Ventilation works by changing the volume of the thorax, which changes pressure inside the lungs and causes air to move. The diaphragm and intercostal muscles do not directly pull or push air; they move structures around the lungs, creating pressure gradients.
The Mechanism
Air moves from higher pressure to lower pressure. When thoracic volume increases, intrapulmonary pressure falls below atmospheric pressure, so air enters; when thoracic volume decreases, intrapulmonary pressure rises above atmospheric pressure, so air leaves.
The lungs follow movements of the thoracic wall because they are held against it by the pleural membranes and the thin layer of fluid between them.
| Process | Muscle and rib movements | Result |
|---|---|---|
| Inhalation | The diaphragm contracts and flattens. The external intercostal muscles contract, moving the ribs upwards and outwards. | Thoracic volume increases, intrapulmonary pressure decreases, and air enters the lungs. |
| Normal exhalation | The diaphragm and external intercostal muscles relax. The diaphragm becomes dome-shaped, while the ribs move downwards and inwards. | Thoracic volume decreases through elastic recoil, intrapulmonary pressure increases, and air leaves. |
| Forced exhalation | The internal intercostal muscles contract, pulling the ribs downwards and inwards. Abdominal muscles contract, pushing the diaphragm upwards. | Thoracic volume decreases more rapidly, forcing air out. |
A common misconception is that inhalation occurs because the lungs actively “suck in” air. In fact, respiratory muscles create a lower pressure inside the lungs, and atmospheric pressure drives air inward. Another misconception is that breathing and cell respiration are the same: ventilation moves air, whereas cell respiration releases energy from carbon compounds to produce ATP.
Exam Technique
This is SL and HL content in B3.1 Gas exchange. For an “explain” question, link each step explicitly: muscle contraction → rib or diaphragm movement → volume change → pressure change → direction of airflow. State that normal exhalation is mainly passive; do not incorrectly claim that the internal intercostal muscles contract during every exhalation.