Blood Gases Level 1 Tutorial: Blood Gases
Tutorial: Blood Gases
Learn an organized approach to arterial blood gas analysis, incorporating serum and urine electrolyte values into your more advanced levels of analysis.
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Tutorial: Blood Gases Acidosis or Alkalosis
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Acidosis or Alkalosis

Arterial blood gases, abbreviated ABG’s, measure the acid base status and the oxygenation level in the arterial blood. There are many pieces of information measured in an ABG. We will discuss some of the information now, and add further details later.

The standard format of writing an ABG is:

pH  /  pCO2  /  pO2  /  HCO3- 

HCO3- is bicarbonate, abbreviated at "bicarb"

For example,   7.40 / 40 / 100 / 24   is an example of a normal ABG and means:
·      the pH is 7.40
·      the partial pressure of carbon dioxide (pCO2) is 40 mmHg
·      the partial pressure of oxygen (pO2) is 100 mmHg
·      the bicarbonate (bicarb) level is 24 mmol/liter
Do not confuse pO2 with the oxygen saturation (SpO2), which is a value expressed in percent. The pO2 and SpO2 are related, but not equal. We will not focus on the pO2 (or SpO2) in this module, as they is not very related to the acid-base status.

Normal Values (memorize these values)
  • The normal pH of the blood is 7.40 and the normal range is 7.35 to 7.45
  • The normal pCO2 is 40
  • The normal  HCO3- is 24
When you do calculations in upcoming lessons, use the values above for your normal values. Some sources will state a range of normal (such as 22-26 for bicarb), but it is harder to use a range of values in an equation when you need a single value. For example, if the bicarb is 19, you will calculate that the bicarb dropped 5 points, based on a normal value of 24.

Any pH that is lower than 7.35 is considered acidotic. Acidosis, a process that causes too much acid, and acidemia, a condition of having acidic blood, are common terms used to describe a low pH in the blood. The stomach has a very low pH and is normally acidic. However, the blood is different.
Any pH that is higher than 7.45 is considered alkalotic. Other common terms include alkalosis (a state of having high pH) and alkalemia (high pH in the blood). Of note, the first parts of the small intestine are normally alkalotic (to buffer the acid coming from the stomach).

Here is where it gets a little bit confusing. Acidosis and alkalosis refer to a process. Usually, an ongoing process will create the same condition in the blood. For example, an acidosis process will cause acidemia. However, you could have 2 opposing processes occuring at the same time which balance each other out. For example, you could have acidosis and alkalosis both together and the pH of the blood is normal. In this case, there would be an acidosis and an alkalosis present, but no acidemia and no alkalemia.

"-osis" versus "-emia": remember that we are measuring the acid-base status directly in the blood. Therefore, the terms alkalotic, alkalosis, and alkalemia are USUALLY interchangable, as are acidotic, acidosis, and acidemia.

Yes, we are splitting hairs here and this might seem like an annoying detail, but wait until you start practicing and you look at a normal pH of 7.40 and realize that there is lots going on behind the scenes. We will train you how to do this.