As the thoracic diaphragm contracts and lowers, the vertical diameter of the thoracic cavity is increased.
This increases the volume of the thoracic cavity. Thus, according to Boyle's law, the pressure of the air in the lungs decreases. The relatively higher atmospheric pressure outside pushes the air into the lungs, and the alveoli are inflated.
At the same time, the thoracic diaphragm produces a pistol-like pressure upon the noncompressible fluid continuum in the abdominopelvic cavity. By Pascal's law, the resulting pressure is distributed equally to the elastic walls of the cavity. As these walls are stretched by the added pressure, they "store" potential energy.
Diaphragmatic Exhalation FAQ:
When the thoracic diaphragm relaxes, it no longer pushes down upon the contents of the abdominopelvic cavity. The potential energy stored in the stretched muscular walls becomes kinetic energy, and the walls rebound.
This energy is sufficient for exhalation during quiet breathing. However, during forced breathing, the muscles of the abdominal wall will contract in accordance with the amount of air to be pushed out.
As the muscles in the abdominal wall rebound, pressure is applied to the fluid continuum in the abdominopelvic cavity. By Pascal's law, this pressure is transferred to the underside of the thoracic diaphragm. The relaxed thoracic diaphragm is thus pushed up into the thoracic cavity.
This decreases the vertical diameter and the volume of the thoracic cavity. The decreased volume results in increased pressure within the lungs (Boyle's law).
Since the air pressure in the lungs is relatively greater than the outside atmospheric pressure, air is forced out through the respiratory passageways.