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In Respiratory Physiology, we use the $P_x{O_2}$ and $P_x{CO_2}$ in blood at different regions of the peripheral circulation. From my Chemistry knowledge I know that $P_x$ of a gas in a solution is the fraction of total pressure caused due to the gas dissolved. But in blood $O_2$ is present mostly as $OxyHb$ and $CO_2$ is also present as $CarbaminoHb$ & $HCO_3^-$. In blood their dissolved state fraction is very low. Now my question is how do we measure the $P_x$ of any gas ($O_2$ or $CO_2$) in blood and what is the logic here? [x means either arterial blood or venous blood]

Here is the data I'm talking about.(https://i.stack.imgur.com/lA1M5.jpg)

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As a notation note, I never seen your $P_a{O_2}$ notation used except for referring to arterial partial pressure ($P_v{O_2}$ would be venous). Generically, partial pressures are usually noted as, for example, $P_{O_2}$.

When we talk about partial pressures in biology, we literally mean the partial pressures: that caused by dissolved gas.

Hemoglobin-bound oxygen is not the same as gaseous oxygen, nor is carbonic acid the same as carbon dioxide. Neither contribute to the partial pressure. They do relate to the total oxygen/carbon dioxide carrying capacity of blood, but this cannot be measured directly through partial pressures.

See this Q&A for a situation this comes up: Why is arterial pO2 normal in carbon monoxide poisoning?

In medicine, it is common to use oxygen saturation as an alternative measure of blood oxygen concentration; this refers to the percentage of oxygen binding sites of Hb that are saturated.

It is important to realize that the partial pressures of dissolved gases will eventually reach equilibrium with the surrounding atmosphere; the lungs are a great gas exchange organ (that's their entire job), so blood leaving the lungs should have partial pressures that approximate those inspired air (in practice, there is some discrepancy because the lungs are very humid and also are constantly refilled with $CO_2$ from blood, so water vapor and carbon dioxide contribute a substantial gas pressure that pushes out other gases: see the similarity between arterial blood and alveolar gas in your data table and the differences between atmospheric gas and alveolar gas).

Clinically/in a lab, we measure these things with a machine that just magically gives the numbers. I looked for a simple description of how these machines actually function, and found https://acutecaretesting.org/en/articles/understanding-the-principles-behind-blood-gas-sensor-technology to be useful. In summary, $CO_2$is measured by exposing a captive solution to the gas and measuring the pH, giving an indirect (but accurate) measure of $CO_2$. $O_2$ is measured with a reducing current. It's also possible to measure concentrations of arbitrary compounds more directly with gas chromatography - colleagues of mine have used this for anesthetic gases, for example.

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  • $\begingroup$ Extremely sorry for using wrong notations. But $CO_2$ is more soluble in water than $O_2$. So then wouldn't that imply $P_{CO_2}>P_{O_2}$ ? $\endgroup$ – A.N. Ψ Jul 29 at 19:03
  • $\begingroup$ @A.N.Ψ No, it would not imply that. If you want you could ask about why that is on the Chemistry stack exchange, since it's unrelated to biology. $\endgroup$ – Bryan Krause Jul 29 at 19:11

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