44 - Understanding Arterial Blood Gases part 1

KYJ 44 understanding Blood gases. Part 1.

In this series of "knowing your jargon (KYJ)", we explore the blood gases, and explain some of the terminology used in interpretation.

We start with some fundamental science and a physics principle called Dalton's Law.

Dalton said that the pressure exerted by a mixture of gases is equal to the sum of all gases in the mix.   Air exerts pressure (air pressure).

Air is a mixture of gases, mostly nitrogen (78%) oxygen (21%), and a minute quantity of CO2, argon, xenon, water vapour, and traces of other environmental contaminants make up the last 1%.

All these gases exert their cumulative pressure in the mix that we call Air, but individually these gases all exert their own pressure.  This is called Partial Pressure, abbreviated to "pp".

Now air pressure as a whole, or atmospheric pressure is relatively constant at sea level but reduces with altitude. At sea level our atmospheric pressure is 760 mmHg (101.3 KPa).
But if you were at Everest base camp at 5000metres, the air is so thin that atmospheric pressure is about half that at sea level (380 mmHg), and only a third at the summit some 8.3km high.

Now back to Dalton's law. The pressure of a mixed gas is the sum of its parts.

So air pressure (760), is the sum of the pressure of nitrogen, oxygen, argon , CO2 etc.

When we know the percentage of a gas in air, we can calculate its partial pressure.  Oxygen makes up 21% ( actually 20.9% but let's not split hairs), of air

If Air is 760mmHg and Oxygen is 21% (or 0.21 of air), then
760 x 0.21 = 159.6

Oxygen at sea level then has a partial pressure of 159.6mmHg.
Expressed as ppO2 = 159.6.
...
Take a sip of your coffee/wine
And do these quick calculations.
...
1. What would the partial pressure of nitrogen (78% of air) be at sea level?

2. What would the partial pressure of Oxygen be at Everest Base Camp?
...
Ok.  Answers at the end of this blog session.
Read on:

Now another physics principle is that many gases are soluble in liquids. An excellent example is oxygen and carbon dioxide(CO2).
Ponder the bubbles in your bottle of champagne / soft drink or beer.  These are CO2 bubbles coming out of solution and returning to gas.  Under pressure the CO2 formed during fermentation (grog) or manufacture (soft drink), dissolves into the liquid.  When you pop the top off these beverages, pressure is released and the dissolved gas reforms into bubbles and fizzes or "off gases".

Blood or more correctly, plasma is the same. It dissolves oxygen and CO2 into solution.  Of course these gases don't travel in the blood in bubbles do they- they'd get stuck.  That is what happens to divers with the bends. But that is another post.

In our next edition we will explore more about oxygen and CO2 gas dissolved in blood.

Answer 1= ppN2 is 600.4 mmHg
Answer 2= ppO2 base camp is only about 80mmHg