Oxygen Cascade

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  • PO2 of inspired air is 20.93% x (760 - 47) = 149mmHg
  • 760mmHg is barometric pressure, 47 is water vapour of moist inspired gas, 20.93% is the fraction of inspired air which is oxygen

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  • PO2 is ~100mmHg by the time it reaches the alveoli
  • This is because O2 is continually being removed by pulmonary capillary blood
  • Stays within ~3mmHg of this level as the tidal volume replenishment is small compared to the total volume in the lung
  • Rate of removal of O2 constant at resting conditions
  • Level of alveolar ventilation largely determines the alveolar PO2 and PCO2
  • PO2 is much lower in tissue capillaries and mitochondria
  • Variable due to the proximity of tissues to vessels, but PO2 can be as low as 1mmHg
  • 4 causes of hypoxaemia - hypoventilation, diffusion limitation, shunt, ventilation-perfusion inequality