15A17: Exam Report

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

63% of candidates passed this question.

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.

F7i / 15A17: Describe the O2 cascade in a person breathing room air at sea level



  • O2 cascade = step wise reduction in partial pressure of O2 from inspired air until Oreaches tissues
  • 2 moves passively around body
  • ∴it requires pressure gradients
  • O2 cascade describes sequential falls in partial pressure

Inspired Dry Air

  • PO2 = FiO2 x PATM

        = 0.21 x 760

        = 159mmHg

Inspired Humidified Air

  • Air breathed in is warmed & humidified
  • SVP of H2O at 37°C is 47mmHg
  • ∴PO2 is diluted by H2O vapour
  • PO2 = FiO2 (PATM – PSVP) = 149mmHg

Alveolar Gas

  • Significant ↓Partial P
  • Due to O2 uptake from alveoli & CO2 excretion of ventilation
  • Described by Alveolar Gas Equation

Arterial Blood

  • Small drop ~95mmHg
  • Due to venous admixture
    • Shunt (anatomical & pathological)
    • Low V/Q units
  • This is the A – a gradient
  • Normal A – a gradient roughly estimated; 100 – (age x 1/3)

End Capillary Blood

  • ↓↓↓ a lot, as O2 uptaken into cells


  • 4 – 22mmHg
  • Wide range, depending on O2 consumption
  • Oxidative phosphorylation can continue down to a mitoch O2 tension of 1mmHg (PASTEUR POINT)
  • If falls any further, aerobic → anaerobic metabolism

Definition Pasteur Point = the critical PO­2 for oxidative phosphorylation ~1mmHg