Partial pressures of gases in the blood.

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For example: PO2 and PCO2 in the blood are measured and have some numerical value in mm of Hg. What does this actually mean and what’s the significance?

In: Chemistry
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The pressure of a gas is the result of the contribution by the different molecules in a gaseous state.
The partial pressure of a gas is the contribution of one kind of gas to the total pressure

As far as mmHg goes it’s a pressure unit based on how high the pressure can push an amount of mercury. It’s a non SI unit and an abomination.

Also, the mm Hg (millimeters of mercury) is just a standardized method of measuring the pressure produced by a gas by the amount of mercury this gas displaces in a column.

Partial pressure measures the concentration of the gas dissolved in the blood plasma (ie, not bound to haemoglobin). This can vary by environment, and also by location in the body (the more time it’s spent away from the lungs, the lower it falls), and it’s important because it influences the binding of oxygen to haemoglobin. In high PO2 (when a lot of oxygen is dissolved in the blood plasma), oxygen readily binds to the haemoglobin, and in low PO2, it tends to dissociate from the haemoglobin. This means that haemoglobin picks up a lot of oxygen at the lungs, where there’s a high PO2, and loses it more and more the further it gets from the lungs, essentially ensuring an even distribution of oxygen to various parts of the body. PCO2 is the same thing but for carbon dioxide (which is also carried bound to haemoglobin, but carried from the tissues back towards the lungs instead).

What this is most relevant to is [dissociation curves](https://upload.wikimedia.org/wikipedia/commons/8/8a/Oxyhaemoglobin_dissociation_curve.png), which are a visual representation of the amount of oxygen saturation of haemoglobin you can expect at any given PO2 under various conditions. For example, if you increase the temperature, the curve shifts right, and if you draw a line up from say, 60 PO2, you’re now getting 82% oxygen saturation instead of the 88% you were getting previously. This is useful for visualising the adaptations of haemoglobin and blood in various animals and stages of life. There are lots of different types of haemoglobin that are effective to varying degrees – some more effective than adult human haemoglobin, some less effective, and it’s useful to be able to represent this on a graph. For example, human foetuses have a haemoglobin molecule that’s more sensitive to oxygen – it achieves higher saturation than adult haemoglobin at lower PO2s. This is because it’s not getting oxygen from the high PO2 blood of the lungs. Instead it’s having to take oxygen from the already lowered PO2 of the blood that has reached the mother’s uterus, so haemoglobin that can absorb more in those oxygen-deficient situations is preferable. The trade-off is that oxygen doesn’t dissociate as easily, so it’s not a good thing to have as an adult when oxygen is now readily available.