pH is the negative log of the hydrogen ion concentration. More hydrogen ion, more acid. Less acid, more alkaline. As the pH goes down you are more acidotic and as the pH goes up you are more alkalotic. So it is actually going inversely, negative log, but the Europeans have already switched over. I don’t think we are going to switch over, I wish we would, and they’ve gone to hydrogen ion concentrations so it is directly – it actually adds and subtracts with all the other ions and frankly I wish we would change over, but I don’t think we are.
Now partial pressure is another concept that is sometimes difficult to deal with and that is, it isn’t a real measurement of anything. It’s just a relationship to the total. Here the barometric pressure is about 730 mmHg or so. So that’s a partial pressure of 100%, whatever that is. And then partial pressures then are the part that a gas plays in that. So if a gas, an inert gas, is 80% then the partial pressure is 80% of whatever that is. If it’s 10% or 20%, it’s whatever that is. So if its partial pressure is 730 mmHg and somebody is 10%, it’s 73. If it’s 20% it’s 146. And since oxygen is right about 20% of room air – that’s approximately the PO2 of room air. It’s about 140 mmHg. It’s very important to remember that this is just a proportion. It doesn’t have any absolute value at all. There’s a lot of other things that come into play, including hemoglobin and so forth that we’ll get into, so it’s just the part that CO2 plays in the total. Same thing with O2, N2 whatever you are measuring.
Now bicarbonate, on the other hand, is something. It is an ion. The problem is, we can’t measure it. There has been no accurate way ever devised to measure it. So all of the bicarbonates that you get will be calculated. The good news is that the calculations are such that it is almost as good as a measured ion and I never even think twice about it. And it almost always works out right. So even though it’s calculated, not measured, it is a good value and is worthy of our attention.
December 22nd, 2009 at 9:42 pm
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