. ▁Though ▁the ▁rad ii ▁of ▁the ▁cap ill aries ▁are ▁very ▁small , ▁the ▁network ▁of ▁cap ill aries ▁have ▁the ▁largest ▁surface ▁area ▁in ▁the ▁v asc ular ▁network . ▁They ▁are ▁known ▁to ▁have ▁the ▁largest ▁surface ▁area ▁( 4 8 5   mm ^ 2 ) ▁in ▁the ▁human ▁v asc ular ▁network . ▁The ▁larger ▁the ▁total ▁cross - section al ▁area , ▁the ▁lower ▁the ▁mean ▁velocity ▁as ▁well ▁as ▁the ▁pressure . ▁ ▁Sub st ances ▁called ▁v aso con str ict ors ▁can ▁reduce ▁the ▁size ▁of ▁blood ▁vessels , ▁thereby ▁increasing ▁blood ▁pressure . ▁Vas od il ators ▁( such ▁as ▁nit rog ly cer in ) ▁increase ▁the ▁size ▁of ▁blood ▁vessels , ▁thereby ▁decre asing ▁ar ter ial ▁pressure . ▁ ▁If ▁the ▁blood ▁vis cos ity ▁increases ▁( gets ▁th icker ), ▁the ▁result ▁is ▁an ▁increase ▁in ▁ar ter ial ▁pressure . ▁C ertain ▁medical ▁conditions ▁can ▁change ▁the ▁vis cos ity ▁of ▁the ▁blood . ▁For ▁instance , ▁an emia ▁( low ▁red ▁blood ▁cell ▁concentration ), ▁reduces ▁vis cos ity , ▁whereas ▁increased ▁red ▁blood ▁cell ▁concentration ▁increases ▁vis cos ity . ▁It ▁had ▁been ▁thought ▁that ▁asp ir in ▁and ▁related ▁" blo od ▁th inner " ▁dru gs ▁decre ased ▁the ▁vis cos ity ▁of ▁blood , ▁but ▁instead ▁studies ▁found ▁that ▁they ▁act ▁by ▁reducing ▁the ▁t endency ▁of ▁the ▁blood ▁to ▁cl ot . ▁ ▁Wall ▁t ension ▁▁ ▁Reg ard less ▁of ▁site