≈ −8 cmH2O, alveolar pressure becomes ≈ −1 cmH2O and ≈500 mL flows in. Quiet expiration is passive: elastic recoil of lung tissue and surface tension raise alveolar pressure to ≈ +1 cmH2O.

Forced breathing recruits accessory muscles. Inspiratory: sternocleidomastoid, scalenes, pectoralis minor (and pectoralis major with the arms fixed), serratus anterior, trapezius, alae nasi. Expiratory: rectus abdominis, external and internal obliques, transversus abdominis and the internal intercostals, which raise intra-abdominal pressure and pull the ribs down. Minute ventilation = tidal volume × rate (≈500 mL × 12 = 6 L/min at rest; > 100 L/min in maximal exercise).