When the diaphragm relaxes, it regains its domed shape. This action causes an increase in thoracic pressure which forces air out of the lungs. To have air forced out of the lungs, the pressure in the thorax must rise above _______________ pressure?

For air to be pushed out of the lungs, the trans thoracic pressure must rise above atmospheric (barometric) pressure. The diaphragm will relax and dome upward and the intercostal muscles will contract and pull the rib cage inward. This action increases the trans thoracic pressure above atmospheric pressure which forces air out.