▁of ▁ 4 0   mm ▁at ▁the ▁root . ▁By ▁the ▁time ▁it ▁becomes ▁the ▁asc ending ▁a ort a , ▁the ▁diameter ▁should ▁be ▁< ▁ 3 5 – 3 8   mm , ▁and ▁ 3 0   mm ▁at ▁the ▁arch . ▁The ▁diameter ▁of ▁the ▁desc ending ▁a ort a ▁should ▁not ▁exceed ▁ 2 5   mm . ▁ ▁The ▁arch ▁of ▁the ▁A ort a ▁lies ▁within ▁the ▁medi ast in um . ▁ ▁Development ▁The ▁a ort ic ▁arch ▁is ▁the ▁connection ▁between ▁the ▁asc ending ▁and ▁desc ending ▁a ort a , ▁and ▁its ▁central ▁part ▁is ▁formed ▁by ▁the ▁left ▁ 4 th ▁a ort ic ▁arch ▁during ▁early ▁development . ▁ ▁The ▁du ct us ▁ar ter ios us ▁connect s ▁to ▁the ▁lower ▁part ▁of ▁the ▁arch ▁in ▁fo etal ▁life . ▁This ▁allows ▁blood ▁from ▁the ▁right ▁vent ric le ▁to ▁mostly ▁by pass ▁the ▁pul mon ary ▁vessels ▁as ▁they ▁develop . ▁ ▁The ▁final ▁section ▁of ▁the ▁a ort ic ▁arch ▁is ▁known ▁as ▁the ▁is th mus ▁of ▁a ort a . ▁This ▁is ▁so ▁called ▁because ▁it ▁is ▁a ▁narrow ing ▁( is th mus ) ▁of ▁the ▁a ort a ▁as ▁a ▁result ▁of ▁decre ased ▁blood ▁flow ▁when ▁in ▁fo etal ▁life . ▁As ▁the ▁left ▁vent ric le ▁of ▁the ▁heart ▁increases ▁in ▁size ▁throughout ▁life , ▁the ▁narrow ing ▁eventually ▁dil ates ▁to ▁become ▁a ▁normal ▁size . ▁If ▁this ▁does ▁not