olar ▁walls ▁there ▁are ▁inter connect ing ▁air ▁pass ages ▁between ▁the ▁al ve oli ▁known ▁as ▁the ▁por es ▁of ▁K ohn . ▁The ▁al ve oli ▁contain ▁some ▁col lagen ▁fib ers ▁and ▁el astic ▁fib ers . ▁The ▁el astic ▁fib res ▁allow ▁the ▁al ve oli ▁to ▁stretch ▁when ▁they ▁fill ▁with ▁air ▁during ▁in hal ation . ▁They ▁then ▁spring ▁back ▁during ▁ex hal ation ▁in ▁order ▁to ▁exp el ▁the ▁carbon ▁dio x ide - rich ▁air . ▁ ▁There ▁are ▁three ▁major ▁types ▁of ▁al ve olar ▁cell . ▁Two ▁types ▁are ▁p neum ocy tes ▁or ▁p neum on ocy tes ▁known ▁as ▁type ▁I ▁and ▁type ▁II ▁cells ▁found ▁in ▁the ▁al ve olar ▁wall , ▁and ▁a ▁large ▁ph ag oc yt ic ▁cell ▁known ▁as ▁an ▁al ve olar ▁macro ph age ▁that ▁moves ▁about ▁in ▁the ▁lum ens ▁of ▁the ▁al ve oli , ▁and ▁in ▁the ▁connect ive ▁t issue ▁between ▁them . ▁Type ▁I ▁cells , ▁also ▁called ▁type ▁I ▁p neum ocy tes , ▁or ▁type ▁I ▁al ve olar ▁cells , ▁are ▁squ am ous , ▁thin ▁and ▁flat ▁and ▁form ▁the ▁structure ▁of ▁the ▁al ve oli . ▁Type ▁II ▁cells , ▁also ▁called ▁type ▁II ▁p neum ocy tes ▁or ▁type ▁II ▁al ve olar ▁cells , ▁release ▁pul mon ary ▁sur fact ant ▁to ▁lower ▁ ▁surface ▁t ension , ▁and ▁can ▁also ▁differenti ate ▁to ▁replace ▁dam aged ▁type ▁I ▁cells . ▁