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1. No signals are sent from the respiratory centre in the brainstem so the muscles are not stimulated to contract – therefore, they relax.
2. The diaphragm relaxes, as do the external intercostal muscles.
3. Remember that at this stage, the lungs have just been filled with air from the atmosphere during inspiration. However, the entry of this air has caused an increase in the alveolar pressure - to the extent that the alveolar pressure is greater than the atmospheric pressure. A pressure gradient now exists and this, combined with the relaxed muscles and the elastic recoil of the alveoli, causes air to rush out of the lungs.
4. By the end of expiration, the alveolar pressure has retuned to equal atmospheric pressure.
5. Inspiration can now begin again, with the contraction of the diaphragm.