How to calculate the fractional concentration of gases in a mixture

Before we dive into the analysis of blood gas values, it's important to review some of the basics about gases in a mixture. In this video, from our Blood Gas Analysis Essentials course, we'll take a deep dive into the fractional concentration of gases and how to calculate this important value.

Master arterial blood gas analysis based on an understanding of relevant physiological principles. You’ll cover the crucial factors that determine the oxygenation of blood in the lungs, as well as oxygen transport and delivery to peripheral tissues. Learn about the interplay between blood gas and acid-base analysis and how carbon dioxide affects arterial pH. This course complements our Acid-Base Essentials course.

Before we dive into the analysis of blood gas values, we're going to take a few moments to review some of the basics of gases in a mixture. First up, fractional concentration. For simplicity, let's look at a mixture of gases A, B, and C, we can express the concentration of gas a in the mixture M as a fractional concentration FMA.

The fractional concentration of gas A can be calculated by the number of molecules of gas A divided by the total number of all gas molecules in the mixture. That is the amount of gas a plus the amount of gas B plus the amount of gas see.

For our mixture here, we would have 10 divided by 10 plus 15 plus five, or 0.33. So the fractional concentration of gas a in this mixture is 0.33.
You could do the same thing for gases B, and C.

Now let's look at something a little more practical, the air around us. This atmospheric air is also a mixture of gases
nitrogen, oxygen, carbon dioxide, argon, and water vapor.

The amounts of argon and carbon dioxide are minimal, as is the amount of water vapor under normal so called dry conditions.
Therefore, from a practical standpoint, atmospheric air can be considered a mixture of oxygen and nitrogen at approximately 21% and 79% respectively.

Just as we did for our example gas mixture, we can express the concentrations of gases in the air around us in terms of fractional concentrations. And since this is the air we breathe in, we can express them as fractional concentrations of inspired gas X fi X for oxygen, FY oh two is 0.21.

And for nitrogen, FY N two is 0.791. More thing we should make note of here is the fact that as indicated previously, atmospheric air is considered dry. That is it has very little water content. However, as it is inhaled, dry atmospheric air is warmed to body temperature, and fully humidified by the upper airways.

You'll need to keep this in mind because this adds an additional component that needs to be factored in when calculating partial pressures of gases within the alveoli, which is what we'll discuss in the next lesson.

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