Pressure is one of the most important physical factors to understand as a scuba diver. The pressure on you underwater consists of two forces that act together: the weight of the water and the weight of the surface atmosphere acting on the surface of the water.
You do not feel much pressure underwater because you are made up mostly of liquids that do not compress. However, they are affected where pressure is exerting a force on any gas cavities.
Pressure can be expressed in pounds per square inch, bar, or atmospheres. At sea level, the pressure is 1 atmosphere (ata), which is approximately equal to 1 bar or 14.7 psi or 760 mm Hg (mercury). You are used to living at 1 atmosphere (ata) of pressure, so you rarely take notice of it. You will notice changes of pressure in your ears when swimming deeper, flying, or driving up in the mountains. This is because your ears and sinuses have air spaces in them; therefore, they are compressible. The gas will compress or expand proportionately to the changes in the pressure exerted on it.
Gas can be compressed. This is wonderful for divers because breathing gases (air, nitrox, etc.) can be squeezed into the diving cylinders so that divers can take a huge volume of air on their dives in a much smaller container. Divers need to understand that as gas is compressed into less space, the pressure inside the space—a scuba cylinder, for example—increases proportionally. This relationship is described by Boyle’s Law.
A potential pressure-related problem comes when you consider your lungs. If you were to take a lungful of compressed gas at depth and were to hold your breath and start to surface, your lungs would overexpand and rupture as the air expanded. This is why divers are told from the very first scuba lesson to never hold their breath when diving. Boyle’s Law is behind the mechanics of lung overexpansion.
Let's say a diver takes a full breath of compressed air at 10 meters/33 feet/2 bar. There would be twice as much pressure on his or her body at that depth as there is on the surface. To overcome that increased pressure and to fill the lungs, the air would have to be twice the density. Scuba regulators help a diver to do this.
As the absolute pressure in the water (the ambient pressure) increases or decreases when you descend or ascend, the volume of gas in your lungs would increase or decrease proportionately. This means that if pressure increases, the volume of gas in your lungs decreases. And so, you have to breathe out to expel the expanding gas as you surface. Grasping this concept is vitally important to you when you are diving.