Humans Exhibit Negative Pressure Breathing
Mammals ventilate their lungs by negative pressure breathing. The thoracic cavity acts like a suction pump, pulling air down into the lungs rather than pushing it in from above. During inhalation, the rib muscles and diaphragm contract. The volume of the thoracic cavity and lungs increases because the diaphragm moves down the rib cage expands upward and outward. It creates a negative pressure in the lungs causing it to inflate. Exhalation occurs when the rib muscles and diaphragm relax. The relaxation of rib muscles and diaphragm restores the thoracic cavity to its smaller volume.