When we breathe in, the outward movement of the chest increases the volume of the lungs and the pressure in them falls below that of the outside world. Therefore, air is drawn in automatically. When we breathe out, some air exits because of the normal elastic recoil of the lungs, but we also force air out by using the muscles of the chest and the diaphragm. Replicating this artificially involves using a device to produce intermittent positive or negative pressure ventilation as described below.