▁vol itional ▁effort ▁what so ever , ▁but ▁occurs ▁whenever ▁a ▁m amm al ▁is ▁in ▁a ▁natural ▁state ▁of ▁relax ation , ▁i . e . ▁when ▁there ▁is ▁no ▁clear ▁and ▁present ▁danger ▁in ▁their ▁environment . ▁When ▁a ▁m amm al ▁perce ives ▁potential ▁danger , ▁e up nea ▁stops , ▁and ▁a ▁much ▁more ▁limited ▁and ▁labor ed ▁form ▁of ▁breath ing ▁– ▁shall ow ▁breath ing ▁– ▁occurs . ▁▁ ▁E up nea ▁is ▁an ▁efficient ▁and ▁effective ▁form ▁of ▁breath ing , ▁which ▁maxim izes ▁air ▁int ake , ▁and ▁minim izes ▁mus cular ▁effort . ▁ ▁During ▁e up nea , ▁neural ▁output ▁to ▁resp ir atory ▁mus cles ▁is ▁highly ▁regular , ▁with ▁rh yth mic ▁burst s ▁of ▁activity ▁during ▁insp iration ▁only ▁to ▁the ▁dia ph rag m ▁and ▁external ▁inter cost al ▁mus cles . ▁ ▁Et ym ology ▁and ▁pron unci ation ▁The ▁word ▁e up nea ▁uses ▁combining ▁forms ▁of ▁eu - ▁+ ▁- p nea , ▁from ▁Greek ▁e up no ia , ▁from ▁eu -, ▁" well " ▁+ ▁p no ia , ▁" bre ath ". ▁ ▁See ▁pron unci ation ▁information ▁at ▁d ys p nea . ▁ ▁See ▁also ▁ ▁List ▁of ▁terms ▁of ▁lung ▁size ▁and ▁activity ▁ ▁Res p ir atory ▁rate ▁ ▁T ach yp nea ▁ ▁Brad yp nea ▁ ▁Ap nea ▁ ▁Foot notes ▁ ▁References ▁ ▁Category : Res p ir atory ▁system <0x0A> </s> ▁Verm ill ion ▁is