suction suction suction is the air pressure differential between areas. removing air from a space results in a pressure differential. suction pressure is therefore limited by external air pressure. even a perfect vacuum cannot suck with more pressure than is available in the surrounding environment. suctions can form on the sea, for example, when a ship founders. when the pressure in one part of a physical system is reduced relative to another, the fluid in the higher pressure region will exert a force relative to the region of lowered pressure, referred to as pressure-gradient force. pressure reduction may be static, as in a piston and cylinder arrangement, or dynamic, as in the case of a vacuum cleaner when air flow results in a reduced pressure region. when animals breathe, the diaphragm and muscles around the rib cage cause a change of volume in the lungs. the increased volume of the chest cavity decreases the pressure inside, creating an imbalance with the ambient air pressure, resulting in suction.