What the Hell is Partial Pressure Anyway?
Dr Otter’s Physiology Boot Camp
Lesson #1 - Partial Pressure
It’s a term that gets bandied around the freediving physiology literature all the time, but Partial Pressure is often poorly understood. Here’s a 2 minute introduction to this fundamental concept: Put simply, Partial Pressure is the pressure of a single gas in a mixture of gasses. Take air for example: Air is made up of a mixture of Oxygen, Nitrogen, Carbon Dioxide and small amounts of gasses like Helium, Argon and Xenon which are called “Noble Gasses”:
Units of Air Pressure
Air pressure can be given in lots of different units. The standard unit for pressure is the Kilo Pascal (kPa) which is used in scientific studies, but others are more commonly used in different situations and different countries around the world. Air pressure at sea level is around 100kPa. The exact value depends on the weather conditions, but never changes by more than a few kPa. Here are a few examples of normal atmospheric air pressure given in different units:
|Unit||Abbreviation||Approximate Air Pressure Value|
|Millimeters of Mercury||mmHg||760|
|Pounds per Square Inch||psi||14.7|
For divers, the most common units to use are bar and atm, but for all the physiology topics on Freedive-Earth, we’ll stick to kPa, because it makes the maths easier later on. Each of the component gasses in the mixture of air, or any mixed gas for that matter, contributes to the total pressure by exactly the same amount that it contributes to the volume. Compare the pie chart below which shows pressures, to the one above which shows percent by volume:
Because of this, we say that the Partial Pressure of Oxygen in atmospheric air is about 21kPa. That’s the same as saying that air is about 21% Oxygen. That’s it, really. That’s partial pressure. At least as far as plain old air goes.
Gasses Dissolved In Liquid
When we breathe air, gasses like oxygen and nitrogen pass through the cells of the alveoli in the lungs and dissolve in the blood. The amount of each gas that ends up dissolved in the blood depends on the solubility (‘dissolvability') of that gas in the blood (sometimes called the liquid or “aqueous phase”), and on the partial pressure of the same gas in the air (sometimes called the “gas phase”). Here’s a simplified example:
Let’s say that the gas we’re considering in these examples is oxygen. In container 1 there are lots of oxygen molecules in the gas phase and much fewer in the aqueous phase. Usually we’d use the word Concentration to describe the amount of a substance dissolved in a liquid but handily, when we’re talking about gasses, we can still use the term Partial Pressure to describe more or less the same thing. So we can say that in example 1 “The Partial Pressure of oxygen in the gas phase is higher than the Partial Pressure of oxygen in the aqueous phase”. See? You’re already talking like a pro! Note also that there is a movement of oxygen into the aqueous phase in this example. The high partial pressure of oxygen in the lungs is pushing oxygen into the blood. In container 2, there are no oxygen molecules in the gas phase at all. We might say that “The Partial Pressure of oxygen in the aqueous phase is higher than the Partial Pressure of oxygen in the gas phase.” Note that here, just like when you open the top of a bottle of coke, gas is moving out of the liquid (sometimes called “coming out of solution”) and into the air. In container 3 we have a state called equilibrium. Here, there is movement of oxygen both into and out of solution and the rate of movement is the same in both directions. That means that the Partial Pressures in both phases remains the same. Notice, though, that the actual amounts of oxygen in each phase are not the same. This is because the solubility of oxygen in the blood is limited. In order to get more oxygen into the blood in this example, we’d have to increase the partial pressure of oxygen in the air and drive more in to the blood. The maximum partial pressure of oxygen we can get into the blood by breathing air at atmospheric pressure is about 12kPa (91.2mmHg), which you’ll notice is quite a lot less than the 21kPa contained in the air itself.
Partial Pressure and Diving
Increasing the partial pressure of oxygen in the air is exactly what happens in the lungs when we dive, because of Boyle’s Law (which is coming up next!). That results in an increase in the Partial Pressure of oxygen in the blood too, as you might expect. It’s a good thing on the whole but, as we’ll see in later lectures, can also cause the problem known as shallow water blackout. Another whole set of problems is caused by the effect of increasing partial pressure of Nitrogen at depth as well… but that’s another story. Got a question for Dr Otter? She’s on the case. Drop her an email at email@example.com, ask us on Facebook or comment below. See you again soon.