oli ▁and ▁the ▁blood ▁in ▁the ▁surrounding ▁cap ill aries . ▁▁ ▁The ▁nucle us ▁of ▁a ▁type ▁I ▁cell ▁occup ies ▁a ▁large ▁area ▁of ▁free ▁cy top las m ▁and ▁its ▁organ elles ▁are ▁cluster ed ▁around ▁it ▁reducing ▁the ▁thick ness ▁of ▁the ▁cell . ▁This ▁also ▁keeps ▁the ▁thick ness ▁of ▁the ▁blood - air ▁bar rier ▁reduced ▁to ▁a ▁minimum . ▁ ▁The ▁cy top las m ▁in ▁the ▁thin ▁portion ▁contains ▁pin oc yt otic ▁ves icles ▁which ▁may ▁play ▁a ▁role ▁in ▁the ▁removal ▁of ▁small ▁partic ulate ▁cont amin ants ▁from ▁the ▁outer ▁surface . ▁In ▁addition ▁to ▁des mos om es , ▁all ▁type ▁I ▁al ve olar ▁cells ▁have ▁occ l ud ing ▁j unction s ▁that ▁prevent ▁the ▁leak age ▁of ▁t issue ▁fluid ▁into ▁the ▁al ve olar ▁air ▁space . ▁ ▁The ▁relatively ▁low ▁sol ub ility ▁( and ▁hence ▁rate ▁of ▁diffusion ) ▁of ▁o xygen , ▁necess it ates ▁the ▁large ▁internal ▁surface ▁area ▁( about ▁ 8 0 ▁square ▁m ▁[ 9 6 ▁square ▁yards ]) ▁and ▁very ▁thin ▁walls ▁of ▁the ▁al ve oli . ▁We aving ▁between ▁the ▁cap ill aries ▁and ▁helping ▁to ▁support ▁them ▁is ▁an ▁ext rac ell ular ▁matrix , ▁a ▁mesh like ▁fabric ▁of ▁el astic ▁and ▁col lagen ous ▁fib res . ▁The ▁col lagen ▁fib res , ▁being ▁more ▁rig id , ▁give ▁the ▁wall ▁firm ness , ▁while ▁the ▁el astic ▁fib res ▁permit ▁expansion ▁and ▁contra