Bite Size Med: Compliance of Lungs and Chest Wall

Introduction

• Focus on understanding the compliance of lungs and chest wall.
• Compliance: Change in volume over the change in pressure.
• Aim to simplify the concept for easier understanding.

Breathing Cycle

• Inspiration & Expiration:
  • Air enters lungs during inspiration; exits during expiration.
  • Volume and pressure changes occur during this cycle.

Key Volume and Pressure Terms

• Tidal Volume:

  • Volume of air breathed in/out during quiet respiration (~500 ml).

• Pressure Gradient:

  • Air moves from high to low pressure.
  • Key pressures: Atmospheric (0), Alveolar (0 at rest), Pleural (-5 cm H2O at rest).

Pressure Changes During Breathing

• Inspiration:

  • Diaphragm contracts, lungs expand.
  • Alveolar pressure: 0 to -1.
  • Pleural pressure: -5 to -7.5.

• Expiration:

  • Lungs recoil passively.
  • Alveolar pressure: 0 to +1.
  • Pleural pressure returns to -5.

Transpulmonary Pressure

• Difference between alveolar and pleural pressure.
• Changes during cycle:
  • Inspiration: +5 to +7.5.
  • Expiration: Back to +5.

Compliance

• High Compliance:

  • Large volume change with small pressure change.
  • Lungs stretch easier.

• Elastic Recoil:

  • Lungs return to shape after stretching (like a rubber band).
  • Influenced by elastin fibers in lung tissue.

Surface Tension & Surfactant

• Surface Tension:

  • Alveoli lined with fluid, causing surface tension.
  • Tends to collapse alveoli.

• Surfactant:

  • Reduces surface tension, increasing compliance.

Compliance Diagram & Hysteresis

• Compliance Diagram:

  • Pressure vs. volume plot; slope indicates compliance.

• Hysteresis:

  • Different paths for inspiration and expiration.
  • Due to surface tension overcoming during inspiration.

Chest Wall and Lung Compliance

• Chest Wall Compliance:

  • Chest wall stretches; combined lower compliance with lungs.

• Functional Residual Capacity (FRC):

  • At rest, lung volume at FRC.
  • Lungs naturally tend to collapse; chest wall tends to expand.

• Balance of Forces:

  • Lungs’ inward pull vs. chest wall’s outward pull.
  • Determines whether the system tends to collapse or expand.

Conclusion

• Understanding compliance helps grasp lung and chest wall mechanics.
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