Mechanics of Breathing

In this lecture, we'll explore the mechanics of breathing, focusing on lung anatomy, pressures, and the processes involved.

Lung Anatomy

• Lungs: Two lungs (right and left).
• Trachea and Bronchi: Trachea branches into right and left primary bronchus.
• Alveoli: Smallest structural unit of the lung.
• Pleura: Lung covering with layers:
  • Visceral Pleura: Thin epithelial tissue covering the lungs.
  • Pleural Cavity: Potential space with pleural fluid.
  • Parietal Pleura: Layer attached to the chest wall.

Importance of Pleural Fluid

• Prevents friction by allowing visceral and parietal pleura to glide against each other.
• Pleurisy: Condition caused by friction due to lack of pleural fluid.

Pressures in Breathing

• Intrapulmonary Pressure (Pressure A): Also known as intraalveolar pressure, found in alveoli (~760 mmHg).
• Intrapleural Pressure (Pressure B): Found in pleural cavity, always negative (~756 mmHg).
• Atmospheric Pressure (Pressure C): Pressure outside the body (~760 mmHg).

Relationships between Pressures

• Intrapulmonary vs Atmospheric Pressure: Determines gas flow; at rest, equivalent to zero mmHg.
• Intrapleural Pressure: Always lower than intrapulmonary pressure due to:
  • Lung Elasticity: Lungs want to recoil.
  • Surface Tension: Alveoli tendency to collapse.
  • Chest Wall Elasticity: Tendency to expand.

Boyle's Law

• Describes pressure-volume relationship: Increase in thoracic cavity volume leads to decrease in pressure.

Factors Affecting Intrapleural Pressure

• Lymphatic Drainage: Prevents excessive fluid accumulation, maintaining pressure.
• Gravity: Affects pressure distribution, causing non-uniform intrapleural pressure.

Trans-Pressures

• Transpulmonary Pressure: Difference between intrapulmonary and intrapleural pressures; positive value allows lung inflation.
• Transthoracic Pressure: Difference between intrapleural and atmospheric pressures; negative value indicates deflation tendency of chest wall.
• Transrespiratory Pressure: Difference from intrapulmonary to atmospheric pressure; at rest is zero.

Summary

• Pressures and Dynamics: Elasticity and tension in lungs and chest wall are crucial for creating necessary pressure differences for breathing.
• Fluid Mechanics: Proper fluid dynamics in the pleural cavity are essential to prevent friction and maintain pressures.

This lecture sets the groundwork for understanding how neural control affects respiratory mechanics in subsequent discussions (Part 2).