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Here is the issue of blood gases partial pressure.

On medical reports is common to see partial pressure of gas on blood, usually oxygen, but there is no gas in equilibrium with blood, so it is confusing. What they mean with partial pressure of a gas on blood is, according to this answer:

the partial pressure which would prevail in a gas phase in equilibrium with the liquid [blood] at the same temperature.

I suppose they use a partially filled vessel with blood and a manometer or something like that, but I would like to know the way they measure it.

Question

How do they measure $p(\mathrm{O}_2)$?

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In principle you could measure it by getting a sample of blood and putting it in contact with gaseous O2 at various pressures. Whatever external pressure of O2 results in the amount of O2 dissolved in the blood neither increasing nor decreasing would equal p(O2) in the blood.

As for how it's actually measured, I believe the usual answer these days is spectroscopically. That is, you shine light through a sample of blood and look for the absorption at certain frequencies which are typical of hemoglobin with O2 bound to it. The amount of absorption is proportional to the concentration of oxygenated hemoglobin, and your software has some internal chart that converts that to partial pressure.

As for why it's measured that way, in part it's probably just tradition, and medicine changes very slowly and conservatively, for obvious reasons (you're experimenting with health and life). In part, though, it may be that it directly relates to something important: the partial pressure of O2 in the atmosphere, which is about 160 mmHg. That's the concentration of O2 in the lungs. What fraction of that reaches the blood at various points in the circulation is a natural indicator of how well oxygen is being taken up, transported, and delivered. That is, you can directly relate this particular measure of O2 concentration to the form of concentration usually used to describe a mixture of gases, in this case the air you breathe, which is the origin and destination of the blood gases.

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  • $\begingroup$ I believe you and nice reasoning, but any resources about what you say on second paragraph? $\endgroup$ – user43021 Jun 8 '18 at 18:33
  • $\begingroup$ BTW because of a higher concentratiom of water vapor on lungs oxygen partial pressure is 150 approx... $\endgroup$ – user43021 Jun 8 '18 at 18:36
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    $\begingroup$ For the casual setting, like the ER, I believe you use one of those pulse oximeters. Wikipedia has an article on those. I looked around for what's done for serious blood gas measurement, and this article: ncbi.nlm.nih.gov/books/NBK371 suggests it's actually done with electrochemistry, by reducing the dissolved O2 and measuring the current required. $\endgroup$ – Christopher Grayce Jun 8 '18 at 22:02
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    $\begingroup$ I'm not sure about your idea of measuring O2 spectroscopically as it does not have an ir spectrum (no dipole) . Oxygen uptake is measured (in your finger) by using visible (red) light and the absorption change O2 binding makes to the electronic absorption spectrum of haemoglobin vs that without O2. $\endgroup$ – porphyrin Jun 12 '18 at 8:01
  • $\begingroup$ Oh excellent point, I made an edit to make that clear, thanks. $\endgroup$ – Christopher Grayce Jun 12 '18 at 9:37

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