▁simple ▁squ am al ▁ep ith el ial ▁cells ; ▁the ▁tun ica ▁media , ▁composed ▁of ▁smooth ▁cell ▁mus cles ▁and ▁el astic ▁fib ers ; ▁and , ▁the ▁tun ica ▁advent it ia , ▁composed ▁of ▁loose ▁col lagen ▁fib ers . ▁Inner v ated ▁by ▁bar ometric ▁n erve ▁terminal s , ▁the ▁a ort ic ▁arch ▁is ▁responsible ▁for ▁sens ing ▁changes ▁in ▁the ▁d ilation ▁of ▁the ▁v asc ular ▁walls , ▁indu cing ▁changes ▁in ▁heart ▁rate ▁to ▁compens ate ▁for ▁changes ▁in ▁blood ▁pressure . ▁ ▁The ▁a ort a ▁begins ▁at ▁the ▁level ▁of ▁the ▁upper ▁border ▁of ▁the ▁second ▁stern oc ost al ▁art ic ulation ▁of ▁the ▁right ▁side , ▁and ▁runs ▁at ▁first ▁up ward , ▁back ward , ▁and ▁to ▁the ▁left ▁in ▁front ▁of ▁the ▁tra che a ; ▁then ▁travel s ▁back ward ▁on ▁the ▁left ▁side ▁of ▁the ▁tra che a ▁and ▁finally ▁passes ▁down ward ▁on ▁the ▁left ▁side ▁of ▁the ▁body ▁of ▁the ▁fourth ▁thor ac ic ▁verte bra . ▁At ▁this ▁point ▁the ▁a ort ic ▁arch ▁continues ▁as ▁the ▁desc ending ▁a ort a . ▁ ▁The ▁a ort ic ▁arch ▁has ▁three ▁branches . ▁The ▁first , ▁and ▁largest , ▁branch ▁of ▁the ▁arch ▁of ▁the ▁a ort a ▁is ▁the ▁bra chio ce phal ic ▁tr unk , ▁which ▁is ▁to ▁the ▁right ▁and ▁slightly ▁anterior ▁to ▁the ▁other ▁two ▁branches ▁and ▁origin ates ▁behind ▁the ▁man ub rium ▁of ▁the ▁stern