Mechanics of Breathing Lecture

• Introduction
  • Mechanics of breathing is a complex topic, especially regarding pressures.
  • Importance of understanding anatomical structures and pressures in the thoracic cavity.

Anatomy Overview

• Lungs and Trachea
  • Two lungs: right and left lung.
  • Trachea branches into right and left primary bronchi, leading to alveoli.
• Pleura and Pleural Cavity
  • Visceral Pleura: Thin epithelial tissue layer clinging to the lung.
  • Pleural Cavity: Potential space with pleural fluid to prevent friction.
  • Parietal Pleura: Layer clinging to the chest wall.

Importance of Pleural Fluid

• Provides lubrication between pleura layers to prevent friction and inflammation.
• Conditions like pleurisy can arise when pleural fluid is inadequate.

Pressures in Breathing

• Types of Pressures: Intrapulmonary, Intrapleural, and Atmospheric.
• Intrapulmonary Pressure (Intra-Alveolar Pressure)
  • Pressure within alveoli, approximately 760 mmHg.
• Intrapleural Pressure
  • Always negative, approximately 756 mmHg.
  • Lower than intrapulmonary pressure by about 4 mmHg.
• Atmospheric Pressure
  • At sea level, approximately 760 mmHg.

Pressure Relationships

• Intrapulmonary vs. Atmospheric
  • Intrapulmonary pressure can be seen as 0 mmHg when equal to atmospheric.
• Intrapleural vs. Atmospheric
  • Negative pressure when atmospheric pressure is subtracted.
• Boyle’s Law
  • Pressure inversely related to volume.
  • Increase in thoracic cavity volume decreases pressure, creating a negative intrapleural pressure.

Factors Affecting Intrapleural Pressure

• Elasticity of Lungs
  • Lungs naturally recoil, pulling visceral pleura away from parietal pleura.
• Surface Tension
  • At the alveolar level, contributing to lung recoil.
• Elasticity of the Chest Wall
  • Encourages expansion, pulling parietal pleura away from visceral pleura.
• Lymphatic System
  • Drains pleural fluid, maintaining appropriate fluid levels.

Variability of Intrapleural Pressure

• Intrapleural pressure is not uniform due to gravity’s effect on lungs.
• Pressure differences from the base to the apex of the lungs.

Transmural Pressures

• Transpulmonary Pressure
  • Difference between intrapulmonary and intrapleural pressures.
  • Positive value indicates lung inflation.
• Transthoracic Pressure
  • Difference between intrapleural and atmospheric pressure.
  • Negative value indicates a natural tendency for chest wall deflation.
• Transrespiratory Pressure
  • Difference between intrapulmonary and atmospheric pressure.
  • Indicates no net airflow when at rest.

Key Concepts

• Elasticity and surface tension promote lung collapse while the chest wall promotes expansion.
• Pleural fluid and lymphatic drainage maintain negative intrapleural pressure.

Conclusion

• Understanding pressures and anatomical interplay is crucial for grasping breathing mechanics.

Stay tuned for Part 2 which will cover how the nervous system affects respiratory mechanics.