Tutorial: The Building Blocks of Aerospace Physiology (Part 1)

La Pression Barométrique…

Published in 1878, some quarter of a century before the first heavier than air flight at Kitty Hawk, Paul Bert’s ‘La Pression Barometrique…’ is one of the fundamental building blocks of our understanding of aerospace physiology. In this text (and other works) he proposed that:

• Functional impairment or death from hypoxia occurs based on inspired oxygen partial pressure, regardless of the combination of barometric pressures and percentages of oxygen used to generate that pressure.
• High oxygen tensions cause toxicity
• Nitrogen bubbling out of solution due to reduced barometric pressure causes decompression sickness

As you will come on to see in our upcoming tutorials on altitude and its physiological effects, these principles form the foundation of a large amount of work in this field to this day.

Like anything in science, aerospace medicine is built upon a variety of building blocks that form the basis of our wider understanding. These range from seminal works to key individuals or fundamental principles. It’s the last of these groups that I want to cover in more detail in this tutorial.

THE BUILDING BLOCKS OF AEROSPACE PHYSIOLOGY

There will always be debate about what fundamentals you need to know before delving into a topic in more detail. In our first series of tutorials we aim to address a number of these including the environment, how aircraft fly and operate, what aircrew do and how we can protect against this. However, it is often hard to know where to start…

The basics of cardiovascular and respiratory physiology have been discussed in detail elsewhere so I won’t focus on those (there are some good videos here, here, here and here if you want a refresher). Instead I think it best to start with some principles that we will run into time and time again as we start to look at how the aviation and space environments affect our physiology. So without further rambling…

Principle One: The Hydrostatic Gradient (THEORY)

Not perhaps where some others might start, but bear with me…

In any column of fluid (and here it is important to remember that gasses are fluids too), there exists a gradient of pressure, increasing the deeper you go. This seems a lot like common sense when you think about it.

When this molecule that looks strangely like Paul Bert is part of a human shaped tower of molecules the amount of ‘pressure’ that he has pressing down on him depends on how many person-shaped molecules are on top of him. As he goes deeper in the tower, the pressure he is under increases in a predictable manner.

Perhaps a more practical way to show this is by taking a bucket and drilling some holes down the side. What you will notice is that the water coming out of the highest holes will be slower and will shoot a shorter distance than the water coming out of the lowest holes (this is known as Torricelli’s theorem).

This response follows consistent laws and hence the pressure gradient can be calculated. Those who have been SCUBA diving may have learnt that for every 10m a diver goes under the surface they are under the influence of a whole extra atmosphere’s worth of pressure (760mmHg, 14.5psi or simply 1 atm*):

But what if the pool was filled with Grade A pure Canadian maple syrup…

At the same depth of 10m, the diver would be under an extra 1.3atm of pressure. Whilst the height of the column above the diver (h) is the same, as is the acceleration acting on that fluid (usually due to gravity, hence ‘g’) the density of the fluid (ρ) has changed, increasing the pressure. These are the three aspects that affect the hydrostatic pressure gradient which can be calculated by the following equation:

Let’s break this down a bit with the help of our friend Paul…

There are two key issues at play. The first of this is how many molecules are sitting on top of you and this depends on two things:

HEIGHT

Molecule Paul needs to reach his copy of Ernsting’s Aviation Medicine which he foolishly left on the top shelf of his empty bookcase. Budget cuts mean his ladder is being used to replace the helmet drop test rig. However, the taller his tower gets (increase in h), the more pressure he has pushing down on him until he has to give up.

• ρ = 1
• g = 1
• h = 0.5
• P = ρgh = 0.5

• ρ = 1
• g = 1
• h = 1
• P = ρgh = 1

So what can we do to help poor old Molecule Paul?

DENSITY

If two columns of fluid are of the same height but one has a lower density than the other, there are fewer molecules between any given point in those columns and the surface. This reduces the pressure of at that point.

• ρ = 1
• g = 1
• h = 1
• P = ρgh = 1

• ρ = 0.5
• g = 1
• h = 1
• P = ρgh = 0.5

This time Molecule Paul has found some lightweight platforms to spread out the other molecules in his stack but still reach the same height. Instead of 4 molecules on top of him he now only has two, reducing the pressure that he has to withstand but it’s still too much.

This still looks a lot like hard work, so what else can be changed? If the first issue is how many molecules are sitting on top of you, the second is how hard those molecules are being pulled down so…

GRAVITY

If Paul is still struggling, we could take him, his friends and the bookcase to the Moon where gravity is 16.6% as strong as it is on Earth…or he could just get a shorter bookshelf.

But why is any of this important?

Principle ONE: THE HYDROSTATIC GRADIENT (APPLICATION)

Fluids are hugely important parts of our lives. We are made up mostly of fluid, we distribute oxygen around our body via a fluid and when you remember that gases are fluids too it’s clear that we need fluids to breath and in fact are constantly bathed in it them too.

So fluids are critical parts of our life. We’ve already talked about how important the environment in which we evolve is to how our bodies function. Pressure is one of these key factors – it drives diffusion in our lungs and at our capillaries, it determines what shape body we have (just look at a blobfish!) and how hard out cardiovascular system has to work. If you mess around with certain pressures you can quite quickly make it very hard to live.

But if you think about those variables that affect the hydrostatic gradient you’ll see notice that these are the very things that change when we get in an aircraft or a rocket. Simply put the atmosphere is one big ‘column’ of fluid. As you’ll see in our upcoming tutorials on altitude, there is a gradient of pressure that decreases as we climb towards the edge of the atmosphere (or the ‘surface’ of the column). On top of that, the density of the fluid changes too.

Your blood is a column of fluid too (in simple terms again…we’ll see why this isn’t entirely true). When you stand up in the morning the acceleration due to gravity shifts to act on the long axis of this column, increasing the hydrostatic gradient (this is why you feel dizzy if you jump out of bed too quickly). When we pull “G” we increase the acceleration on our blood as well but to many multiples of Earth’s gravity. We will talk about what this significant affect on the hydrostatic gradient does when we come on to tutorials about Long Duration Acceleration. And of course, space travel does the opposite. By reducing the normal +1Gz pull on blood towards your feet, your blood volume is redistributed headwards, causing mucosal congestion, central diuresis and possibly even contributing toward thromboses. The CSF in the subarachnoid space of the spinal column forms a column too – headward shift in this may cause raised intracranial pressure leading to long term visual symptoms experienced by astronauts.

Hopefully you can see why it is worth dwelling on the basics. Whilst this is a bit of a long way round, proper understanding of the building blocks will make things quicker in the long run…I promise.

There will be plenty of opportunity for more equations in Part 2 when we discuss Gas Laws and more some fundamental principles of aerospace physiology. Until then…

Yours,

JB

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