▁to ▁vessel ▁radius , ▁vessel ▁length , ▁and ▁blood ▁vis cos ity . ▁ ▁In ▁a ▁first ▁approach ▁based ▁on ▁flu ids , ▁as ▁indicated ▁by ▁the ▁H agen – Po ise u ille ▁equation . ▁The ▁equation ▁is ▁as ▁follows : ▁▁▁ ∆ P : ▁pressure ▁drop / gradient ▁▁ µ : ▁vis cos ity ▁ ▁l : ▁length ▁of ▁t ube . ▁In ▁the ▁case ▁of ▁vessels ▁with ▁infinitely ▁long ▁lengths , ▁l ▁is ▁replaced ▁with ▁diameter ▁of ▁the ▁vessel . ▁ ▁Q : ▁flow ▁rate ▁of ▁the ▁blood ▁in ▁the ▁vessel ▁ ▁r : ▁radius ▁of ▁the ▁vessel ▁ ▁In ▁a ▁second ▁approach , ▁more ▁real istic ▁of ▁the ▁v asc ular ▁resistance ▁and ▁coming ▁from ▁experimental ▁observations ▁on ▁blood ▁flows , ▁according ▁to ▁Th ur ston , ▁there ▁is ▁a ▁pl asma ▁release - cell ▁layer ing ▁at ▁the ▁walls ▁surrounding ▁a ▁plug ged ▁flow . ▁It ▁is ▁a ▁fluid ▁layer ▁in ▁which ▁at ▁a ▁distance ▁ δ , ▁vis cos ity ▁ η ▁is ▁a ▁function ▁of ▁ δ ▁written ▁as ▁ η ( δ ), ▁and ▁these ▁surrounding ▁layers ▁do ▁not ▁meet ▁at ▁the ▁vessel ▁centre ▁in ▁real ▁blood ▁flow . ▁Instead , ▁there ▁is ▁the ▁plug ged ▁flow ▁which ▁is ▁hyper vis c ous ▁because ▁holding ▁high ▁concentration ▁of ▁R BC s . ▁Th ur ston ▁assemble d ▁this ▁layer ▁to ▁the ▁flow ▁resistance ▁to ▁describe ▁blood ▁flow ▁by ▁means ▁of ▁a ▁vis cos ity ▁ η ( δ ) ▁and ▁thick ness ▁ δ ▁from