Pressure and volume are inversely related. When the pressure goes up, the volume goes down. When the volume goes up, the pressure goes down. Swimming on the ocean surface, a diver breathes in air by increasing the chest volume, causing lower pressure in the lungs and higher pressure in the air, so the air is "pulled into" the lungs. Breathing out, the diver decreases the volume of the lungs, increasing the pressure of the air in the lungs, and the breath is expelled to the outside air. If the diver holds the breath, and swims down 33 feet, the body is exposed to double the pressure than at sea level, and the air in the diver's lungs is compressed in half. Rising to the surface, the pressure decreases and the air in the lungs expands to its original volume. These inverse relationships can be related by the following formula:
(Pressure 1)(Volume 1) = (Pressure 2)(Volume 2)
which means that the initial conditions (P1V1) will are constant with the final conditions (P2V2). So if the pressure increases, (P2 is greater than P1) the volume (V2) must decrease. If the volume increases (V2 is greather than V1) the pressure (P2) must decrease.
At 33 feet underwater, the pressure on the diver has doubled, and the volume of the lungs has halved. If the diver takes a full breath from a compressed air source, twice the amount of air will enter the lungs than at sea level; breathing out, twice the amount of air will be expelled, but to the diver, it feels like normal, full breathing, just like at sea level. One of the dangers in diving is that if the diver held the breath underwater and swam to the surface, the pressure would return to normal (decrease) but the air in the lungs would still be twice the amount, and the pressure will increase, and the diver could rupture the lungs.