46 - Understanding ABGs part 3

KYJ 46 - Acidosis. ABG series part 3.
In this series I walk you through the fundamentals of analysing arterial blood gases.

Part 1 looked at partial pressure
Part 2 looks at oxygen and CO2 values.
In today's episode we look at pH of blood, and discuss acid vs alkali.
Potenz Hydrogen or pH is a measure of acidity or alkalinity.

Fun Science fact: all liquids can be measured for pH on a scale between 1 - 14.  Pure sterile distilled water is neutral at 7.0

If a liquid is less than 7.0 it is called an acid, and the closer to 1 it is, the stronger acid it is.  Examples include sulphuric acid in your car battery, hydrochloric acid in your stomach, vinegar and lemon juice.  Some of you will remember an advert for skin cream or shampoo claiming it is pH balanced at 5.5, the same pH of your skin.

Conversely, a liquid with a pH higher than 7.0 is called a base or alkali.  Bile, caustic soda, soap, and Sodium Bicarbonate that we have in our pantries for cooking. Infarct anything that is called an Antacid eg Mylanta or Gaviscon.

Arterial Blood has a pH of 7.35-7.45 but under certain circumstances may fluctuate either side of this normal range.   Notice that blood is actually slightly alkaline. Despite this, it is called neutral or normal in this range (that must really irritate the nerdy Sheldon types).

If the pH of blood should drop below 7.35 it is called acidosis. If it swings above 7.45 it is called alkalosis.

Now there are a few principles to understand before we discuss how acidosis or alkalosis occurs.  The first is the effect of CO2 being produced.

All cells produce two waste products at part of their normal metabolism. Carbon Dioxide and water.
These waste products must be removed from the body and we do that two ways. We urinate and we breathe.
CO2 and water form a weak acid in blood. It is called carbonic acid (CO2+H2O =H2CO3).
So blood is constantly becoming acidotic, and without being able to breathe off CO2 it would accumulate in minutes to cause dangerous levels of acid in your blood.

CO2 is also converted inside red blood cells to an alkali called bicarbonate (HCO3).  This acts in blood to buffer or counteract the effect of acidic CO2 rise.
As HCO3 levels rise driving blood into the alkalosis end of normal pH, your kidneys work harder to pee off the bicarb.  Normal HCO3 levels are 22-26mEq

Remember
HCO3 is alkali
CO2 is acid
We breathe off CO2 to prevent acidosis, and pee off bicarb to prevent alkalosis.

Our lungs and kidneys are therefore organs that help to keep our pH between 7.35 -7.45

In the next session we will pull these all together and look at where CO2, pH and bicarb fit into the basics of ABG analysis.