Egan+CH+12+Gas+Exchange+NEW

Oxygen Transport and Gas Exchange Fundamentals

• Definition of Respiration: The process entails moving oxygen ($O_2$) to tissues for aerobic metabolism and the subsequent removal of carbon dioxide ($CO_2$) from the body.

  • Lungs: Gas exchange occurs here to bring $O_2$ from the atmosphere into the bloodstream.
  • Tissues: Gas exchange occurs here to deliver $O_2$ for metabolism and collect $CO_2$ for transport back to the atmosphere.

• The Pressure Cascade:

  • Oxygen Movement: Shows a downward "cascade" of partial pressures starting from the atmospheric level down to the cellular level.
  • Carbon Dioxide Movement: Moves in the opposite direction; pressures are highest in the cells and cascade downward toward the atmosphere.

• Alveolar-Capillary Diffusion Barriers:

  • The Alveolar-Capillary (A/C) membrane consists of three main barriers:
    • Alveolar epithelium.
    • Interstitial space and its associated structures.
    • Capillary endothelium.
  • The Red Blood Cell (RBC) membrane also acts as a final barrier to gas entry into hemoglobin.

Clinical Conditions Impeding Gas Diffusion

• Diffusion-Limited Problems: Various clinical states decrease the rate of gas diffusion by altering the membrane or volume:
  • Alveolar-Capillary Destruction: Example includes Emphysema.
  • Alveolar Collapse: Known as Atelectasis.
  • Interstitial Edema: Fluid accumulation in the space between the alveolus and capillary.
  • Thickening of Alveolar Wall: Example includes Alveolar Fibrosis.
  • Alveolar Consolidation: Example includes Pneumonia.
  • Frothy Secretions: Typical of Pulmonary Edema.

Determinants of Alveolar Gas Tensions

• Alveolar Carbon Dioxide ($PACO_2$):

  • Direct Relationship: $PACO_2$ varies directly with the body’s production of carbon dioxide ($VCO_2$).
  • Inverse Relationship: $PACO_2$ varies inversely with alveolar ventilation ($V_A$).
  • Normal Range: Maintained at approximately $35\text{ to }45\,mmHg$.
  • Hypercapnia Causes: $PACO_2$ increases if $CO_2$ production rises while ventilation remains constant, or if there is an increase in dead space (gas not participating in exchange).
  • Hypocapnia Causes: $PACO_2$ decreases if $CO_2$ production falls or alveolar ventilation increases.
  • Compensation: During exercise or fever ($\uparrow CO_2$ production), ventilation automatically increases to keep $PACO_2$ within normal limits.

• Alveolar Air Equation and Oxygen Tension ($PAO_2$):

  • As air enters the lungs, it is diluted by water vapor and $CO_2$.
  • Water Vapor Pressure: Gas fully saturated at BTPS (Body Temperature, Pressure, Saturated) is $47\,mmHg$.
  • Dalton’s Law: The sum of all partial pressures must equal the Total Barometric Pressure ($P_B$). Since $CO_2$ and water vapor are present, the $PO_2$ in the alveoli is lower than in the atmosphere.
  • Partial Pressure of Nitrogen ($PAN_2$): Nitrogen is inert but occupies space.
    • $PAN_2 = P_B - (PAO_2 + PACO_2 + PH_2O)$
  • Inverse Oxygen-Carbon Dioxide Relationship: At a constant $FIO_2$, the $PAO_2$ varies inversely with $PACO_2$.

• Calculation Practice Data:

  • Scenario 1: $P_B = 760\,mmHg$, $PH_2O = 47\,mmHg$, $FIO_2 = 70\%$, $PaCO_2 = 50\,mmHg$.
  • Scenario 2: $P_B = 750\,mmHg$, $PH_2O = 47\,mmHg$, $FIO_2 = 45\%$, $PaCO_2 = 47\,mmHg$.
  • Scenario 3: $P_B = 750\,mmHg$, $PH_2O = 47\,mmHg$, $FIO_2 = 30\%$, $PaCO_2 = 55\,mmHg$.

Mechanisms and Time Limits of Diffusion

• Pressure Gradients: Diffusion occurs primarily along pressure gradients. In healthy individuals, the area and distance of the A/C membrane are constant, so diffusion depends mostly on these gradients.

• Fick’s Law: The volume of gas diffusion is greater when there is a larger surface area, a higher diffusion constant, and a steeper pressure gradient.

• Time Limits and Transit Time:

  • Normal Resting Transit: Pulmonary blood is exposed to alveolar gas for approximately $0.75\text{ seconds}$.
  • Equilibration Time: Oxygen equilibration normally occurs within the first $0.25\text{ seconds}$.
  • Exercise/Stress: Total transit time decreases. If it falls below $0.25\text{ seconds}$, or if a diffusion limitation exists (e.g., Alveolar Thickening), equilibration may be inadequate.

• Normal Gradients at the A/C Interface:

  • Alveolus: $O_2 ≈ 100\,mmHg$, $CO_2 ≈ 40\,mmHg$.
  • Venous Capillary: $O_2 ≈ 40\,mmHg$, $CO_2 ≈ 46\,mmHg$.
  • Arterial Capillary (Post-Equilibration): $O_2 ≈ 100\,mmHg$, $CO_2 ≈ 40\,mmHg$.

Shunting and the Alveolar-Arterial Gradient

• The A-a Gradient ($P(A-a)O_2$): This measures the difference between Alveolar and Arterial $PO_2$ and indicates the efficiency of gas exchange.

  • Normal Room Air ($21\% FIO_2$): $5\text{ to }10\,mmHg$.
  • Normal on $100\% FIO_2$: $25\text{ to }65\,mmHg$.
  • $V/Q$ Mismatch: $66\text{ to }300\,mmHg$.
  • Shunt: $> 300\,mmHg$.

• Anatomical Shunts (Right-to-Left): Even in normal lungs, $PaO_2$ is $5\text{ to }10\,mmHg$ lower than $PAO_2$ because of deoxygenated blood bypassing the lungs:

  • Thebesian Venous Drainage: Vessels of the left ventricular myocardium drain directly into the left ventricle.
  • Bronchial Venous Drainage: Some bronchial and mediastinal veins drain into pulmonary veins, diluting arterial oxygenation.

Ventilation/Perfusion ($V/Q$) Relationships

• Ideal Ratio: A ratio of $1$ where ventilation ($V$) and perfusion ($Q$) are in perfect balance.
• Dead Space ($V/Q > 1$): Ventilation exists without blood flow.
  • Alveolar Deadspace: Caused by Pulmonary Embolism (PE), partial vascular obstruction, destroyed vasculature (COPD), or reduced cardiac output.
  • Anatomic Deadspace: The portion of the Tidal Volume ($V_T$) that does not reach the alveoli.
• Shunt ($V/Q < 1$): Perfusion exists without ventilation.
  • Pneumonia Example: Ventilation decreases in the affected lobe. Perfusion initially stays the same ($V/Q < 1$), but hypoxic vasoconstriction eventually occurs, decreasing perfusion as well.
• Positional Effects: In an upright person, $V/Q$ is increased at the top of the lung (apex) due to gravity decreasing pulmonary blood flow relative to ventilation.

Oxygen Transport in Blood

• **Phys