MadSci Network: Biochemistry Query:

Hi Liz,

I'm sorry that it has taken so long to answer your question. In thinking about your question, my first instinct was that you were mistaken about the boiling point of blood being lower about the boiling point of water, and after poking around a but, I can't find any evidence contrary to my instinct.

At the heart of the issue is a phenomenon called Boiling Point Elevation. The boiling point of water is elevated when solutes are dissolved in the water. The boiling point also depends on the atmospheric pressure, but for the this question, I'm going to assume that we are at one atmosphere of pressure so that the boiling point of water is 100 degrees centigrade.

Some people put salt in water when they make pasta, believing that the dissolved salt raises the boiling point of the water, and that the water is actually hotter than 100 degrees centigrade when the pasta is added. I say that they believe this because I'm doubt people add enough salt to result in a significant elevation in the boiling point. For water, the boiling point is elevated by 0.513 degrees centigrade for each mole of dissolved solute-ions per kilogram of solution. A mole of salt (NaCl) is 58.44 grams, and since one mole of NaCl disassociates into two moles of ions (Na+ ions and Cl- ions), 58.44 grams of salt (or 2 ounces) would be required to raise the boiling point of one liter (kilogram) of water (at one atmosphere of pressure) to 101.026 degrees. That would be some pretty salty water.

There is a related phenomenon known as Freezing Point Depression, in which the freezing point of water is lowered by 1.86 degress centigrade for each mole of solute-ions per kilogram of solution. We exploit the phenomenon of Freezing Point Depression by using salt to melt snow and ice on roads in the winter and to make ice-cream.

Now, how does this relate to your question about the boiling point of blood? Well blood is basically a water solution. The dissolved solutes are more complex than a simple NaCl solution, but the principle of boiling point elevation (and freezing point depression) should still apply. The salt (NaCl) concentration of blood is 0.9%. This is approximately 0.154 moles of NaCl per kilogram (liter), or 0.308 moles of dissolved solute per liter. This should result in a boiling point elevation of a whopping 0.158 degrees centigrade.

Now, there are other molecules dissolved in blood besides Na+ and Cl- ions, so the boiling point may be slightly different, but I think that you can see that it isn't going to be noticeably higher than 100 degrees (at one atmosphere of pressure).

So the answer to your question is basically, at the same temperature that water does. I hope that isn't too disappointing. If you want more information on Boiling Point Elevation or Freezing Point Depression, you should be able to find it in any college-level chemistry text book.

Keep asking questions!

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