Inspiration is initiated by a stimulus in the respiratory center of the brain.
The signal is transmitted to the diaphragm via the phrenic nerve. The impulse causes the diaphragm to contract or flatten. This causes intrapulmonic pressure to fall below atmospheric pressure and air is drawn into the lungs like a vacuum. The ribs elevate and expand, the alveoli inflate, and oxygen and carbon dioxide diffuse across the membrane. During expiration, the stretch receptors in the lungs signal the respiratory center of the brain to inhibit the inspiration impulse.