Question 17

Last updated 02/01/2020 - 22:56
 Topic: Respiratory system
Pass rate
63%
Other SAQs in this paper:
Other SAQs in this topic:

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.

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

References

Biro, George P. "From the atmosphere to the mitochondrion: the oxygen cascade." Hemoglobin-Based Oxygen Carriers as Red Cell Substitutes and Oxygen Therapeutics. Springer, Berlin, Heidelberg, 2013. 27-53.

Luft, U. C. "Aviation physiology-the effects of altitude." Handbook of Physiology 2 (1965): 1099-1145.

Ortiz-Prado, Esteban, et al. "Partial pressure of oxygen in the human body: a general review." American journal of blood research 9.1 (2019): 1.

Plotnikow, Gustavo Adrián, et al. "Humidification and heating of inhaled gas in patients with artificial airway. A narrative review." Revista Brasileira de terapia intensiva 30.1 (2018): 86-97.

Cohen, R. O. B. E. R. T., EDWARD M. Overfield, and JOHANNES A. Kylstra. "Diffusion component of alveolar-arterial oxygen pressure difference in man." Journal of applied physiology 31.2 (1971): 223-226.

Carreau, Aude, et al. "Why is the partial oxygen pressure of human tissues a crucial parameter? Small molecules and hypoxia." Journal of cellular and molecular medicine 15.6 (2011): 1239-1253.

Vanderkooi, Jane M., Wayne W. Wright, and Maria Erecinska. "Oxygen gradients in mitochondria examined with delayed luminescence from excited-state triplet probes." Biochemistry 29.22 (1990): 5332-5338.

Dmitriev, Ruslan I., and Dmitri B. Papkovsky. "Optical probes and techniques for O 2 measurement in live cells and tissue." Cellular and molecular life sciences 69.12 (2012): 2025-2039.

Ortiz-Prado, Esteban, et al. "Partial pressure of oxygen in the human body: a general review." American journal of blood research 9.1 (2019): 1.

Engelhardt, W. A. "On the dual role of respiration." Molecular and cellular biochemistry 5.1 (1974): 25-33.

Rhoads, Donald C., and John W. Morse. "Evolutionary and ecologic significance of oxygen‐deficient marine basins." Lethaia 4.4 (1971): 413-428.

Wilson, David F., et al. "The oxygen dependence of mitochondrial oxidative phosphorylation measured by a new optical method for measuring oxygen concentration." Journal of Biological Chemistry 263.6 (1988): 2712-2718.

Sharma, Sandeep, Bracken Burns., and William Gossman. "Alveolar Gas Equation." StatPearls (2019)

Helmholz, H. Frederic. "The abbreviated alveolar air equation." Chest 75.6 (1979): 748.

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