Breathing is an active process ■ To inha|e Contraction of external intercostal 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 Air OutBreathing is an active process ◼ To inhale – Contraction of external intercostal 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