Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Inhalation
What is Inhalation?
The phase of ventilation in which air moves into the lungs. The external intercostal muscles contract while the internal intercostal muscles relax, so the ribs are pulled upwards and outwards. At the same time the muscle of the diaphragm contracts, so the diaphragm flattens and moves downward. Both movements increase the volume of the thoracic cavity, and because the same quantity of air now occupies a larger space, the pressure inside the lungs falls below atmospheric pressure. Air then moves into the lungs down that pressure difference. Inhalation is therefore an active process, requiring muscular contraction, and the air is drawn in by the pressure difference rather than pulled in by the lungs themselves.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.