▁in ▁a ▁fine ▁mesh ▁of ▁cap ill aries ▁covering ▁about ▁ 7 0 % ▁of ▁its ▁area . ▁The ▁diameter ▁of ▁an ▁al ve ol us ▁is ▁between ▁ 2 0 0 ▁and ▁ 5 0 0 ▁ μ m . ▁ ▁Micro an atom y ▁▁ ▁The ▁al ve oli ▁consist ▁of ▁an ▁ep ith el ial ▁layer ▁of ▁simple ▁squ am ous ▁ep ith el ium ▁( very ▁thin , ▁fl atten ed ▁cells ), ▁and ▁an ▁ext rac ell ular ▁matrix ▁surrounded ▁by ▁cap ill aries . ▁The ▁ep ith el ial ▁l ining ▁is ▁part ▁of ▁the ▁al ve olar ▁memb rane , ▁also ▁known ▁as ▁the ▁resp ir atory ▁memb rane , ▁that ▁allows ▁the ▁exchange ▁of ▁g ases . ▁The ▁memb rane ▁has ▁several ▁layers ▁– ▁a ▁layer ▁of ▁l ining ▁fluid ▁that ▁contains ▁sur fact ant , ▁the ▁ep ith el ial ▁layer ▁and ▁its ▁bas ement ▁memb rane ; ▁a ▁thin ▁inter st it ial ▁space ▁between ▁the ▁ep ith el ial ▁l ining ▁and ▁the ▁cap ill ary ▁memb rane ; ▁a ▁cap ill ary ▁bas ement ▁memb rane ▁that ▁often ▁f uses ▁with ▁the ▁al ve olar ▁bas ement ▁memb rane , ▁and ▁the ▁cap ill ary ▁end oth el ial ▁memb rane . ▁The ▁whole ▁memb rane ▁however ▁is ▁only ▁between ▁ 0 . 2 ▁ μ m ▁at ▁its ▁th inn est ▁part ▁and ▁ 0 . 6 ▁ μ m ▁at ▁its ▁thick est . ▁ ▁In ▁the ▁al ve