Mechanics of Breathing: Inhalation & Exhalation Physiology

Ventilation is the mechanical movement of air into and out of the lungs, driven by pressure gradients.

Inhalation (Inspiration) – Active Process

• Diaphragm contracts: Flattens, descends 1–2 cm → ↑ thoracic volume (vertical)
• External intercostals contract: Elevate ribs → ↑ volume (anteroposterior + lateral)
• Accessory muscles (if forced): Scalenes, SCM, pectoralis minor → further ↑ volume
• Intrapulmonary pressure (P_pulm): Drops below atmospheric (-1 to -3 cmH₂O)
• Air flows in down pressure gradient

Exhalation (Expiration) – Passive (at rest)

• Muscles relax → elastic recoil of lungs/chest wall
• P_pulm rises above atmospheric (+1 to +3 cmH₂O)
• Air flows out

Forced Exhalation: Internal intercostals, abdominal muscles (rectus abdominis, obliques) → active compression.

Key Pressure Relationships

• P_pulm = P_atm + P_el (elastic recoil)
• Transpulmonary pressure (P_tp = P_alv - P_pl) keeps alveoli open
• Work of breathing = ∫ P dV

Clinical Relevance

• Obstructive disease: ↑ resistance → ↑ work, dynamic hyperinflation
• Restrictive disease: ↓ compliance → ↑ work, rapid shallow breathing
• Mechanical ventilation: Manipulates pressure/volume to unload respiratory muscles

Breathing is a pressure-driven pump; understanding its mechanics underpins respiratory care.