The amount of any gas that can be dissolved in a liquid depends on the partial pressure of the gas over the solution and the nature and temperature of the liquid. If you increase the pressure of the gas, more gas will dissolve in solution. As for the nature of the solvent, water dissolves a different amount of gas than mineral or cooking oil does. If you increase the temperature of the liquid, less gas will dissolve. Another factor relevant to scuba diving is time: The longer you are at a given depth (pressure), the more nitrogen will dissolve in solution.
Underwater, your body must deal with two major issues: pressure and temperature. Pressure affects the amount of nitrogen and oxygen gases that dissolve in your blood and tissues. Pressure also affects your ears and sinuses. The ability of water to absorb your body heat can lower your body temperature and put you at risk for hypothermia.
Problems: Dissolved Gases Under Pressure
The air we breathe is a mixture of mostly nitrogen (78 percent) and some oxygen (21 percent). When you inhale air, your body consumes the oxygen, replaces some of it with carbon dioxide and does nothing with the nitrogen. At normal atmospheric pressure, some nitrogen and oxygen is dissolved in the fluid portions of your blood and tissues. As you descend under the water, the pressure on your body increases, so more nitrogen and oxygen dissolve in your blood. Most of the oxygen gets consumed by your tissues, but the nitrogen remains dissolved. Increased nitrogen pressure has two problematic effects on your body: nitrogen narcosis and residual nitrogen.
First, when the nitrogen partial pressure reaches high levels, usually those experienced when you reach depths of about 100 ft (30 m) or more, you experience a feeling of euphoria called nitrogen narcosis. The feeling of euphoria is like that experienced when a dentist or anesthesiologist gives you nitrous oxide (laughing gas). Nitrogen narcosis can impair your judgement and make you feel relaxed or even sleepy -- meaning you could start to ignore your instruments, your dive buddy and even drown. Narcosis comes on suddenly and without warning, but can be relieved by ascending to a shallower depth because the nitrogen starts to come out of solution as pressure decreases.
Second, the amount of excess nitrogen in your tissues depends on how deep you dive and the amount of time you spend at those depths. The only way that you can rid your body of residual nitrogen, excess nitrogen in your tissues, is to ascend to the surface, which relieves the pressure and allows the nitrogen to come out of solution. If you ascend slowly, the nitrogen comes out of solution slowly. However, once you reach the surface, you still have residual nitrogen in your system, so you must relax before your next dive and give your body time to get rid of the residual nitrogen before you dive again.
In contrast, if you ascend rapidly, the nitrogen comes out of your blood quickly, forming bubbles. It's like opening a can of soda: You hear the hiss of the high-pressure gas and you see the bubbles caused by the gas rapidly coming out of solution. This is what happens in your blood and tissues. When nitrogen bubbles form in your system, a condition known as decompression sickness or "the bends", they block tiny blood vessels. This can lead to heart attacks, strokes, ruptured blood vessels in the lungs and joint pain (one of the first symptoms of decompression sickness is a "tingling" sensation in your limbs).
The best way to avoid decompression sickness is to minimize residual nitrogen by adhering to the "no decompression" depths and bottom times provided by dive tables. If you violate the "no decompression" limits, you have to stay underwater longer, for various times at pre-set depths (determined by dive tables), to allow the nitrogen to come out of your system slowly. This can present problems because you're dealing with a limited air supply; and if you ignore the decompression guidelines, you will suffer "the bends," have to be airlifted to a decompression chamber and be decompressed under emergency medical conditions. It's a life-threatening situation.
We have talked about nitrogen under pressure, but what about oxygen?