Human survival at great depths

Have you ever wondered why divers don’t just breathe through a long tube extending to the surface? No? Well, we’re going to tell you now!

10 meters of water column

The atmosphere creates pressure on us constantly, because the column of air from the earth to space has an impressive weight: at sea level, an area of just one square meter has pressure from 10 tons of air! But since the air pressure inside the body cavities is often equal to the pressure from the outside, we do not notice anything.

Pressure under water works in a similar way, but there is a nuance: water is 800 times heavier than air, and much denser. And the pressure of the «water column» on the human body increases proportionally.

At a depth of only 10 meters, the pressure increases by a whole atmosphere: that is, equivalent to 100 kilometers of air column. It turns out that, at a depth of 100 meters, you will already be under a pressure of11 atmospheres (one «air» plus ten «water»). And when you dive to 1 kilometer:101 atmospheres. At the bottom of the Mariana Trench, which is 11 kilometers deep: 1101 atmospheres of pressure!

But if fish can live at a much greater depth than a kilometer, why do people have difficulty breathing through a tube at a depth of half a meter?

When a sumo wrestler sits on your lungs

Most of the human body, like a fish, is made up of water. What is different about humans, apart from the fact that they need more oxygen?

The thing is, humans have many cavities filled with air, which have their own pressure. Our lungs, intestines, stomach, ears, sinuses, and even our teeth—everywhere, there is air with a pressure of one atmosphere, which is lower than the pressure at a depth of 1 meter (1.1 atmospheres).

That’s why we are so sensitive to pressure changes when we fly in an airplane, climb mountains, or dive into a swimming pool—our ears pop when the external pressure is higher or lower than our internal pressure.

The most noticeable effect of under water pressure is on our lungs—they gradually shrink like a sponge with the air squeezed out, and it becomes increasingly difficult for the muscles to expand the chest, as they are compressed by more and more water.

Diver Ahmed Gabr, who holds the world record for scuba diving (332 meters). The dive itself took just 12 minutes, and all those gas cylinders were needed for the 15-hour ascent: frequent and long stops helped him avoid decompression sickness. Photo: Guinness World Records

At a depth of half a meter, the pressure on the chest is already 135 kilograms: as if a sumo wrestler was sitting on your chest. If you try to breathe through a tube stretched up to the surface, your muscles will have to push back out these 135 kilograms, in order to allow you to take a breath—a difficult task, and at a depth of one meter and greater, almost impossible.

This happens, among other things, because you are trying to breathe air that is under normal atmospheric pressure, while your lungs are being compressed at a higher pressure.

But even if your muscles are strong enough to breathe, there will also be pressure inside the tube—not water, but from the atmosphere, i.e. the air column above you. And with each exhalation, part of the carbon dioxide will be deposited inside of the tube instead of leaving through it—that is, over time, you will be breathing not fresh air, but a gas mixture that can suffocate you.

Let’s imagine a woman swimming with a very long tube. The blue arrows show water pressure, which prevents her lungs from spreading properly. The green arrow is the air pressure from above.
Valerya Milovanova / Windy.app

And other dangers in the search for Cthulhu

That’s why scuba divers use a regulator that monitors external pressure, and delivers a gas mixture to their mouths at exactly the pressure that surrounds them at the moment.

This means that the muscles don’t have to work so hard to push your chest back out. However, the deeper you go, the more the water will squeeze the air in your lungs, and sooner or later, at a depth that is different for each person depending on their body, you will be physically unable to breathe, even with the most perfect scuba and gas mixture.

An aqualung is not an ideal solution, because at a high enough pressure of the gas mixture, nitrogen from the cylinder will dissolve better and better in the blood, which will lead to nitrogen poisoning—at safe levels, similar to mild intoxication, but at greater depths, leading to loss of critical thinking, hallucinations, paralysis, and even death.

The current record for scuba diving is 332 meters, while the record for freediving (without scuba equipment) is 214 meters. At greater depths, there can be worse effects than nitrogen poisoning. The difference in pressure between water and body cavities can lead to tissue rupture, and literally explosive pressure equalization between the two environments, leading to serious damage to the body, called barotraumas. Of course, a person is unlikely to be torn apart directly, and it is possible to survive a barotrauma, but it is really unpleasant.

And only pressurized spacesuits, submarines and minisubmarines allow people to dive many kilometers.

Text: Jason Bright, a journalist, and a traveler

Cover photo: Jesse van Vliet / Unsplash