Role of the Intercostal Muscles & Diaphragm
The movement of air is governed by the inverse relationship between volume and pressure; as the volume of a sealed container increases, the internal pressure decreases.
Air naturally moves from an area of higher pressure (the atmosphere) to an area of lower pressure (the lungs) until equilibrium is reached.
To inhale, the body must expand the thorax to drop internal pressure below atmospheric pressure ().
To exhale, the body must compress the thorax to raise internal pressure above atmospheric pressure ().
| Feature | Inhalation | Normal Exhalation | Forced Exhalation |
|---|---|---|---|
| Diaphragm | Contracts (Flattens) | Relaxes (Domes) | Relaxes (Domes) |
| External Intercostals | Contract | Relax | Relax |
| Internal Intercostals | Relax | Relax | Contract |
| Rib Movement | Up and Out | Down and In | Pulled Down/In Harder |
The "Pull" Rule: Always remember that muscles can only pull, never push. Therefore, inhalation requires active pulling by the diaphragm and external intercostals.
Sequence Logic: When describing ventilation, always follow the sequence: Muscle Action Volume Change Pressure Change Air Movement.
Synonym Awareness: Be prepared to use "Inspiration" for inhalation and "Expiration" for exhalation, as these terms are used interchangeably in exams.
Common Error Check: Ensure you do not confuse the internal and external intercostals; the External muscles are for breathing In (External = Entrance), while Internal are for forced breathing Out.