Mechanics of Breathing

Introduction

• Topic: Mechanics of Breathing
• Main focus: Understanding lung pressures and anatomy.
• Importance: Complex topic for some due to pressures.

Lung Anatomy

• Lungs: Two (Right and Left lung).
• Trachea: Branches into right and left primary bronchus.
• Alveoli: Smallest structural unit of lungs.
• Layers of the Lung:
  • Visceral Pleura (#1): Thin epithelial tissue layer.
  • Pleural Cavity (#2): Potential space with fluid (pleural fluid) to prevent friction.
  • Parietal Pleura (#3): Layer clinging to the chest wall.
• Pleural Fluid: Serous fluid to reduce friction between visceral and parietal pleura.
• Condition: Pleurisy from too much friction due to insufficient pleural fluid.

Pressures in the Lung

• Three Main Pressures:
  • A (Intrapulmonary/Intra-alveolar Pressure): Pressure in alveoli (approx. 760 mmHg).
  • B (Intrapleural Pressure): Pressure in the pleural cavity (approx. 756 mmHg, always negative).
  • C (Atmospheric Pressure): Barometric pressure at sea level (approx. 760 mmHg).

Relationship Between Pressures

• Intrapulmonary Pressure (P_pul): 760 mmHg ≈ 0 mmHg (relative to atmospheric).
• Intrapleural Pressure (P_ip): 756 mmHg ≈ -4 mmHg (relative to atmospheric).
• Boyle's Law: P1V1 = P2V2
  • Increase in volume → Decrease in pressure.
  • Decrease in volume → Increase in pressure.
• Negative Intrapleural Pressure:
  • Reasons:
    • Elasticity of the lungs.
    • Surface tension in alveoli.
    • Elasticity of the chest wall.
  • Effect: Prevents lung collapse and helps in breathing.

Pressure Differences Across Lung Structures

• Transpulmonary Pressure (TP): Difference between intrapulmonary and intrapleural pressures (P_pul - P_ip).
  • Example: 0 mmHg - (-4 mmHg) = +4 mmHg (inflation pressure).
• Transthoracic Pressure (TTP): Difference between intrapleural and atmospheric pressures (P_ip - P_atm).
  • Example: -4 mmHg - 0 mmHg = -4 mmHg (deflating pressure).
• Transrespiratory Pressure (TRP): Difference between intrapulmonary and atmospheric pressures (P_pul - P_atm).
  • Example: 0 mmHg - 0 mmHg = 0 mmHg (no gas flow at rest).

Summary of Intrathoracic Dynamics

• Intrapulmonary pressure = intrapleural pressure + 4 mmHg.
• Elastic forces: Lungs and chest wall create dynamic interplay creating lower intrapleural pressure.
• Lymphatic System: Drains excess pleural fluid.
• Gravity's Role: Affects pressure distribution within pleural cavity (Pressure varies from base to apex).

Conclusion

• Understanding the complex interplay of pressures is crucial for understanding the mechanics of breathing.
• Next step: How the nervous system influences these pressures during the respiratory cycle.