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Breathing Mechanics and Pleural Anatomy

Sep 15, 2025

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Overview

This lecture covers the anatomical structures and pressures involved in the mechanics of breathing, with a focus on pleural layers, pressure relationships, and the factors maintaining negative intrapleural pressure.

Lung and Pleural Anatomy

  • The lungs are divided into right and left, served by the trachea and bronchi, ending at alveoli (gas exchange sites).
  • Lung surface is covered by the visceral pleura, a thin epithelial tissue.
  • The pleural cavity is a potential space with pleural fluid, reducing friction between pleural layers.
  • The parietal pleura lines the chest wall.
  • Pleural fluid prevents inflammation by minimizing friction during breathing.
  • Excess or deficient pleural fluid can cause pleurisy (painful inflammation).

Respiratory Pressures

  • Three main pressures: intraulmonary (intra-alveolar), intrapleural, and atmospheric (barometric).
  • Intraulmonary pressure (P_pul) ≈ 760 mmHg (or 0 mmHg relative to atmosphere).
  • Intrapleural pressure (P_IP) ≈ 756 mmHg (or -4 mmHg relative to atmosphere), always negative.
  • Atmospheric pressure (P_atm) ≈ 760 mmHg at sea level.

Relationships and Concepts

  • Pressures are compared to atmospheric pressure (zero, positive, negative).
  • Intrapleural pressure is negative due to:
    • Lung elasticity: tendency to recoil/snap smaller.
    • Surface tension: alveoli’s tendency to collapse.
    • Chest wall elasticity: tendency to expand outwards.
  • Dynamic interplay increases pleural cavity volume, lowering intrapleural pressure (Boyle’s law: increased volume, decreased pressure).
  • Lymphatic vessels drain excess pleural fluid to maintain pressure.

Transmural Pressures and Formulas

  • Transpulmonary pressure (TP) = intraulmonary pressure – intrapleural pressure (P_pul - P_IP).
    • At rest: TP = 0 - (-4) = +4 mmHg (positive means lungs can inflate).
  • Transthoracic pressure (TTP) = intrapleural pressure – atmospheric pressure (P_IP - P_atm).
    • At rest: TTP = -4 - 0 = -4 mmHg (deflating pressure on chest wall).
  • Transrespiratory pressure (TRP) = intraulmonary pressure – atmospheric pressure (P_pul - P_atm).
    • At rest: TRP = 0 - 0 = 0 mmHg (no air movement at rest).

Gravity and Pleural Pressure Differences

  • Gravity alters intrapleural pressure: lower at apex (top) and higher at base (bottom) of lung.
  • Intrapleural pressure is not uniform throughout pleural cavity.

Key Terms & Definitions

  • Visceral Pleura — inner membrane covering the lungs.
  • Pleural Cavity — potential space with fluid between visceral and parietal pleura.
  • Parietal Pleura — outer membrane lining the chest wall.
  • Intraulmonary Pressure (P_pul) — pressure inside alveoli.
  • Intrapleural Pressure (P_IP) — pressure in pleural cavity.
  • Atmospheric Pressure (P_atm) — pressure outside the body.
  • Transpulmonary Pressure (TP) — difference between intraulmonary and intrapleural pressures.
  • Transthoracic Pressure (TTP) — difference between intrapleural and atmospheric pressures.
  • Transrespiratory Pressure (TRP) — difference between intraulmonary and atmospheric pressures.
  • Pleurisy — inflammation due to friction between pleural layers.

Action Items / Next Steps

  • Study how pressure changes during inspiration by reviewing part two.
  • Review key pressure formulas and practice calculating pressure differences.

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