How Breathing Changes Lung Volume: Mechanics of Ventilation

The Breathing Cycle

Inhalation (Active)

• Diaphragm contracts → flattens, descends.
• External intercostals contract → ribs elevate (bucket handle + pump handle).
• Thoracic volume increases → intrapleural pressure becomes more negative.
• Alveolar pressure drops below atmospheric → air flows in.
• Lungs expand (elastic recoil overcome).

Exhalation (Passive at Rest)

• Diaphragm relaxes → returns to dome shape.
• External intercostals relax; elastic recoil of lungs/chest wall compresses alveoli.
• Alveolar pressure rises above atmospheric → air flows out.
• Lungs decrease in volume.

Forced Exhalation (Active)

• Internal intercostals + abdominal muscles contract → further decrease thoracic volume.

Key Determinants of Ventilation

• Lung compliance: Ease of expansion (reduced in fibrosis, edema).
• Airway resistance: Diameter, length, turbulence (increased in asthma, COPD).
• Respiratory muscle strength.

Clinical Relevance

Understanding these mechanics guides ventilator management, pulmonary rehab, and interpretation of spirometry (FVC, FEV1, TLC).