Mechanics of Breathing Lecture

Introduction

• Topic: Mechanics of breathing
• Importance: Understanding pressures in the respiratory system

Lung Anatomy and Pleura

Lung Structure

• Two lungs: right and left
• Trachea branches into primary bronchi, leading to alveoli
• Alveoli: Smallest structural units of the lung

Pleura Layers

1. Visceral Pleura
   • Thin epithelial tissue covering the lung
2. Pleural Cavity
   • Potential space with pleural fluid
   • Allows movement with minimal friction
3. Parietal Pleura
   • Layer attached to chest wall

Function of Pleural Fluid

• Reduces friction between pleural layers
• Prevents pleurisy (inflammation from friction)

Pressures in the Thoracic Cavity

Types of Pressures

• Pressure A (Intrapulmonary/Intraalveolar Pressure)
  • Pressure within alveoli
  • Approximately 760 mmHg
• Pressure B (Intrapleural Pressure)
  • Pressure in pleural cavity
  • Always less than intrapulmonary pressure
  • Approximately 756 mmHg (negative pressure)
• Pressure C (Atmospheric Pressure)
  • External atmospheric pressure
  • Approximately 760 mmHg

Negative Pressure Concept

• Intrapleural pressure is negative compared to intrapulmonary
• Maintained by lung elasticity, surface tension, and chest wall elasticity
• Role of lymphatic vessels in draining excess pleural fluid

Boyle’s Law

• Relationship between pressure and volume
  • Increase in volume decreases pressure (and vice versa)

Trans-Pressures

• Transpulmonary Pressure (TP)
  • Difference between intrapulmonary and intrapleural pressure
  • Positive pressure indicates ability to inflate lungs
• Transthoracic Pressure (TTP)
  • Difference between intrapleural and atmospheric pressure
  • Negative pressure indicates deflating tendency
• Transrespiratory Pressure
  • Difference between intrapulmonary and atmospheric pressure

Factors Affecting Intrapleural Pressure

1. Elasticity of the Lungs
   • Lungs naturally recoil, pulling visceral pleura away
2. Surface Tension
   • Alveoli try to collapse, increasing pleural volume
3. Chest Wall Elasticity
   • Expands chest, pulling parietal pleura away
4. Gravity
   • Affects pleural pressure distribution (not uniform throughout)

Conclusion

• Understanding these pressures is critical for understanding respiratory mechanics
• Prepares for understanding the role of the nervous system in breathing (to be covered in part two)