Lung Mechanics: How Inhalation and Exhalation Move Air

Air moves due to pressure gradients created by changing thoracic volume. The diaphragm and intercostal muscles drive these volume changes.

Inhalation (Active)

1. Diaphragm contracts → flattens, descends → ↑ vertical thoracic dimension.
2. External intercostals contract → ribs elevate (bucket handle) & sternum lifts (pump handle) → ↑ anteroposterior & lateral dimensions.
3. Thoracic volume ↑ → Intrapleural pressure ↓ (more negative) → Alveolar pressure ↓ below atmospheric.
4. Pressure gradient (Patm > Palv) → Air flows IN.

Exhalation (Passive at Rest)

1. Diaphragm relaxes → recoils upward (elastic recoil).
2. External intercostals relax → ribs depress.
3. Lung elastic recoil (elastin/collagen + surface tension) → volume ↓.
4. Alveolar pressure ↑ above atmospheric → Air flows OUT.

Forced Exhalation (Exercise, Disease)

• Internal intercostals pull ribs down.
• Abdominal muscles (rectus, obliques) push diaphragm up.
• Generates positive alveolar pressure > atmospheric.

Key Concepts

• Boyle's Law: P ∝ 1/V (at constant T).
• Compliance: ΔV/ΔP — ease of expansion. Reduced in fibrosis, edema, ARDS.
• Resistance: Airway radius⁴ (Poiseuille). Increased in asthma, COPD, obstruction.
• Work of Breathing: Elastic (compliance) + Resistive (airway) + Inertial.

This pressure-volume mechanics underpins all respiratory physiology and ventilator management.