Overview
This lecture explains the mechanics of breathing, focusing on lung and chest wall anatomy, the pleural membranes, and the key pressures involved in ventilation.
Lung Anatomy & Pleural Membranes
- The lungs are formed by multiple alveoli, the smallest structural unit for gas exchange.
- The visceral pleura is a thin epithelial layer covering the lung's surface.
- The parietal pleura lines the inner chest wall.
- The pleural cavity is a potential space between these membranes containing pleural fluid, which reduces friction during breathing.
- Pleural fluid is drained by lymphatic vessels to prevent fluid accumulation.
Key Pressures in Breathing
- Three main pressures: intraulmonary (intra-alveolar), intrapleural, and atmospheric.
- Intrapulmonary pressure (P_pul) ≈ 760 mm Hg (or 0 relative to atmosphere).
- Intrapleural pressure (P_ip) ≈ 756 mm Hg (or -4 mm Hg relative to atmosphere); always less than intra-alveolar.
- Atmospheric pressure (P_atm) ≈ 760 mm Hg at sea level.
Pressure Comparisons & Significance
- Pressures are compared to atmospheric pressure; "zero" means equal to atmosphere, "negative" means below atmospheric.
- Intrapleural pressure is negative due to: lung elasticity (recoil), surface tension in alveoli, and chest wall elasticity.
- Increasing thoracic cavity volume (via chest wall expansion) decreases intrapleural pressure (Boyle's Law: ↑volume = ↓pressure).
Special Pressure Differences Across Structures
- Transpulmonary pressure (TP): difference between intra-alveolar and intrapleural pressures; keeps lungs inflated (TP = P_pul - P_ip = +4 mm Hg at rest).
- Transthoracic pressure (TTP): difference between intrapleural and atmospheric pressure (TTP = P_ip - P_atm = -4 mm Hg at rest); indicates the chest wall's tendency to recoil inward.
- Transrespiratory pressure: difference between alveolar and atmospheric pressure (TRP = P_pul - P_atm = 0 mm Hg at rest).
Other Considerations
- Gravity causes regional differences in intrapleural pressure (more negative at apex, less at base).
- Proper pleural fluid balance is essential to maintain pressure dynamics and avoid lung collapse (atelectasis) or pleurisy.
Key Terms & Definitions
- Visceral pleura — membrane covering the lung surface.
- Parietal pleura — membrane lining the chest wall.
- Pleural cavity — potential space with lubricating pleural fluid.
- Intrapulmonary (alveolar) pressure — air pressure within alveoli.
- Intrapleural pressure — pressure within the pleural cavity.
- Atmospheric pressure — external air pressure.
- Transpulmonary pressure (TP) — P_pul minus P_ip; keeps lungs inflated.
- Transthoracic pressure (TTP) — P_ip minus P_atm; pressure across chest wall.
- Transrespiratory pressure — P_pul minus P_atm; pressure difference driving airflow.
- Boyle's Law — pressure and volume are inversely related in a closed system.
Action Items / Next Steps
- Review the roles of the nervous system in breathing mechanics in Part 2.
- Ensure you understand each key pressure and their relationships before moving on.