You Are Like a Multi-Layered Sponge:
I suggested that you imagine that you have a scrubby sponge for a reason. The different layers of the sponge absorb water a different rates. For example, the loosely-woven, rough side of the sponge becomes saturated with water quickly, while more time is required for water to leach into the yellow, dense side of the sponge. When drying the sponge, the opposite is true. The scratchy green side dries quickly, while the yellow puffy part takes a bit longer to dry.
Just as the layers of the sponge absorb and release water at different speeds, various parts of a diver's body absorb and release nitrogen at different rates. While some parts of a diver's body can “dry” of nitrogen quickly, other parts remain “wet” with absorbed nitrogen for hours or even days.
Most People Are Merely Damp Sponges:
Now imagine that you bring your yellow and green scrubby sponge to an extremely humid environment, like a steam bath. (Hey, it beats scrubbing the dishes!) In the steam bath, the sponge is always exposed to some water in the air so it is always a bit damp. If some sweaty naked dude picks up the sponge, it isn't going to drip all over the place. The sponge absorbs just enough water from the air to keep it damp.
A person always has a very, very small amount of nitrogen dissolved in his system. This nitrogen comes from the air (which is 78% nitrogen). The miniscule amount of nitrogen in a person's system is normal; the human body naturally holds a certain amount of nitrogen in its tissues and fluids. A person breathes nitrogen in and out with each breath, but the very small amount of nitrogen in his system remains constant. This nitrogen has no adverse effect on his body.
Scuba Divers Are Wet Sponges:
Following our sponge analogy, now imagine that the sponge is submerged in water in slow motion. Bit by bit, water starts to penetrate the sponge. It completely soaks the green part first, and seeps into the yellow portion more slowly. The sponge keeps absorbing water until it is completely soaked and can't hold a drop more. At this point, the sponge is saturated with water.
During a dive, a diver's body absorbs nitrogen in a similar way. The nitrogen already present in his body from the surface and the nitrogen in the tank air are compressed by the increasing water pressure as the diver descends. (Confused? Click here to read about pressure-depth relationships in scuba diving) The increasing pressure forces the nitrogen molecules close together to the point that they occupy less space.
The diver's body fills the space left by by the compression nitrogen by absorbing more nitrogen (also compressed) from the tank air. The diver's body continues absorbing nitrogen until it can hold no more, just like a sponge will keep absorbing water until it is totally saturated. Time is required for a diver to become saturated with nitrogen (usually longer than a recreational dive), but given long enough or deep enough dives, it will happen. Just like the sponge, some parts of the diver's body will become saturated with nitrogen more quickly than others. Remember, any nitrogen a diver absorbs underwater is more than he would normally have in his body on the surface.
Don't Be a Drippy Sponge:
If the sponge is taken out of the water too quickly, it drips all over the place. The absorbed water does not have time to drain out of the sponge. However, if a sponge is removed from the water very slowly, enough of the water drains from the sponge that it does not drip.
Just as a sponge can absorb more water than it can retain on the surface, a diver can end up with more nitrogen in his system than his body can safely hold. On ascent, the compressed nitrogen gas in a diver's body begins to expand, occupying more space. (If you don't understand why gasses expand as a diver ascends, click here.) During a slow ascent, this expanding nitrogen leaves the body's tissues when the tissues no longer have enough space to hold the expanded gas. The nitrogen travels in the blood to the lungs and is released when the diver breathes out.
However, if a diver does not ascend slowly enough to allow sufficient time for his body to remove the expanding nitrogen gas, the nitrogen will form bubbles in the diver's blood and tissues. These bubbles may travel through his arteries and block off blood flow to various parts of the body, or stay in his tissues and cause damage. This phenomena causes decompression sickness.
Sponges Don't Dry Immediately:
Imagine that you remove the sponge from the water very slowly, draining as much water as possible. Even though the sponge doesn't drip, it is still wetter than before it was submerged. Time is required for the small amount of excess water in the sponge to evaporate before it returns to its original “slightly damp” state. The green scrubby part of the sponge will probably reach this state first, and the denser, more absorbent part will reach this state a bit later.
A diver's body works the same way. Even if he ascends slowly enough to avoid decompression sickness, he still has some extra nitrogen in his system when he reaches the surface. After a dive, a scuba diver's body is still working hard to eliminate this excess nitrogen. Some of the tissues return to their pre-dive state quickly, while others can take quite a while to release the extra nitrogen. Depending upon the length and depth of the dive, eliminating all the nitrogen may take many hours or days.
Because a diver's body is eliminating excess nitrogen for some time after a dive, heavy exercise and flying after diving are not recommended. These activities are akin to wringing the sponge on the surface. They can force nitrogen out of the system so quickly that it forms bubbles and leads to decompression sickness.
Sponges Get Soaked Faster if They Are Already Wet:
If a wet and a dry sponge are submerged in water, which becomes saturated faster? The wet sponge, of course! The wet sponge already has water in it, so it doesn't have to absorb as much water to reach a soaked, saturated state.
If a diver makes two dives in a row, he will have more nitrogen in his system than a diver who makes only one dive. On the second dive, the diver starts the dive with some nitrogen left over in his system from the first dive. A diver who engages in repetitive dives must account for the extra nitrogen in his system when planning his dive profiles.
In Some Circumstances, Sponges Must Be Squeezed:
If a sponge has absorbed too much water, it may be impossible to remove it from the water slowly enough to avoid drips. In this case, the sponge must be squeezed while it is still underwater. Squeezing the sponge can force enough water out that the sponge will not drip on the surface.
A diver may absorb so much nitrogen that he can not swim straight to the surface without risking decompression sickness, no matter how slowly he ascends. Ways that scuba divers absorb high amounts of nitrogen include engaging in very deep or long dives (with bottom times that exceed the no-decompression limit for a given depth). In this case, a diver must allow his body extra time to eliminate the high amount of nitrogen in his system by making a safety stop or decompression stop (a pause during the ascent at a certain depth for a predetermined amount of time). In technical diving, some divers breathe gas mixtures with a higher than normal ratio of oxygen to nitrogen. This is akin to squeezing the sponge. It helps the body to eliminate nitrogen more quickly than normal, and shortens the length of required decompression stops.