In what stage of respiration does the exchange of gases between the lungs and the blood occur?

External

Explanation:

External or expiration is brought about by the external intercostal muscles relax while the internal intercostal muscles contract causing the rib cage to move downwards and inwards. At the same time the muscles of the diaphragm relax and the diaphragm regains its original dome shape. These movements decrease the volume of the thoracic cavity and increase the pressure inside it thus air is forced out of the lungs into the atmosphere and gaseous exchange is completed.