org es ▁the ▁corpor a ▁ca vern osa , ▁which ▁expand ▁to ▁hold ▁ 9 0 % ▁of ▁the ▁blood ▁involved ▁in ▁an ▁ere ction , ▁increasing ▁both ▁in ▁length ▁and ▁in ▁diameter . ▁The ▁function ▁of ▁the ▁cor pus ▁sp ong ios um ▁is ▁to ▁prevent ▁compression ▁of ▁the ▁u re th ra ▁during ▁ere ction . ▁ ▁Blood ▁can ▁leave ▁the ▁ere ct ile ▁t issue ▁only ▁through ▁a ▁d rain age ▁system ▁of ▁ve ins ▁around ▁the ▁outside ▁wall ▁of ▁the ▁cor pus ▁ca vern os um . ▁The ▁expand ing ▁sp ong y ▁t issue ▁press es ▁against ▁a ▁surrounding ▁dense ▁t issue ▁( t un ica ▁al bug inea ) ▁con str ict ing ▁these ▁ve ins , ▁prevent ing ▁blood ▁from ▁leaving . ▁The ▁pen is ▁becomes ▁rig id ▁as ▁a ▁result . ▁The ▁gl ans ▁pen is , ▁the ▁expanded ▁cap ▁of ▁the ▁cor pus ▁sp ong ios um , ▁remains ▁more ▁m alle able ▁during ▁ere ction ▁because ▁its ▁tun ica ▁al bug inea ▁is ▁much ▁th inner ▁than ▁elsewhere ▁in ▁the ▁pen is . ▁ ▁Add itional ▁images ▁ ▁References ▁ ▁External ▁links ▁▁ ▁- ▁" The ▁Male ▁Per ine um ▁and ▁the ▁Pen is : ▁Pen is " ▁ ▁Category : M amm al ▁male ▁re product ive ▁system ▁Category : H uman ▁pen is ▁an atom y <0x0A> </s> ▁The ▁Women ' s ▁ 8 0 0 m ▁athlet ics ▁events ▁for ▁the ▁ 2 0 1 6 ▁Summer ▁Par al ym