Breathing is an active process – requiring the contraction of skeletal muscles. The primary muscles of respiration include the external inter costal muscles (located between the ribs) and the diaphragm (a sheet of muscle located between the thoracic & abdominal cavities).
The external inter costals plus the diaphragm contract to bring about inspiration. To inhale:
Contraction of external inter costal muscles elevation of ribs & sternum > increased front- to-back dimension of thoracic cavity > lowers air pressure in lungs > air moves into lungs
Contraction of diaphragm > diaphragm moves downward > increases vertical dimension of thoracic cavity > lowers air pressure in lungs > air moves into lungs.
The external inter costals plus the diaphragm relax to bring about exhale. To exhale:
Relaxation of external inter costal muscles & diaphragm > return of diaphragm, ribs, & sternum to resting position > restores thoracic cavity to pre inspiratory volume > increases pressure in lungs > air is exhaled