What force moves air in and out of the lungs?

The primary force that moves air in and out of the lungs is the pressure difference created by the diaphragm and intercostal muscles during breathing. When the diaphragm contracts, it expands the thoracic cavity, reducing pressure in the lungs and drawing air in (inhalation). Conversely, when the diaphragm relaxes, the thoracic cavity decreases in size, increasing lung pressure and pushing air out (exhalation). This process is often referred to as the mechanics of ventilation.