I understand that breathable air behaves differently underwater due to higher ambient pressure, and that for some reason it causes bubbles in your blood vessels if you ascend too quickly. But I couldn’t understand why that is.
Additionally, given the same supply of air tank, why is it the case that if you are living in a underwater habitat, then you can dive for longer hours as opposed to someone who came down from the surface?
In: Biology
When you breathe air under pressure nitrogen (the main gas in air) gets dissolved into your blood. As you rise and the pressure reduces that nitrogen comes out of solution and forms gas bubbles. These are bad news. You come up slowly (decompress) to allow the nitrogen to be cleared before it can form big bubbles.
Saturation diving refers to spending long periods under pressure, such that your blood has lots of dissolved nitrogen (or other gases depending on the depth of operation). You would need to spend a long time decompressing, so instead you stay at high pressure inside a tank on the dive boat. At the end of the job you would decompress carefully.
I won’t go into ~~oxygen~~ *nitrogen* narcosis as I don’t know much about it.
The lungs have a membrane which allows gasses to pass through but not blood. So any gas can freely pass between your blood stream and the air in your lungs. The reason for this is that any oxygen in the air can freely enter the blood stream and be carried away to the cells and any carbon dioxide that have been dissolved into the blood will be released into the air. But the process is not limited to those gasses but any gas in the air will dissolve into the air, although usually in smaller quantities.
When you dive under water you increase the pressure. This means that your lungs need more air to fill the same volume which increase the amount of oxygen and nitrogen in your lungs. Oxygen is actually toxic in large concentrations and if the concentration is high enough even nitrogen is toxic. So divers usually change the mixture and might even use another gas like helium to pressurize with. But all these gasses will dissolve into the blood stream. And since there is much more gas in the high pressures there will be more gasses dissolving into your blood. This is not a big problem as it does not really take up any space in the blood stream and is relatively harmless. The problem is when you try to ascend back to normal pressures. A commonly used demonstration is when you open a pressurized soda bottle and see all the bubbles form in the soda as the dissolved gasses comes out of the solution. This is pretty accurate as to what is happening in your blood if you ascend too fast. The solution to this is to spend more time when heading for the surface so that the dissolved gasses will slowly get out of your blood stream through the lungs.
The problem to this is that it takes a lot of time. Especially if you have been working at deapths for some time and allowed a lot of gasses to dissolve into your blood. If you go deep enough you will often spend more time and oxygen ascending then actually working on the bottom. The solution to this is satturation diving. You use a diving bell which can keep the pressure at the same level as the deapths you are working at. But when the diving bell is closed it can be raised to the surface and even connected to other pressurized habitats where the divers will be living with their blood satturated with gasses under pressure at all times. This allows a team of divers relative comfort and the ability to go to and from the work site without having to spend a lot of time depressurizing between dives. These dives often takes place far out to sea so when the work is done they have a lot of time as they head back to land when they can slowly release the pressure in the habitat so the gasses that is dissolved in the divers blood have plenty of time to get out throug the lungs. It is a very dangerous job though as any issues with the equipment might release all the pressure at once killing the divers either by the strong winds created and related debris or by having all the dissolved gasses in their blood release from the blood. So today most deep sea dives is done exclusively using robots.
Your blood and tissues absorb nitrogen at any altitude where there’s air pressure. As you descend in the water, pressure increases. Correspondingly, your blood and tissues are able to and do absorb more nitrogen as the pressure increases. It takes time for the nitrogen to accumulate (nitrogen saturation) in your system. As you ascend, your blood loses its ability to hold the nitrogen. It can come out of solution much the same way that a soda can releases its CO2 when it’s opened and the pressure is released. If you ascend slowly, the Nitrogen is gassed off and removed through respiration. If you ascend too quickly, the nitrogen bubble can form too quickly and end up in the tissues or bloodstream. This can cause a stroke, heart attack, or bends.
There’s not a problem going down because there’s little harm while the body accumulates the nitrogen. The problem is when you ascend. If you dive down from the surface, you need to ascend (come back up) slowly on return to the surface. If you plan to stay at depth under pressure (“In a habitat for instance), then there’s not a limit to your dive time because you’re not returning to the surface. When you do finally return to sea level, you’d need to ascend slowly to safely off-gas the nitrogen.
There is a condition known as Nitrogen Narcosis. Some people experience symptoms akin to those caused by Nitros Oxide at depth. This can be a very dangerous condition in that you’re underwater in an altered mental state.
Below 100 feet, there’s a risk of oxygen toxicity due to increased oxygen saturation. Deep divers use a special mixture of air to avoid this (nitrox)
Decompression occurs because more nitrogen dissolves in your blood at high pressures. When you start coming up, the nitrogen comes out of your blood as bubbles. If you do it slowly, the nitrogen can come out through your lungs like normal. If you come up too fast, the bubbles don’t have time to reach the lungs before they start collecting in your joints. This puts pressure on your joints and causes pain, known as “the bends” or decompression sickness.
Think of a can of Pringles. If you take it up to a mountain top too quickly, the cap pops open because the pressure from when it was filled with air at sea level is much higher than the air pressure at the mountain top. This is because air sits on air, so it’s thickest and heaviest at the lowest altitudes.
When you go diving, the same thing happens. The water pressure is much higher when you go lower, so the air that goes into your lungs is at a much higher pressure than your lungs would normally be at in order to expand to their regular size for taking a breath.
So it’s not the high pressures that hurt your lungs (to a point) but the changes in pressure while you have air in your lungs. You don’t want to ascend or descend too quickly and have your lungs explode/collapse as a result, like the can of Pringles on the mountain top.
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