Suction is the force that a partial vacuum exerts upon a solid, liquid, or a gas.[better source needed] 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.
When the pressure in one part of a system is reduced relative to another, the fluid in the higher pressure region will exert a force relative to the region of lowered pressure. 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.