Blood returns to the heart in venules and veins which will give rise to the pulmonary veins. 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.