B. Respiratory cycle To inhale normally, the external intercostals contract , lifting the rib cage up and out. The diaphragm contracts , losing its domed shape and flattening out. The overall result of these two actions is an increase in the volume of the thoracic cavity. This results in a lower intrathoracic pressure than atmospheric pressure . Gases move from areas of high to low pressure.
Air moves down the respiratory passages and the lungs expand.
In a normal exhalation, all of the actions are passive . The diaphragm relaxes , returning to the domed shape. The external intercostals relax , lowering the rib cage. Theses actions result in a decrease in the volume of the thoracic cavity, leading to an intrathoracic pressure which is greater than atmospheric pressure. The internal intercostals forcefully pull the rib cage down and the abdominal muscles contract, increasing intra-abdominal pressure pushing upward on the diaphragm.