Overview
This lecture covers the mechanisms and factors influencing diffusion and perfusion in respiration, focusing on oxygen and carbon dioxide transport, and distinguishes between external and internal respiration.
Structure of the Respiratory System and Transport Mechanisms
- The respiratory tree consists of conducting airways (trachea, bronchi, terminal bronchioles) and respiratory (acinus) airways (respiratory bronchioles, alveolar ducts, alveolar sacs).
- Two transport mechanisms: bulk flow (movement of air in conducting airways) and diffusion (movement of gases in respiratory airways).
Gas Diffusion Principles
- Diffusion is the movement of oxygen and carbon dioxide down their partial pressure gradients.
- Fick's Law of Diffusion: Rate of diffusion ∝ (surface area × pressure gradient × solubility) / (thickness × √molecular weight).
- Factors increasing diffusion (numerator) enhance rate; factors in denominator decrease rate if increased.
Factors Affecting Gas Exchange
- Alveolar-capillary barrier thickness and surface area affect diffusion; thinner and larger area improve exchange.
- Oxygen partial pressure gradient across alveoli and blood: ~60 mmHg; for CO₂: ~6 mmHg.
- CO₂ diffuses 20 times faster than O₂ due to higher solubility.
- Exercise increases diffusion capacity by recruiting more capillaries and increasing surface area.
- Conditions like fibrosis or edema increase barrier thickness, reducing diffusion.
Respiratory Cycle Effects
- During inspiration: alveolar surface area increases, barrier thickness decreases, alveolar O₂ tension rises, CO₂ tension falls.
- During expiration: surface area decreases, thickness increases, O₂ tension falls, CO₂ tension rises.
External vs Internal Respiration
- External respiration: exchange between alveolar air and blood in lungs, O₂ moves into blood, CO₂ moves into alveoli.
- Internal respiration: exchange between systemic blood and tissues, O₂ moves into tissues, CO₂ moves into blood.
- Equilibration time for O₂ and CO₂ is 0.25 seconds; blood transit time through pulmonary capillary is 0.75 seconds.
Hypoxia and Pathological Responses
- Hypoxia: insufficient tissue oxygen; types include hypoxic, anemic, ischemic, and histotoxic.
- Hypoxemia: low arterial O₂ tension, often due to ventilation-perfusion mismatch (V/Q < 1).
- Anatomic and physiologic shunts decrease arterial O₂; only physiologic shunt may improve slightly with 100% O₂.
- Low alveolar O₂ causes vasoconstriction; high O₂ causes vasodilation in pulmonary vessels.
- High alveolar CO₂ dilates bronchioles; low CO₂ constricts bronchioles.
Key Terms & Definitions
- Diffusion — movement of gases from high to low partial pressure across a barrier.
- Fick's Law — equation describing factors affecting gas diffusion rate.
- External Respiration — gas exchange between lungs and blood.
- Internal Respiration — gas exchange between blood and tissues.
- Hypoxia — reduced oxygen available to tissues.
- Hypoxemia — reduced oxygen in arterial blood.
- Shunt — passage of blood bypassing ventilated alveoli, causing decreased oxygenation.
Action Items / Next Steps
- Review Guyton and Hall Chapter 39 and relevant chapters in BRS, West Respiratory Physiology, and videos in Osmosis.
- Study the table differentiating internal and external respiration and memorise key partial pressures.
- Understand causes and consequences of hypoxia and effects of V/Q mismatch.