▁Development ▁ ▁Res p ir atory ▁bron chi oles , ▁the ▁earliest ▁structures ▁that ▁will ▁contain ▁al ve oli , ▁have ▁formed ▁by ▁ 1 6 ▁weeks ▁of ▁gest ation ; ▁the ▁cells ▁that ▁will ▁become ▁the ▁al ve oli ▁begin ▁to ▁appear ▁at ▁the ▁end ▁of ▁these ▁ ▁bron chi oles . ▁Ar ound ▁week ▁ 2 0 , ▁f etal ▁breath ing ▁movements ▁may ▁begin . ▁Al ve olar ▁sac s ▁are ▁formed ▁at ▁ 3 2 ▁weeks ▁of ▁gest ation , ▁and ▁these ▁air ▁sac s ▁continue ▁to ▁form ▁until ▁ 8 ▁years ▁of ▁age ▁and ▁possibly ▁into ▁the ▁te en age ▁years . ▁ ▁Function ▁ ▁Type ▁I ▁cells ▁ ▁Type ▁I ▁cells ▁ ▁are ▁thin ▁and ▁flat ▁ep ith el ial ▁l ining ▁cells , ▁that ▁form ▁the ▁structure ▁of ▁the ▁al ve oli . ▁They ▁are ▁squ am ous ▁( g iving ▁more ▁surface ▁area ▁to ▁each ▁cell ) ▁and ▁their ▁long ▁cy top las mic ▁extensions ▁line ▁more ▁than ▁ 9 5 % ▁of ▁the ▁al ve olar ▁surface . ▁ ▁Type ▁I ▁cells ▁are ▁involved ▁in ▁the ▁process ▁of ▁gas ▁exchange ▁between ▁the ▁al ve oli ▁and ▁blood . ▁These ▁cells ▁are ▁extremely ▁thin ▁– ▁sometimes ▁only ▁ 2 5   nm ▁– ▁the ▁electron ▁mic ros cope ▁was ▁needed ▁to ▁prove ▁that ▁all ▁al ve oli ▁are ▁l ined ▁with ▁ ▁ep ith el ium . ▁This ▁thin ▁l ining ▁enables ▁a ▁fast ▁diffusion ▁of ▁gas ▁exchange ▁between ▁the ▁air ▁in ▁the ▁al ve