Respiratory Mechanics: Diaphragm and Chest Wall Dynamics During Breathing

Breathing relies on coordinated pressure changes generated by the diaphragm, intercostal muscles, and chest wall compliance.

Inhalation (Active Process)

• Diaphragm: Contracts, flattens, descends 1-3 cm → increases vertical thoracic dimension by ~2-3 cm.
• External intercostals: Elevate ribs → "bucket handle" (lateral expansion) and "pump handle" (anteroposterior expansion) motions.
• Accessory muscles (if forced): Scalenes, sternocleidomastoids, pectoralis minor elevate upper ribs.
• Result: Intrapleural pressure drops to -5 to -8 cmH₂O (rest) or -30 cmH₂O (forced); alveolar pressure falls below atmospheric → air flows in.

Exhalation (Passive at Rest)

• Diaphragm: Relaxes, returns to dome shape via elastic recoil.
• Chest wall: Elastic recoil of lungs and chest wall returns ribs to resting position.
• Internal intercostals/abdominals (forced): Actively depress ribs and compress abdomen.
• Result: Alveolar pressure exceeds atmospheric → air flows out.

Summary Table

Phase	Diaphragm	Chest Wall	Pressure Gradient
Inhalation	Contracts, descends	Expands (ribs up/out)	Palv < Patm
Exhalation (quiet)	Relaxes, ascends	Recoils passively	Palv > Patm
Exhalation (forced)	Relaxes	Active compression	Palv ≫ Patm

Clinical Relevance

• COPD: Flattened diaphragm → mechanical disadvantage; pursed-lip breathing improves expiration.
• Neuromuscular disease: Diaphragm weakness → orthopnea, hypercapnia.
• Obesity: Reduced chest wall compliance → increased work of breathing.