Millimeters of mercury is the way we measure pressure - anywhere in the body where pressure can be measured. This is the way blood pressure is measured, in mm of Hg. Have you ever seen the old timey blood pressure machines that hang on the wall and have the silver liquid (mercury) that rises up when you pump up the cuff and then lowers as you let out the cuff pressure? We don't say 120/80 mm Hg, but that is what it is.
Anyway, the number that you cited in your first post is the partial pressure or tension of CO2 (the amount of carbon dioxide gas dissolved in the blood). No, there is no mercury in the lungs. I'm not sure in what context you are reading about PCO2, but anyway, that is what it means.
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