A draw-over system is one in which the gases move on a 'demand-flow' basis. This means the work of moving the gas in the patient breathing system comes from the patient's own inspiratory effort during spontaneous breathing, or by manual operation of a self-inflating bag (SIB) or bellows during IPPV.

Oxygen from a cylinder or oxygen concentrator is added into a reservoir so that an air/oxygen mixture is drawn through a low resistance vaporizer on the way to the patient.