How does the decompression in BENDS occur?

[imstillcurious]

More specifically, I am confused about why the increased underwater pressure would have an effect on the internal pressure in a person's body.  Idk maybe I'm an idiot and the answer is too obvious for most people to ask, but hey here I am! 

[Endy0816]

The air you're breathing is compressed when you are underwater and exposed to sea pressure.

As pressure is reduced during ascent it expands again.

Edited August 12, 2023 by Endy0816

[studiot]

1 hour ago, imstillcurious said:

More specifically, I am confused about why the increased underwater pressure would have an effect on the internal pressure in a person's body.  Idk maybe I'm an idiot and the answer is too obvious for most people to ask, but hey here I am! 

 

1 hour ago, Endy0816 said:

The air you're breathing is compressed when you are underwater and exposed to sea pressure.

As pressure is reduced during ascent it expands again.

More specifically it is the nitrogen in the air.
This is why to dive deeper, where there is greater pressure, special gas mixtures without nitrogen are used.

Endy does have the right idea about expansion being the cause of the the trouble so to answer your query more fully, you need to know two things.

1.  As pressure increases more gas dissolves in a liquid (water, blood, cellular fluid etc).
   Dissolved gas is no longer a gas but part of the fluid.
   If the pressure is then reduced the additional dissolved gas returns to being a gas, as Endy says.
    
2. All the fluids in you body have to be at the same pressure as the water at any particular depth or they would be squashed into a smaller space.
   This is not like a submarine where there is a rigid (metal) shell to resist the difference between the external water pressure and the air pressure inside the shell.
    

So the air you breath when diving is at higher pressure than in the open atmosphere.

So more of the gas dissolves in your blood stream.

As you return towards the surface the pressure regulator in your air supply reduces the air pressure to match that of the surrounding water at your depth
(obviously it also increases the air pressure as you dive deeper  -   this is all automatic)

So on return some of the gas emerges in your bloodstream as bubbles, nitrogen being the particular problem.

Your body can only clear these bubbles at a limited rate so if you ascend too fast they build up.

The Bends occur when uncleared bubbles start to acumulate within the rest of the body, particularly around the joints in the skeleton.

This is why you ascend slowly and in stages if you have dived to any significant depth.

 

Does this help?
Feel free to ask for amplification of any point.

 

 

[exchemist]

2 hours ago, imstillcurious said:

More specifically, I am confused about why the increased underwater pressure would have an effect on the internal pressure in a person's body.  Idk maybe I'm an idiot and the answer is too obvious for most people to ask, but hey here I am! 

The solid tissues of your body are flexible, apart from the bones. So your body cannot prevent the ambient pressure from compressing it.

However, since most of your body is made of liquids, mainly water-based, and as liquids are virtually incompressible, what happens is the compression does not deform your body when the pressure goes up. However all the fluids in your body become pressurised, to the same pressure as the ambient environment.

The exception to all this is the body cavity of the lungs, which is filled with gases. These most definitely do reduce a lot in volume when the pressure goes up. So if you were not given compressed air to breathe, your lungs would be collapsed by the water pressure and you would asphyxiate. The pressure regulator in SCUBA gear adjusts the pressure of air you are fed from the cylinder so that it is equal to the pressure of the environment, i.e the water pressure at whatever depth you are. This is also the pressure of all the fluids in your body.

So your blood is at the same pressure as the water and so is the nitrogen in the air you are breathing. At higher pressure your blood dissolves more nitrogen than at atmospheric pressure and if you ascend too fast from great depth it does not have time to come gradually out of solution and be breathe out through the lungs. So you can get bubbles - and then you have a bent diver on your hands.  

Edited August 12, 2023 by exchemist