▁The ▁cap ill aries ▁connect ▁to ▁ven ules , ▁and ▁the ▁blood ▁then ▁travel s ▁back ▁through ▁the ▁network ▁of ▁ve ins ▁to ▁the ▁right ▁heart . ▁The ▁micro - cir cul ation ▁— ▁the ▁ar ter i oles , ▁cap ill aries , ▁and ▁ven ules ▁— const it utes ▁most ▁of ▁the ▁area ▁of ▁the ▁v asc ular ▁system ▁and ▁is ▁the ▁site ▁of ▁the ▁transfer ▁of ▁O 2 , ▁gl uc ose , ▁and ▁en zym e ▁substr ates ▁into ▁the ▁cells . ▁The ▁ven ous ▁system ▁returns ▁the ▁de - ox y gen ated ▁blood ▁to ▁the ▁right ▁heart ▁where ▁it ▁is ▁p ump ed ▁into ▁the ▁l ungs ▁to ▁become ▁o xygen ated ▁and ▁CO 2 ▁and ▁other ▁g ase ous ▁was tes ▁ex changed ▁and ▁exp elled ▁during ▁breath ing . ▁Blood ▁then ▁returns ▁to ▁the ▁left ▁side ▁of ▁the ▁heart ▁where ▁it ▁begins ▁the ▁process ▁again . ▁ ▁In ▁a ▁normal ▁circul atory ▁system , ▁the ▁volume ▁of ▁blood ▁returning ▁to ▁the ▁heart ▁each ▁minute ▁is ▁approximately ▁equal ▁to ▁the ▁volume ▁that ▁is ▁p ump ed ▁out ▁each ▁minute ▁( the ▁card iac ▁output ). ▁Because ▁of ▁this , ▁the ▁velocity ▁of ▁blood ▁flow ▁across ▁each ▁level ▁of ▁the ▁circul atory ▁system ▁is ▁primarily ▁determined ▁by ▁the ▁total ▁cross - section al ▁area ▁of ▁that ▁level . ▁ ▁This ▁is ▁mathemat ically ▁expressed ▁by ▁the ▁following ▁equation : ▁▁ ▁v ▁= ▁Q / A ▁where ▁▁ ▁v ▁= ▁velocity ▁( cm / s ) ▁ ▁Q