Intermediate 9 minutes
How the body responds to depth
Compression, expansion, gas absorption: the physiological summary of what going down means
Going down is easy: the body tolerates compression fairly well, and all you have to do is equalize the air spaces. It's going up that has to be done carefully, and the reason is that the body spent the entire bottom phase loading up on a gas it now has to release without rushing.
This article gathers and organizes what you've seen in the previous chapters: what happens to each system on descent, how the body absorbs and eliminates inert gas, what effects appear at greater depth, and why the ascent concentrates almost all the physiological risk.
This is the article that closes out the physiology category and connects it to physics, planning, and safety. If you keep just one mental map of what depth does to the body, this is the one.
The air spaces
The first thing the body notices on descent is the compression of its gas-filled cavities.
The body is mostly water, and water is practically incompressible: pressure doesn't affect it appreciably. What does respond to pressure are the spaces that contain gas.
Those spaces are three in the body (the lungs and airways, the middle ear and the paranasal sinuses) and two added by your gear: the volume of air inside the mask and, for anyone using one, the air in a dry suit.
On descent, all of them tend to shrink in volume according to the pressure-volume relationship formalized by Boyle's Law, covered in the diving physics category. The lungs resolve themselves because the regulator delivers gas at ambient pressure; the ear and sinuses require the equalization you saw in their respective articles; and the mask is equalized by exhaling through the nose.
On ascent the opposite happens: all that gas expands, and the problem stops being about getting air in and becomes about letting it out. That's why most pressure-related complications (pulmonary overexpansion, reverse block in the ear and sinuses) belong to the ascent phase.
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