Gas Exchange

  • This diagram represents a normal cluster of alveoli with a normal capillary:
    • Delivering carbon dioxide (CO2) and picking up oxygen (O2)
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
CO2 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
lungs with arrows depicting minute ventilation and respiratory rate
  • 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
 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

#Arrangoiz #Surgeon #Teacher

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