diffusion of pulmonary gases

DIFFUSION OF PULMONARY GASES

Ventilation vs. Respiration

• Ventilation: Movement of air into and out of the lungs.
• Respiration: Gas exchange at the alveolar level.

Diffusion

• Definition: Movement from an area of high concentration to an area of low concentration until equilibrium is reached.

Composition of the Atmosphere

• Common gases: Nitrogen (N2), Oxygen (O2), Carbon Dioxide (CO_2), Inert Gases.
• Atmospheric Pressure: N2 + O2 + CO2 = PB

Dalton's Law of Partial Pressures

• Definition: Total pressure (PT) of a gas mixture is the sum of the partial pressures (P1, P2, P3) of individual gases:
  PT = P1 + P2 + P3 + \text{…}

Atmospheric Composition Breakdown

• Atmospheric pressure (P_B) is approximately 760\ \text{mmHg}.
• Partial pressures (approximate):
  • P_{N2} = 593\ \text{mmHg (78.08\%)}
  • P_{O2} = 159\ \text{mmHg (20.95\%)}
  • P_{CO2} = 0.2\ \text{mmHg (0.03\%)}

Impact of Altitude on Atmospheric Pressure

• Altitude changes pressure, but not the percentage of gases.
• As altitude increases, pressure decreases, potentially reducing oxygen tension.

Gas Diffusion Mechanism

• Diffusion gradient: Concentration difference driving movement from high to low concentration (e.g., O2 from alveoli to blood, CO2 from blood to alveoli).

Gaseous Exchange in Capillaries

• Alveolar Partial Pressures:
  • PAO_2 = 100\ \text{torr}
  • PACO_2 = 40\ \text{mmHg}
• Blood Gas Tensions:
  • Nonoxygenated Blood: P{O2} = 40\ \text{torr}, P{CO2} = 46\ \text{mmHg}
  • Reoxygenated Blood: P{O2} = 100\ \text{torr}, P{CO2} = 40\ \text{mmHg}

Alveolar-Capillary Membrane Structure

• Layers include: Alveolar epithelium, basement membranes (alveolar and capillary), fluid layer, capillary endothelium, erythrocyte membrane and intracellular fluid.

Laws of Diffusion

• Fick's Law: V{\text{gas}} = A \times D \times (P1 - P_2) / T
  • A = Surface Area, (P1 - P2) = Pressure gradient, T = Tissue thickness, D = Diffusion Constant.
• Henry's Law: Amount of gas dissolved in a liquid is proportional to its partial pressure.
• Graham's Law: Rate of diffusion is directly proportional to solubility coefficient and inversely proportional to the square root of molecular weight.

Solubility Coefficient

• Values: O2 = 0.0244\ \text{ml/mmHg/ml H2O}; CO2 = 0.592\ \text{ml/mmHg/ml H2O}
• CO2 is ~24 times more soluble than O2.

Diffusion Rates

• CO2 diffuses approximately 20 times faster than O2.

Fick's and A-a Gradient

• A-a Gradient Formula: P{\text{A-a}}O2 = PAO2 - PaO2
• Normal gradient: 5-10\ \text{mmHg}, increases with age.

Ideal Alveolar Gas Equation

• Formula: PAO2 = (PB - PH2O)FIO2 - PaCO2(1.25)

Respiratory Exchange Ratio (RER)

• Approximately 200\ \text{ml/min } CO2 (capillaries to alveoli) vs. 250\ \text{ml/min } O2 (alveoli to capillaries).
• Calculation: RER = \frac{250\ O2}{200\ CO2} = 1.25

Increased A–a Gradient

• Indicates ventilation abnormalities like mucus plugging, airway edema, capillary shunts, or perfusion abnormalities.

Limits to Diffusion

• Perfusion-Limited Gas Flow: Occurs under increased demand (exercise) where transit time in capillaries decreases.
• Diffusion-Limited Gas Flow: Occurs with alveolar thickening (e.g., pneumonia, pulmonary edema) or interstitial lung disease.

Common Diffusion-Limited Problems

• Associated with atelectasis, alveolar fibrotic thickening, emphysema (destruction of alveolar-capillary membranes), and pulmonary edema.

DLCO Testing

• Evaluates alveolar-capillary gas exchange efficiency using carbon monoxide.
• Normal subjects: ~25\ \text{ml/min/mmHg}.