Gas diffusion, gas transport and haemoglobin

Gas Diffusion and Gas Transport

Overview of Gas Exchange Mechanisms

  • Principles of Gas Diffusion:

    • Vgas = gas diffusion

    • A = Area

    • T = Thickness

    • D = diffusion constant

    • P1 - P2 = partial pressure difference

    • Fick’s Law of Gas Diffusion is fundamental for understanding the mechanisms involved in gas exchange, particularly in the pulmonary system.

Structure of the Pulmonary System

  • Key Components:

    • Alveolus: Primary site of gas exchange in the lungs.

    • Terminal Bronchiole: Conducting air passage leading to alveoli.

    • Respiratory Bronchiole: Connects to alveoli, site of some gas exchange.

    • Branch of Pulmonary Vein and Artery: Vessels carrying blood to and from the lungs respectively.

    • Smooth Muscle: Present in bronchi and bronchioles regulating airway diameter.

Diffusion of Gases through Alveolar-Capillary Barrier

  • Composition of Alveolar-Capillary Barrier:

    • Alveolar Epithelium: Thin layer for gas exchange.

    • Capillary Endothelium: Lines blood vessels and plays a role in gas diffusion.

    • Basement Membranes: Thin layers that support the epithelium and endothelium.

    • Interstitial Space: Space between the alveoli and capillaries through which gases diffuse.

    • Fluid and Surfactant Layer: Reduces surface tension in alveoli aiding lung expansion.

Fick’s Law of Diffusion

  • Equation:

    • V_{gas} = D imes A imes \frac{(P1 - P2)}{T}

    • Where:

    • D = diffusion constant

    • A = surface area available for diffusion

    • P1 - P2 = partial pressure difference across the barrier

    • T = thickness of the barrier

Gas Solubility and Diffusion Rates

  • Diffusion Characteristics:

    • The diffusion of gas is influenced by:

    • Molecular Weight (MW): For example,

      • CO2 is 20 times more diffusible than O2

      • Solubility Values:

      • CO2 solubility = 0.7 ml/L blood/mmHg

      • O2 solubility = 0.03 ml/L blood/mmHg

      • Molecular Weights: CO2 = 44, O2 = 32

Gas and Blood Interaction

  • Key Measurements:

    • At rest:

    • DL(O_2) = 31 ml/min/mm Hg

    • Efficiency of O_2 diffusion across the alveolar-capillary barrier.

    • During exercise, DL increases due to an increase in area available for diffusion, whilst it can decrease in conditions like interstitial pulmonary fibrosis due to thickening of the barrier.

Partial Pressures and Gas Concentrations

  • Definition of Partial Pressure:

    • The pressure exerted by one gas in a mixture indicates its concentration.

    • Example with atmospheric pressure:

    • If P_{total} = 1000 mm Hg

    • P_{O2} = 1000 imes 0.21 = 210 ext{ mm Hg}

    • P_{N2} = 1000 imes 0.79 = 790 ext{ mm Hg}

    • For normal atmospheric pressure (760 mm Hg):

    • P_{O2} ext{(approx)} = 760 imes 0.21 ext{ mm Hg} ≈ 160 ext{ mm Hg}

Blood Flow and Ventilation Influences on Gas Exchange

  • At Rest vs. During Exercise:

    • At rest, venous blood P_{O2} = 100 ext{ mm Hg} leading to efficient gas exchange.

    • Exercise increases blood flow leading to a faster perfusion rate, which can result in incomplete equilibration of P_{O2} along capillaries.

    • The diagram illustrates the equilibration of alveolar P{O2} and blood P{O2} levels.

Oxygen Transportation in Blood

  • Components of Oxygen Content in Blood:

    • 1 liter of arterial blood consists of:

    • 3 ml O2 physically dissolved (1.5%)

    • 197 ml O2 bound to hemoglobin (98.5%)

    • Total of 200 ml O2 per liter.

  • Oxygen Delivery Rate:

    • Cardiac output is typically 5 L/min.

    • Oxygen carried to tissue/min = 5 L/min imes 200 ext{ ml O2/L} = 1000 ext{ ml O2/min}

Hemoglobin and Oxygen Binding Dynamics

  • Hemoglobin Formula:

    • Hb + O2 ightarrow HbO2

    • Each hemoglobin subunit binds to one O_2 molecule; this binding enhances the binding capabilities of other subunits due to the allosteric effect.

Oxygen Dissociation Curve

  • Characteristics of the Curve:

    • Sigmoidal shape:

    • Reflects the cooperative binding of O_2 to hemoglobin.

    • Indicates a safety mechanism for adequate oxygen supply even at varied P_{O2} levels.

    • Plateau Effect:

    • At high P{O2}, a slight change results in minimal effect on O2 binding.

    • Provides a buffer against fluctuations in oxygen supply.

    • At tissue capillaries:

    • Generally 75% saturated, with the capacity to release more O_2 when required.

Factors Shifting the Oxygen Dissociation Curve

  • Rightward Shift Causes:

    • Increase in H+ concentration (Bohr Effect), causing decreased O_2 affinity.

    • Carbamino Effect: Increased CO2 levels binding to hemoglobin, further reducing affinity for O_2.

  • Example Shifts:

    • Increased levels of 2,3-DPG (a byproduct of anaerobic metabolism) resulting in decreased affinity.

Clinical Implications

  • Anemia: Reduced hemoglobin concentration decreases oxygen carrying capacity.

  • CO Poisoning: Carbon monoxide has a higher affinity for hemoglobin, limiting oxygen binding and shifting the dissociation curve to the left, complicating oxygen unloading.

  • Arterial Oxygen Content Calculation:

    • CaO_2 = (SaO2 imes Hb imes 1.34) + 0.003(PaO2)

  • Components of Formula:

    • SaO2 = % saturation of hemoglobin in arterial blood

    • Hb = hemoglobin concentration (normal ~15 g/100 ml blood)

    • 1.34 ml O2/g = oxygen capacity of hemoglobin

    • 0.003 ml O2/100 ml blood = solubility of oxygen in plasma.

Summary of Carbon Dioxide Transport

  • CO2 Transport Modes:

    • 10% physically dissolved in blood

    • 30% binds to hemoglobin as carbamino compounds

    • 60% in the form of bicarbonate in plasma

  • Formation of Carbonic Acid:

    • The reaction CO2 + H2O <=> H2CO3 <=> HCO3^- + H^+ is catalyzed by carbonic anhydrase located inside red blood cells.

Conclusion

  • Understanding the dynamics of gas exchange, transport mechanisms, and physiological factors is crucial for comprehending respiratory health and its implications for conditions such as anemia and hypoxia.