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Module title = Tutorial: Blood Gases

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Lesson title = Shifting chemistry

This is lesson 2 of 8 in this module

The acid base balance is dictated by following equation:

H

^{+} is the

**hydrogen ion concentration**. Focus your attention on him.

HCO

_{3}^{-} is the

**bicarbonate**, or simply "bicarb" concentration.

H

_{2}O and CO

_{2}, 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 HCO

_{3}^{-} and CO

_{2} 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, HCO

_{3}^{- }and CO

_{2} 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, HCO

_{3}^{-} and CO

_{2}. 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!

**H**^{+} Ion Concentration Dictates the pH:- 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 HCO
_{3}^{-} and CO_{2}. - so start thinking: what will happen to the H
^{+} concentration if we change HCO_{3}^{-} and CO_{2}?

**Acidosis:****decreasing the HCO**_{3}^{-} (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 HCO_{3}^{-} levels.**increasing the CO**_{2} will also shift the equation to the left and generate acidosis. This is called **respiratory **acidosis because your ventilation controls your CO_{2} 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 CO**_{2} 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**:

a) **metabolic acidosis**

b) **respiratory acidosis**

c) **metabolic alkalosis**

d) **respiratory alkalosis**

These 4 processes will form the fundamental basics of your understanding of ABG analysis. If you are confused with any of these details, it is

**really important** to ensure you stop and re-study this lesson. If you are confused about ANYTHING on this page, you will be confused in future lessons also.