a) Since there is a higher concentration of carbon dioxide in the blood than in the alveoli, carbon dioxide diffuses into the alveoli.

b) The internal intercostal muscles contract, moving the ribcage downwards and inwards.

c) At the same time, the abdominal muscles contract, pushing the diaphragm upwards towards the lungs.

d) The combination of the latter 2 muscle contractions decrease the volume of the thorax. This increases the pressure of the gases within the lungs.

e) When this pressure is higher inside the lungs than outside the body, air is forced from the lungs to the external air. This outward movement continues until an equilibrium (equal air pressures) is reached between the concentrations of air in the lungs and external air.

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