Pressure is the first and most important factor that must be overcome for divers to be able to breathe underwater. The pressure on you underwater consists of two forces that act together: the weight of the water and the weight of the atmosphere over 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 or in Newton’s per square meter. Pressure of the atmosphere at sea level can hold up 760 mm of mercury in a vacuum tube. So, at sea level, the pressure is 1 atmosphere (ata), which is equal to 1 bar or 14.7 psi or 760 mm Hg. You are used to living at 1 atmosphere (ata) of pressure, so you rarely take notice of it. You do notice changes of pressure in your ears when swimming down, flying, driving up in the mountains, going in an elevator, etc. This is because your ears or sinuses have air spaces in them; therefore, they are compressible. The gas will compress proportionately to the amount of pressure exerted on it.
According to Boyle’s law, gases can be compressed into a smaller volume. This is what happens when your cylinders are filled and when the air is squeezed together and is taking up less space, increasing the pressure inside the cylinder. The problem comes when we look at your lungs: They do not like to expand and contract very much, so the volume of the lung remains much the same. When you take a full breath of compressed air at 10 meters/33 feet/2 bar, you will breathe twice as much air as you do at the surface due to the increased pressure. But your lungs will remain the same size or volume as normal.
As absolute pressure increases on a gas (with descent), the volume of gas in your lungs will decrease proportionately. For example, when you descend to 10 meters/33 feet, the pressure is twice as much as that on the surface, so the volume of gas would be half. Your lungs cannot work with this, so we need to breathe more air per breath to maintain the volume.