Describe the oxygen cascade, from the atmosphere to the mitochondrion, in a patient breathing room air.

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College Answer

This topic is a core aspect of respiratory physiology. The vast majority of the candidates 
could draw the oxygen cascade, but were let down by not having sufficient breadth and/or 
depth of information (e.g. alveolar gas equation was either omitted, inaccurate or poorly 
described in relation to the oxygen cascade) to describe the physiological principals at each 
step of the cascade

Discussion

The main points in the oxygen cascade:

  • Atmospheric air:
    • 21% oxygen = PaO2 of 159 mmHg
  • Airway gas mixture:
    • Diluted by water vapour = PaO2 of 149 mmHg
  • Alveolar gas mixture:
    • Diluted by CO2 = PaO2 of 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= PaO2 of 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