Oxygen Cascade

Describe and explain the oxygen cascade

The oxygen cascade describes the transfer of oxygen from air to mitochondria.

  • In each step of the cascade the PaO2 falls
    It demonstrates that oxygen delivery to tissues relies on the passive transfer of gas down partial pressure gradients.
  • The steps of the cascade are:
    • Dry atmospheric gas
    • Humidified tracheal gas
    • Alveolar gas
    • Arterial blood
    • Mitochondria
    • Venous blood

Remember:

  • Partial pressure determines rate and extent of gas transfer
  • Oxygen content is what is important for cellular function

Atmospheric Gas

Atmospheric partial pressure of oxygen is a function of barometric pressure and the FiO2:

  • , where:
    • is 760mmHg
    • is 0.21
  • Therefore, = 160mmHg

Humidified Tracheal Gas

  • Gas is humidified during inspiration
  • Gas in the proximal trachea is heated to 37°C and has 100% relative humidity
  • The saturated vapour pressure of water at 37°C is 47mmHg
  • Therefore:
    , where:
    • and are as above
    • is 149mmHg

Alveolar Gas

Ideal alveolar PO2 is calculated using the alveolar gas equation:
, where:

  • is the alveolar partial pressure of oxygen
  • is the inspired partial pressure of oxygen
  • is the arterial partial pressure of carbon dioxide
  • is the respiratory quotient, where
    • R is used in the alveolar gas equation to correct for the change in inspired relative to expired volume
      As generally less CO2 is produced than O2 consumed, expired volumes are typically less than inspired volumes
    • R is dependent on the metabolic substrates used for metabolism:
      • Pure fat ≈ 0.7
      • Pure protein ≈ 0.9
      • Pure carbonhydrate ≈ 1
      • The normal value for a Western diet is quoted as 0.8
  • is a correction factor, usually equal to ~2mmHg, and is given by

Alveolar oxgygen is therefore dependent on:

  • PiO2, which is a function of:
    • FiO2
    • Air pressure
  • Alveolar ventilation
    As .

Arterial Blood

The difference in partial pressure of oxygen between alveolar and arterial blood is called the A-a gradient:

  • A normal A-a gradient is
  • Normal arterial PO2 is 100mmHg
  • It occurs due to:
    • Shunt/VQ scatter
      A small shunt is normal due to blood from the bronchial circulation and thebesian veins.
    • Diffusion abnormality

Mitochondria

  • PO2 varies with metabolic activity, but typically quoted as 5mmHg
  • The Pasteur point is the partial pressure of oxygen at which oxidative phosphorylation ceases, and is ~1mmHg

Venous Blood

  • PO2 is greater than mitochondrial PO2
    Mixed venous blood typically quoted as 40mmHg.
  • Higher than mitochondria as not all arterial blood travels through capillary beds

References

  1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
  2. Brandis K. The Physiology Viva: Questions & Answers. 2003.
Last updated 2017-10-03

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