Medical aspirators are small suction machines used to remove mucus and other bodily fluids from a patient's chest. The thoracic cavity (or chest cavity) is the chamber of the human body (and other animal bodies) that is protected by the thoracic wall (thoracic cage and associated skin, muscle, and fascia). These pumps used for pumping or moving fluids typically have an inlet where the fluid enters the pump and an outlet where the fluid comes out. The inlet location is said to be at the suction side of the pump. The outlet location is said to be at the discharge side of the pump. Operation of the pump creates suction (a lower pressure) at the suction side so that fluid can enter the pump through the inlet. Pump operation also causes higher pressure at the discharge side by forcing the fluid out at the outlet. There may be pressure sensing devices at the pump's suction and/or discharge sides which control the operation of the pump. For example, if the suction pressure of a centrifugal pump is too low, a device may trigger the pump to shut off to keep it from running dry; i. e. with no fluid entering.