Acid/base and arterial blood gases-part 1

First a little refresher...
pH is a measure of how acidic or alkaline (also known as basic) something is. 0 is really acidic, 14 is really basic, 7 is neutral.

The human body is a little on the alkaline side; the normal range is 7.35 to 7.45

PaCO₂ is the partial pressure of CO₂ gas (carbon dioxide) in the arteries. Partial pressure means CO₂ makes up a portion of the total pressure of all the gases in your body, O₂ (oxygen) makes up a portion, etc. Add them all up and you get the total pressure.

The normal levels of PaCO₂ are 35-45 mmHg. Hmm, same numbers as the pH. Kinda makes it easy to remember.

HCO₃^- is bicarbonate (half of what makes up baking soda). The minus sign after the HCO₃ indicates a charge; bicarbonate is an ion. It wants another hydrogen ion to be complete (to be neutral).

The normal levels of HCO₃^- are 22-26 mEq/L (those are milli-equivilants per liter. When it's an ion, that's how you measure it)

So once again, the normal values are:
pH 7.35-6.45
PaCO₂ 35-45 mmHg
HCO₃^- 22-26 mEq/L

Part 2
Your body works hard to keep the pH within the normal range. It does this through buffer systems in your blood, through breathing, and through urination. The chemical formula for what is happening is:

H⁺ + HCO₃^- <--> H₂CO₃ <--> CO₂ + H₂0

Not terribly important to remember, but it does help with test questions and diagnosis to understand what is happening.

On the left hand side, the lonely bicarbonate gets it's extra hydrogen and now it is happy as H₂CO₃ (also known as carbonic acid). It doesn't stop there though; it can break apart into CO₂ and water.

This reaction goes both ways. Water combines with carbon dioxide to form carbonic acid which splits apart into the lone hydrogen atom and bicarbonate.

Part 3
If you have more lonely H⁺ ions floating around, you are more acid. Have less and you are more basic. Since hydrogen pairs up with bicarbonate, that means bicarbonate is basic. If you have more bicarbonate, you are more basic. Have less and you are more acidic.

Similarly, if you take more CO₂ out of the equation (by breathing faster), that means less CO₂ to combine with water --> less H₂CO₃ --> less free H⁺ --> more basic (raise the pH level).

Breath slower and the opposite happens-more CO₂ combining with water --> more H₂CO₃ --> more free H⁺ --> more acidic (lower the pH level).

Pee more bicarbonate out? You get more acidic. Retain more bicarbonate and you get more basic.

Continue to part 2