Soluble and non-soluble gasses
Oxygen poisening to the CNS
Micro-bubbles or supersaturation
Why should a recreational diver lear Nitrox:
- More repeats
- No down side
- More Lucid
Nitrogen is not used by the body, so it can be switched out with Helium.
In basic scuba, everyone learned that the deeper one goes, the more nitrogen the body absorbs. Depth, or rather pressure, is only one factor of the equation. Other factors include solubility, the amount of gas that solid can absorb, permeability, the readiness of a gas to enter a solid, and contact area that, along with permeability determines how much gas is absorbed in a given time for a given pressure.
Solubility: A piece of bread can soak up a lot more water than a piece of cheese. Each part of your body, or tissue, has a different solubility to a given gas. For example, teeth aren't capable of absorbing anywhere near as much nitrogen as fat cells.
Permeability: The tissue in your lungs is designed to be permeable, whereas your teeth are designed to be impermeable. Each tissue in your body, from your muscles to your bones, has different permeability.
Contact Area: Blood does not flow to all parts equally. Teeth and fat cells have just a trickle compared to muscles and lungs. The amount of blood contact to a tissue, along with that tissues permeability, determine how quickly that tissue absorbs gas.
Your lungs are in constant and considerable contact with your breathing gas. Therefore your lungs are instantly permeated to the point of saturation. A tissue that has absorbed all the gas it can carry is called saturated. Your blood is in significant contact with your lungs, so, in general, your blood is very quickly saturated. Your bones and muscles are in somewhat major contact with your blood, but it takes the gas a little while longer saturate these tissues because they are less permeable. Every tissue has a different rates of saturation from the instantaneous lungs to slow-going fat cells; which have fairly minimal blood contact.
Because nitrogen is very soluble in fat, it can soak up a whole heap of nitrogen. However, fat cells have minimal blood contact and poor permeability. Therefore fat is usually the last bits of you to become saturated. For these same reasons, fat is also the slowest to release gas.
The solubility, permeability, and blood contact for a given tissue, say a kidney, is called a "tissue compartment." Dive computers use these "tissue compartments" to simulate the effect of a gas on your body at depth. The more "tissue compartments" a computer measures, the more accurate (theoretically) the computer can gauge the effect of a dive on your body. 9 to 12 compartments is fairly standard, but a few consumer models have 16 or more.
When technical divers need to go down very deep, they mix in Helium to their breathing gas which is much more soluble than Nitrogen. Helium allows much more rapid decompression.