The pH of blood is a way of describing how much like an acid blood is - or how much acid there is in blood. It is usually in the range of 7.35 to 7.45, which means that the concentration of hydrogen ions is between 0.000000035 and 0.000000045. Because water has a pH of 7, this means that blood usually has slightly less hydrogen floating around than normal water does.
The reason that this is so important is that the rate at which enzymes work depends on pH, and most of the enzymes in the body work best at a pH of 7.35 to 7.45. This means that the pH of blood has to be tightly managed.
The pH of blood is controlled by a number of things which are dissolved in the plasma, including the haemoglobin that you find in red blood cells. Haemoglobin, like the other things, acts as a buffer, which means that any time something tries to change the acidity of blood, they try to cover it up by releasing hydrogen or taking it on board.
However, the most important buffer in the blood is the carbonic acid equilibrium, which uses the lungs and the kidneys to deal with any changes in the pH.
The carbonic acid equilibrium is an equilibrium in which carbonic acid splits up into two other particles. However, carbonic acid is able to split up in two different ways. It can either split up into carbon dioxide and water, or it can split up into bicarbonate and a hydrogen ion.
Because carbonic acid can split up into bicarbonate and a hydrogen ion, it is a hydrogen ion donor - that is, an acid. If there aren't many hydrogen ions floating around (i.e. the blood is not as acidic as it should be), the carbonic acid equilibrium can fix the problem by shifting towards the bicarbonate end - releasing more hydrogen into the blood.
However, if the opposite is true, and there's too much hydrogen floating around, then it can bind with bicarbonate to form carbonic acid, and then split up into carbon dioxide and water, to get rid of it. This reduces the amount of hydrogen floating around and can reduce the acidity.
Why is this useful? Well, it means that the body can use the lungs and the kidneys to make changes to the acidity of blood. It doesn't need to go through a complicated manufacturing process to make up lots of extra acid, or to do a complicated filtering process to get rid of it. It can simply use this equilibrium.
Basically you need to understand that the lungs get rid of carbon dioxide and water. If you breathe more quickly, you'll get rid of more; if you breathe more slowly, you can build these up. The second thing you need to remember is that the kidneys are able to reabsorb bicarbonate, and it can change this amount depending on how acidic the blood is. Following so far? Let me explain how the body uses these to change the pH of blood.
Imagine the situation where you have too much acid - there are too many hydrogen ions floating around. Le Chatelier's Principle says that the equilibrium will shift to oppose that change - that is, more carbonic acid will be made to try to use up some of these extra hydrogen ions. If that happens, it will lead to more carbonic acid splitting up into water and carbon dioxide. This build up of carbon dioxide prompts the lungs to breathe more quickly, which gets rid of the extra gas - and so the lungs have helped to bring the pH back to normal.
The kidneys can also help the issue by reabsorbing more bicarbonate. This provides more bicarbonate for the hydrogen to bind to when it forms carbonic acid and ultimately water and carbon dioxide. The system obviously requires that both the kidneys and the lungs are working properly - and if either of them isn't, this can lead to problems with the pH of blood.
Just to make the point clear, imagine the opposite situation - the blood does not have enough hydrogen in it, or it is too alkaline. The equilibrium shifts to oppose that change, so it uses up more carbon dioxide and water, forming carbonic acid which splits into hydrogen and bicarbonate. You fix the problem with the hydrogen, but you now have two other problems - carbon dioxide levels are low, and bicarbonate levels are high. The lungs slow down to build up the carbon dioxide to normal levels, and the kidneys reabsorb less bicarbonate. Again, they've both helped to fix the problems with the pH of blood.
Of course, the carbonic acid equilibrium doesn't just affect hydrogen ion concentrations. A change of carbon dioxide concentration due to any problems with the lungs can also be compensated for by this equilibrium - if there's too much carbon dioxide, it can shift across to the left to produce more bicarbonate and hydrogen ions for the kidney to deal with. If you're breathing too quickly, bicarbonate and hydrogen ions can bind and separate to replace the carbon dioxide. It's a slightly different way of thinking about it, but it's important to be able to understand this and consider it from both perspectives in order to understand how to interpret an arterial blood gas.
The Henderson-Hasselbalch equation is a way of working out the pH of a solution without knowing the concentration of hydrogen ions. It looks at the concentrations of a conjugate acid-base pair in a buffer system. Because the carbonic acid equilibrium is the main buffer system in blood, it can be used to work out the pH of blood by simply looking at the concentrations just mentioned.
If you put the figures in, you'll be looking at an equation that includes the logarithm to the base 10 of bicarbonate concentration divided by carbonic acid concentration.
However, in real life, the concentration of carbonic acid is very, very small because it splits up into water and carbon dioxide. Therefore, it's a lost more useful to look at the concentration of carbon dioxide in solution. The equation therefore looks at the pKa, plus the logarithm to the base 10 of the bicarbonate concentration divided by the concentration of carbon dioxide in solution.
Most of the time, the concentration of carbon dioxide isn't measured - instead, we get a partial pressure of carbon dioxide. We can work out what the concentration is by multiplying this partial pressure by the solubility coefficient, which is measured in concentration per pressure. For example, the solubility coefficient of carbon dioxide is 0.23 mmol per litre per kPa or 0.03 mmol per mmHg.
Finally, the pKa of carbonic acid at 37°C is 6.1, which we can put into the equation to work out the pH. If the temperature is different, then the pKa will be slightly different and the pH changes - which is why it's important to know the temperature of a patient when you carry out an arterial blood gas.