Oxygen and Respiration — Page 45, Lesson 66

I want to walk you through the start of the Oxygen and Respiration chapter. We're going to begin with the foundation that every pilot needs to understand: the standard atmosphere, the composition of the air we breathe, and the gas laws that govern how that air behaves at altitude. Let's start with the ICAO Standard Atmosphere. This is a model — a reference — that gives us consistent numbers to work with, because real atmospheric conditions vary. The ICAO standard defines the following at Mean Sea Level, or MSL: a temperature of +15°C, a pressure of 1013.25 hPa — which is also 760 mm Hg — and a density of 1225 grams per cubic metre. Then, as you climb, the temperature doesn't stay constant. There's a defined lapse rate of 1.98°C per 1000 feet, or 6.5°C per kilometre, and that rate holds true all the way up to 36,090 feet, which is 11 kilometres. Above that altitude, the temperature stops decreasing and remains constant at -56.5°C all the way up to 65,617 feet, or 20 kilometres. Now, because pressure decreases with altitude, the standard atmosphere also gives us some handy reference points. The altitude at which pressure drops to three-quarters of its MSL value is approximately 8,000 feet. At half of MSL pressure, you're at about 18,000 feet. And at one-quarter of MSL pressure, you're at roughly 36,000 feet. There's an important note here: atmospheric pressure decreases at a faster rate at low altitudes than at higher altitudes. So the pressure drop between sea level and 8,000 feet is steeper than the drop between 18,000 and 36,000 feet. Next, let's look at the composition of the atmosphere. The air around us is a mixture of gases. By volume, it's made up of 21.0% oxygen, 78.0% nitrogen, 0.93% argon, 0.03% carbon dioxide, and 0.04% rare gases. These percentages — these volume percentages — remain constant all the way up to about 70,000 feet, which is well within the altitudes at which conventional aircraft operate. So the proportion of oxygen in the air doesn't change as you climb. For the pilot, oxygen is the most important of these gases, and we'll see why shortly. Now, a couple of definitions about water vapour in the air. Absolute humidity is the weight of water vapour in a unit volume of air, usually expressed in grams per cubic metre. Relative humidity is a bit different: it's the amount of water vapour actually present in a volume of air, divided by the maximum amount of water vapour that same volume could hold at that temperature, expressed as a percentage. So relative humidity tells you how saturated the air is, relative to its temperature. Finally, we need to cover the gas laws that are directly relevant to aviation physiology. First, Boyle's Law. It states: "Providing the temperature is constant, the volume of a gas is inversely proportional to its pressure." In plain terms, if you increase the pressure on a gas, its volume shrinks — and if you decrease the pressure, its volume expands — as long as the temperature doesn't change. This law is directly linked to things like otic and gastrointestinal tract barotrauma — that's ear and gut pain from pressure changes — and aerodontalgia, which is tooth pain caused by trapped gas expanding at altitude. Mathematically, it's expressed as P1 over P2 equals V2 over V1, where P1 is the initial pressure, P2 is the final pressure, V1 is the initial volume, and V2 is the final volume. Next is Dalton's Law. This one states: "The total pressure of a gas mixture is equal to the sum of its partial pressures." In other words, each gas in a mixture — oxygen, nitrogen, argon, and so on — contributes its own individual pressure, called its partial pressure, and if you add all those partial pressures together, you get the total pressure of the mixture. This law is directly relevant to hypoxia — oxygen deficiency — and night vision, because the partial pressure of oxygen is what drives how much oxygen gets into your bloodstream. Mathematically, it's written as Pt = P1 + P2 + P3 ... Pn, where Pt is the total pressure of the mixture, and each P with a number is the partial pressure of an individual gas. Here's a diagram of the air passages in the lungs that will help you visualise where these gases go. So, to summarise what we've covered: we have the ICAO standard atmosphere giving us reference values for temperature, pressure, and density at sea level and with altitude. We know the composition of the air is constant up to 70,000 feet. We've defined absolute and relative humidity. And we've introduced Boyle's Law — pressure and volume relationship — and Dalton's Law — total pressure equals the sum of partial pressures. These are the building blocks for understanding how oxygen gets to your body and what happens when it doesn't.