aries . ▁Ac ross ▁the ▁memb rane ▁o xygen ▁is ▁diff used ▁into ▁the ▁cap ill aries ▁and ▁carbon ▁dio x ide ▁released ▁from ▁the ▁cap ill aries ▁into ▁the ▁al ve oli ▁to ▁be ▁bre athed ▁out . ▁ ▁Al ve oli ▁are ▁particular ▁to ▁m amm al ian ▁l ungs . ▁D ifferent ▁structures ▁are ▁involved ▁in ▁gas ▁exchange ▁in ▁other ▁verte br ates . ▁ ▁Str ucture ▁▁ ▁The ▁al ve oli ▁are ▁located ▁in ▁the ▁al ve olar ▁sac s ▁of ▁the ▁l ungs ▁in ▁the ▁pul mon ary ▁lo b ules ▁of ▁the ▁resp ir atory ▁zone , ▁representing ▁the ▁smallest ▁functional ▁units ▁in ▁the ▁resp ir atory ▁tract . ▁They ▁are ▁also ▁present ▁in ▁the ▁resp ir atory ▁bron chi oles ▁as ▁scattered ▁out po ckets , ▁extending ▁from ▁their ▁lum ens . ▁The ▁resp ir atory ▁bron chi oles ▁lead ▁into ▁al ve olar ▁du ct s ▁which ▁are ▁deeply ▁l ined ▁with ▁al ve oli . ▁Each ▁resp ir atory ▁bron chio le ▁gives ▁rise ▁to ▁between ▁two ▁and ▁eleven ▁al ve olar ▁du ct s . ▁Each ▁du ct ▁opens ▁into ▁five ▁or ▁six ▁al ve olar ▁sac s ▁into ▁which ▁clusters ▁of ▁al ve oli ▁open . ▁New ▁al ve oli ▁continue ▁to ▁form ▁until ▁the ▁age ▁of ▁eight ▁years . ▁▁ ▁A ▁typical ▁pair ▁of ▁human ▁l ungs ▁contain ▁about ▁ 3 0 0 ▁million ▁al ve oli , ▁producing ▁ ▁of ▁surface ▁area . ▁Each ▁al ve ol us ▁is ▁wrapped