▁and ▁under ▁the ▁n ails . ▁▁ ▁During ▁development ▁of ▁the ▁baby , ▁the ▁inter at rial ▁sept um ▁develop s ▁to ▁separate ▁the ▁left ▁and ▁right ▁at ria . ▁However , ▁a ▁hole ▁in ▁the ▁sept um ▁called ▁the ▁for amen ▁ov ale ▁allows ▁blood ▁from ▁the ▁right ▁at rium ▁to ▁enter ▁the ▁left ▁at rium ▁during ▁f etal ▁development . ▁This ▁opening ▁allows ▁blood ▁to ▁by pass ▁the ▁non function al ▁f etal ▁l ungs ▁while ▁the ▁fet us ▁obtain s ▁its ▁o xygen ▁from ▁the ▁pla cent a . ▁A ▁layer ▁of ▁t issue ▁called ▁the ▁sept um ▁sec und um ▁acts ▁as ▁a ▁val ve ▁over ▁the ▁for amen ▁ov ale ▁during ▁f etal ▁development . ▁After ▁birth , ▁the ▁pressure ▁in ▁the ▁right ▁side ▁of ▁the ▁heart ▁dro ps ▁as ▁the ▁l ungs ▁open ▁and ▁begin ▁working , ▁causing ▁the ▁for amen ▁ov ale ▁to ▁close ▁entirely . ▁In ▁about ▁ 2 5 % ▁of ▁adult s , ▁the ▁for amen ▁ov ale ▁does ▁not ▁entirely ▁se al . ▁In ▁these ▁cases , ▁any ▁elev ation ▁of ▁the ▁pressure ▁in ▁the ▁pul mon ary ▁circul atory ▁system ▁( due ▁to ▁pul mon ary ▁hy pert ension , ▁tempor arily ▁while ▁c ough ing , ▁etc .) ▁can ▁cause ▁the ▁for amen ▁ov ale ▁to ▁remain ▁open . ▁ ▁Types ▁ ▁The ▁six ▁types ▁of ▁at rial ▁sept al ▁defect s ▁are ▁differenti ated ▁from ▁each ▁other ▁by ▁whether ▁they ▁involve ▁other ▁structures ▁of ▁the ▁heart ▁and ▁how ▁they