Question 17

Describe the oxygen cascade in a person breathing room air at sea level.

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College Answer

This question could be answered with a description or a diagram but required an ordered 
journey from the atmosphere to the mitochondria. This is commonly available in many texts 
and most candidates answered the question well. Most candidates said the alveolar PO2 fell 
solely because CO2 came out of the pulmonary capillary. Very few talked about oxygen 
uptake into the capillary. Another common omission was failure to state normal values for the 
A-a gradient.


The main points in the oxygen cascade:

  • Atmospheric air:
    • 21% oxygen = PaOof 159 mmHg
  • Airway gas mixture:
    • Diluted by water vapour = PaOof 149 mmHg
  • Alveolar gas mixture:
    • Diluted by CO2 = PaOof 99 mmHg
    • Also, some oxygen is taken up by the capillaries,
      which decreases the alveolar PaO2
      Alveolar PO2 is calculated by the alveolar gas equation:
      PAO2 = (FiO2 × (Patm - PH2O)) - (PaCO2 × RQ)
  • Endcapillary blood
    • Essentially the same as alveolar gas, in health
  • Arterial blood
    • Diluted by venous admixture= PaOof 92 mmHg
    • The difference between alveolar and arterial gas is the A-a gradient
    • Normal A-a gradient is 7mmHg in the young, and 14mmHg in the old 
  • Tissue oxygen tension
    • Drops due to diffusion distance
    • Varies from tissue to tissue, but is usually around 10-30 mmHg
  • Mitochondrial oxygen tension
    • Drops due to diffusion distance
    • Usually between 1-10 mmHg

In diagram form:

the oxygen cascade


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