▁smaller ▁radius ▁and ▁lowest ▁velocity ▁compared ▁to ▁other ▁vessels , ▁the ▁Reyn old ' s ▁number ▁at ▁the ▁cap ill aries ▁is ▁very ▁low , ▁resulting ▁in ▁l amin ar ▁instead ▁of ▁tur bul ent ▁flow . ▁ ▁Vel ocity ▁O ften ▁expressed ▁in ▁cm / s . ▁This ▁value ▁is ▁in vers ely ▁related ▁to ▁the ▁total ▁cross - section al ▁area ▁of ▁the ▁blood ▁vessel ▁and ▁also ▁diff ers ▁per ▁cross - section , ▁because ▁in ▁normal ▁condition ▁the ▁blood ▁flow ▁has ▁l amin ar ▁characteristics . ▁For ▁this ▁reason , ▁the ▁blood ▁flow ▁velocity ▁is ▁the ▁fast est ▁in ▁the ▁middle ▁of ▁the ▁vessel ▁and ▁slow est ▁at ▁the ▁vessel ▁wall . ▁In ▁most ▁cases , ▁the ▁mean ▁velocity ▁is ▁used . ▁There ▁are ▁many ▁ways ▁to ▁measure ▁blood ▁flow ▁velocity , ▁like ▁vide oc ap ill ary ▁micro scop ing ▁with ▁frame - to - frame ▁analysis , ▁or ▁las er ▁Do pp ler ▁an em ometry . ▁Blood ▁veloc ities ▁in ▁ar ter ies ▁are ▁higher ▁during ▁syst ole ▁than ▁during ▁di asto le . ▁One ▁parameter ▁to ▁quant ify ▁this ▁difference ▁is ▁the ▁p uls at ility ▁index ▁( PI ), ▁which ▁is ▁equal ▁to ▁the ▁difference ▁between ▁the ▁peak ▁syst ol ic ▁velocity ▁and ▁the ▁minimum ▁di ast ol ic ▁velocity ▁divided ▁by ▁the ▁mean ▁velocity ▁during ▁the ▁card iac ▁cycle . ▁This ▁value ▁decre ases ▁with ▁distance ▁from ▁the ▁heart . ▁ ▁Blood ▁vessels ▁ ▁V asc ular ▁resistance ▁ ▁Res istance ▁is ▁also ▁related