Gas Exchange

• This diagram represents a normal cluster of alveoli with a normal capillary:
  • Delivering carbon dioxide (CO2) and picking up oxygen (O2)

• Carbon dioxide:
  • Travels dissolved in the blood
• Approaching the alveolus:
  • The CO2:
    • Easily crosses through the blood:
      • Across the capillary wall, and into the alveolus

• Because CO2 crosses so readily into the alveolus from the serum:
  • Ventilation occurs readily:
    • This means that the major determinants of CO2 in the blood are:
      • The rate of production:
        • Increased with:
          • Elevated metabolic demand:
            • Such as in:
              • Sepsis
              • Exercise
      • The rate of elimination:
        • Largely determined by:
          • The minute ventilation:
            • Which is the amount of air moving through the lungs in one minute:
              • Quantified as:
                • The tidal volume:
                  • Green shaded arrows and
                • The respiratory rate:
                  • Represented by the back and forth arrows
            • The higher the minute ventilation:
              • The lower the CO2
            • Normal minute ventilation is about:
              • 6 to 8 L/min
            • In times of stress:
              • With increased CO2 production:
                • The minute ventilation may be:
                  • 10 to 15 L/min

• Conversely:
  • The pathway for oxygen is less simple:
    • Oxygen is transported largely bound to:
      • Hemoglobin inside the red blood cells:
        • The hemoglobin in this schematic:
          • Demonstrates the four binding sites per hemoglobin molecule inside the red blood cells
        • Oxygen is represented by small blue dots
        • The concentration of oxygen is high in the alveoli, and:
          • It diffuses down the concentration gradient:
            • Into the capillary:
              • Into the RBC, and
                • Binds with Hgb

• While this binding allows for great efficiency in carrying oxygen:
  • The multiple steps for oxygen transport:
    • As compared to the simplicity of CO2:
      • Explains some of the differential clinical effects seen with ventilation and oxygenation
• A small amount of oxygen is carried dissolved in the plasma, but compared to the amount bound to hemoglobin, this amount is trivial:
  • The oxygen-carrying capacity of the blood is described by the equation:
    • Delivery of Oxygen = Cardiac Output x (Hgb x 1.39 x Oxygen Saturation) + (PaO2 x 0.003):
      • This equation intuitively makes sense:
        • As the more Hgb available to carry oxygen:
          • The more oxygen that can be delivered

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