Breathing in is active work done mainly by the diaphragm. Quiet breathing out is mostly passive, as the lungs and chest wall recoil.
Diaphragm
The diaphragm is a dome-shaped muscle that flattens when it contracts, enlarging the chest and drawing air in. It is supplied by the phrenic nerve from C3 to C5, so a high spinal cord injury can stop breathing.
Intercostals
The external intercostals lift the ribs during inhalation. The internal intercostals help pull the ribs down during forced exhalation.
Accessory muscles
In respiratory distress, the sternocleidomastoid, scalenes and pectoral muscles lift the chest further. The abdominal muscles push air out forcefully during coughing or when the airways are narrowed.
Muscle fatigue
Breathing muscles can tire like any other muscle. Rising effort followed by slower, weaker breathing is a sign of impending respiratory failure.
| Muscle | Phase | When used |
|---|---|---|
| Diaphragm | Inhalation | Every breath |
| External intercostals | Inhalation | Every breath |
| Sternocleidomastoid, scalenes | Inhalation | Distress |
| Abdominals, internal intercostals | Exhalation | Cough, forced exhalation |
At the bedside
Look for accessory muscle use, retractions and abdominal paradox when you assess breathing. These signs show a high work of breathing even before oxygen saturation falls.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).