Understanding Breathing Mechanics: Inspiration & Expiration

Pulmonary Ventilation: How Pressure Changes Drive Airflow

Breathing (pulmonary ventilation) relies on pressure gradients between the atmosphere and the alveoli, created by changing thoracic volume.

Inspiration (Active Process)

1. Diaphragm contraction: The dome-shaped diaphragm contracts and flattens, moving downward.
2. External intercostal contraction: Ribs elevate (pump handle and bucket handle motions), expanding thoracic cavity anteroposteriorly and laterally.
3. Thoracic volume increases: Lung volume expands due to pleural adhesion.
4. Intrapulmonary pressure drops: Falls below atmospheric pressure (~ -1 to -3 cm H₂O).
5. Air flows in: Atmospheric pressure pushes air down the pressure gradient through airways into alveoli.

Expiration (Passive at Rest)

1. Diaphragm relaxation: Returns to dome shape, moving upward.
2. Elastic recoil: Lungs and chest wall passively return to resting positions.
3. Thoracic volume decreases.
4. Intrapulmonary pressure rises: Exceeds atmospheric pressure (~ +1 to +3 cm H₂O).
5. Air flows out: Down the pressure gradient through airways to atmosphere.

Forced Expiration (Active)

• Internal intercostals and abdominal muscles contract
• Further decreases thoracic volume
• Generates higher positive pressures for rapid exhalation (exercise, coughing)

Key Pressure Relationships

• Atmospheric pressure (Patm): ~760 mmHg at sea level (reference zero)
• Intrapulmonary pressure (Palv): Varies with breathing phase
• Intrapleural pressure (Pip): Always subatmospheric (~ -5 cm H₂O at rest)
• Transpulmonary pressure (Palv - Pip): Keeps lungs inflated

Understanding these mechanics is fundamental to respiratory physiology, mechanical ventilation, and managing respiratory disorders.