BOYLE'S LAW: For any gas at a constant temperature, the volume will vary inversely with the absolute pressure, while the density will vary directly with the absolute pressure.

Simply stated it means that as the pressure increases, the volume decreases, and the density increases in proportion to the pressure. At 33 feet or 2 ATM the volume of air in a container will be half its original, while its density is two times the original. The fact that air is compressible, while water is not, can easily be demonstrated. At the surface a diver seals two one-gallon containers, one with water, the other with air. By taking them down with him, he can see the effects of the increasing pressure. As depth and pressure increase the air-filled container will collapse while no changes will be noted in the water-filled container.

The body has air containers in it. The lungs, sinuses, and digestive system are all examples of hollow air containing spaces. It is the job of the regulator to maintain these air spaces, by providing air equal in pressure to the surrounding water. This is to prevent these spaces from being squeezed, as the air-filled container was.

Air - Volume - Density Relationship

Boyle's Law states that as pressure is decreased, the gas expands. To the diver this is very significant. A one gallon container filled at four atmospheres with compressed air will have a surface equivalent of four gallons, since the density is four times greater at four ATM. As the container ascends and if kept closed, it will burst since it could not hold that large of a volume of air. However when the container is left open, the excess air vents off into the surrounding water, and rupture is avoided. Lungs can be closed or open containers. When a diver holds his breath, they are closed, when he, breathes normally, then they are open.

The average lungs hold about 5 liters of air. At 66 feet or 3 ATM, they have a surface equivalent of 3 X 5 or 15 liters. During ascent the lungs can rupture if they do not vent.

When the lungs over pressure and rupture, many problems may ensue, but the worst is called an air embolism. An embolus are some blockages. Therefore, an air embolism is a blockage created by air. Air Embolism is always in the arteries, and labeled ARTERIAL GAS EMBOLUS (AGE). Where the blockage occurs dictates the seriousness of the embolism, because it can cut off the blood supply to any tissue being fed by an artery (all tissues are fed by arteries).

For complications due to pressure to transpire certain conditions must exist:

There must be a gas filled space

It must be rigid or have semirigid walls

It must be enclosed

There has to be a change in ambient pressure

During a free ascent, a diver comes up from depth, on a single breath. He has more than enough air to reach the surface due to Boyle's Law. To prevent lung rupture, divers vent air by exhaling as they free ascend. The natural tendency to hold ones breath all the way up should be over ridden. There are however effects of pressure that are not as obvious. These are the effects of gas mixtures on the body. They affect the tissues and reason rather than air spaces. Luckily these effects are as predictable as the effect of pressure on air spaces. Because these effects can be predicted, divers are given time and depth limits, The Dive Tables. The way these gases affect the body comes from a combination of DALTON'S, and HENRY'S LAW. Dalton's Law deals with the partial pressures of gases in mixtures, and Henry's with solubility of gases in a liquid..