The lungs are bounded on their sides, front, and rear by the ribs. The inside of the thoracic cavity is lined by what is called a pleura , which also forms an outer lining of the lungs. It secretes a lubricating fluid which allows the lungs to slide on the inner thoracic cavity walls as they expand and contract during breathing. The rubbing of inflamed pleurae can be very painful. Inflamed pleura can also secrete enough pleural fluid to put pressure on the lungs, and restrict breathing.
Quiet expiration is primarily passive.
The relaxing inspiratory muscles drop the rib cage under the force of gravity, the diaphragm moves up, and elastic fibers in the lungs recoil, which decreases the size of the chest cavity and the lungs - forcing air out. On the other hand, forced expiration is caused mainly by contraction of the abdominal muscles transversus abdominis and obliques . These contractions decrease chest cavity volume, and out goes the air. A unique pons breathing function is to limit inspiration, thereby preventing over inflation of the lungs. Another pons function is to cause breath holding.