Mechanical event in breathing
When the diaphragm contracts, it becomes flatter and moves downward. That movement enlarges the thoracic cavity, especially in the vertical dimension. The lungs expand because they are mechanically linked to the thoracic wall through the pleural membranes.
Diaphragm contraction produces inhalation: thoracic volume increases, intrapulmonary pressure decreases below atmospheric pressure, and air flows into the lungs.
Pressure and airflow
Air moves according to pressure differences. As the thoracic cavity enlarges, lung volume increases. At approximately constant temperature, pressure varies inversely with volume, which is commonly summarized as
\[ P \propto \frac{1}{V} \]
The increase in lung volume lowers intrapulmonary pressure. Once intrapulmonary pressure falls below atmospheric pressure, air enters through the airways and fills the lungs. The contracted diaphragm therefore corresponds to inspiration rather than expiration.
Accurate breathing diagram
Direct physiological consequences
| Variable | Change when the diaphragm contracts | Biological outcome |
|---|---|---|
| Diaphragm shape | Flattens and descends | Thoracic cavity enlarges |
| Thoracic cavity volume | Increases | Lungs expand |
| Intrapulmonary pressure | Decreases | Becomes lower than atmospheric pressure |
| Direction of airflow | Into the lungs | Inhalation occurs |
Related muscle action
Diaphragm contraction is often accompanied by contraction of the external intercostal muscles during quiet inspiration. Those muscles elevate the rib cage and increase the transverse and anteroposterior dimensions of the thorax. The diaphragm remains the principal muscle of normal inhalation, but the mechanical effect is amplified by the coordinated movement of the thoracic wall.
Common confusion
Diaphragm contraction is sometimes incorrectly associated with expiration because the diaphragm moves downward. The opposite is true. Downward movement increases internal thoracic space, and that increase in volume is the basis of inhalation. Expiration during quiet breathing usually occurs when the diaphragm relaxes and returns to a more domed shape.
Concise statement
The correct biological statement is that when the diaphragm contracts, it flattens and moves downward, thoracic volume increases, lung pressure falls, and air enters the lungs.