Module title = Tutorial: Blood Gases 1

Lesson title = Shifting chemistry

Please see lesson #1 (Acidosis or Alkalosis) for the podcast on this lesson.

The acid base balance is dictated by following equation:
  


H⁺ is the hydrogen ion concentration. Focus your attention on him.
HCO₃^- is the bicarbonate, or simply "bicarb" concentration.
H₂O and CO₂, as you probably know, are water and carbon dioxide. Just adding this for completeness ... not to insult you.

Understanding this equation is required for understanding how HCO₃^- and CO₂ change the pH.
Memorize this equation! This equation will make your life much easier in the acid base world.

There is a quantitative (mathematical) relationship between pH, HCO₃^- and CO₂ and it is described by the Henderson Hasselbach equation:



Do not memorize this equation. Simply understand that there is a fixed and well-described relationship between the 3 variables of pH, HCO₃^- and CO₂. Given 2 of the values, we can calculate what the 3^rd value is. In fact, machines that measure blood gases do this because it is easier/cheaper to measure 2 things vs. 3 things.
 

"Shifting" the Acid Base Equation:

• ok, let's go back to high school chemisty class
• decreasing the concentration of one chemical on the left side of the equation will shift the equation to the left
• when you shift left, all OTHER chemical(s) on the left side of the equation will increase, and all chemicals on the right side will decrease
• increasing the concentration of any chemical on the right side of the equation will also shift the equation to the left
• this means that on the right side, the other chemical(s) will decrease and all chemicals on the left side will increase
• and the opposite is true to shift the equation to the right:
• increase any chemical on the left, or decrease any chemical on the right will shift the equation to the right
• ok, there is a lot of right and left going on here and sorry if it is confusing
• bottom line: make sure you understand what happens to all chemicals in the equation if you increase or decrease one chemical. If you understand this, you are good to go!

• Given that pH is defined by the H⁺ concentration, we are primarily concerned about how changes in the other chemicals will change the H⁺ concentration. Memorize these 2 rules:
  • a higher H⁺ concentration produces acidosis (low pH)
  • a lower H⁺ concentration produces alkalosis (high pH)
• Since the "concentration of water" doesn't change, the 2 chemicals of interest are HCO₃^- and CO₂.
• so start thinking: what will happen to the H⁺ concentration if we change HCO₃^- and CO₂?

• decreasing the HCO₃^- (bicarb) concentration will shift the equation to the left and increase H⁺ ion concentration and generate acidosis. This process is called metabolic acidosis. The kidneys mostly control your HCO₃^- levels.
• increasing the CO₂ will also shift the equation to the left and generate acidosis. This is called respiratory acidosis because your ventilation controls your CO₂ levels.

 
Everything discussed for acidosis can be applied in reverse, to describe processes that generate alkalosis.

Alkalosis:

• increasing the bicarb concentration will shift the equation to the right and generate alkalosis. This is metabolic alkalosis.
• decreasing the CO₂ will also shift the equation right and generate alkalosis. This is respiratory alkalosis.

Let’s do a quick recap. There are 4 primary acid base disturbances: