Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Exhalation
What is Exhalation?
The phase of ventilation in which air moves out of the lungs. In quiet breathing it is a passive process: the external intercostal muscles relax so the ribs move downwards and inwards under their own weight and the elastic recoil of the tissues, and the diaphragm relaxes and returns to its dome shape. Both changes decrease the volume of the thoracic cavity, so the pressure inside the lungs rises above atmospheric pressure and air moves out down that pressure difference. In forced exhalation, such as during coughing or vigorous exercise, the internal intercostal muscles also contract to pull the ribs downwards and inwards more strongly and more quickly, so that air is expelled faster and in greater volume.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Common mistakes with Exhalation
- Antagonistic, not identical. The external and internal intercostals work against each other, like the biceps and triceps at the elbow. Externals raise the ribs; internals lower them. In quiet breathing only the externals do any work — quiet exhalation is caused by muscles relaxing, not by the internals contracting.
Questions students ask about Exhalation
Do the internal intercostal muscles work during every breath out?
No. In quiet breathing, exhalation happens because the external intercostal muscles and the diaphragm relax and the tissues recoil, so the internal intercostals stay relaxed too. They contract only during forced exhalation, such as coughing, blowing hard or breathing out during vigorous exercise, when they pull the ribs downwards and inwards more strongly.