Rules of th Deep

The effects of pressure
Being underwater means being under pressure, and that pressure increases rapidly as you dive deeper. That means that the various gas laws come into play, and those laws can impact the "function of living organisms and their parts," i.e. YOU. So let's quickly revisit those laws:

Boyle's law says that as gas pressure increases... , gas volume decreases. Charles' law says that with constant pressure, gas volume increases and decreases with its temperature. Dalton adds that in a mix of gasses, the total gas pressure is equal to the sum of their individual pressures. And Henry, finally, figured that the amount of gas that will dissolve in a liquid is a function of its partial pressure and how easily the liquid absorbs gas.

The most important issue here is that, in accordance with Henry's law, the increasing pressure correlates to increased absorption of nitrogen. Once the pressure decreases as you ascend, it takes that nitrogen time to get safely released from the body. If we ascend too quickly, the nitrogen bubbles can get too large to safely be eliminated through breathing, and that can have dire consequences. We'll discuss all this in some detail.

Unfortunately, there's more. Other indirect effects of underwater pressure include shallow water blackout, oxygen toxicity, carbon monoxide toxicity, nitrogen narcosis, and full blown decompressions sickness.
Shallow Water Blackout
This is an odd but dangerous syndrome based on the little-known fact that it is not the lack of oxygen that creates in us the urge to breathe, but the build-up of carbon dioxide. Normally, as the oxygen level in the blood falls, the carbon dioxide level rises and we feel a need to breathe. When diving, things can be a different. For example, a breath-hold diver may rapidly take a number of deep breaths before diving. That hyperventilation removes carbon dioxide and thus increases the period of time before s/he'll be overcome with the urge to breathe again. Once underwater, the diver may use up oxygen without feeling a need to breathe, and eventually s/he may black out from insufficient oxygen in the blood, a state called hypoxia. A similar thing may happen when a breathe-hold diver goes deeper. Upon ascent, the oxygen pressure drops below the level to sustain consciousness. This is called deep water blackout.