▁( e . g . ▁to ▁make ▁a ▁fluid ▁flow ) ▁increases ▁with ▁the ▁size ▁of ▁the ▁surface ▁of ▁the ▁material ▁A ., ▁the ▁magnitude ▁of ▁this ▁force ▁F ▁is ▁proportional ▁to ▁the ▁area ▁A ▁of ▁the ▁portion ▁of ▁the ▁surface . ▁Therefore , ▁the ▁quantity ▁( F / A ) ▁that ▁is ▁the ▁force ▁per ▁unit ▁area ▁is ▁called ▁the ▁stress . ▁The ▁she ar ▁stress ▁at ▁the ▁wall ▁that ▁is ▁associated ▁with ▁blood ▁flow ▁through ▁an ▁ar tery ▁depends ▁on ▁the ▁ar tery ▁size ▁and ▁geometry ▁and ▁can ▁range ▁between ▁ 0 . 5 ▁and ▁ 4 ▁Pa . ▁ ▁. ▁Under ▁normal ▁conditions , ▁to ▁avoid ▁a ther ogen esis , ▁th rom b osis , ▁smooth ▁mus cle ▁pro l ifer ation ▁and ▁end oth el ial ▁apo pt osis , ▁she ar ▁stress ▁maintain s ▁its ▁magnitude ▁and ▁direction ▁within ▁an ▁acceptable ▁range . ▁In ▁some ▁cases ▁occurr ing ▁due ▁to ▁blood ▁ham mer , ▁she ar ▁stress ▁reaches ▁larger ▁values . ▁While ▁the ▁direction ▁of ▁the ▁stress ▁may ▁also ▁change ▁by ▁the ▁reverse ▁flow , ▁depending ▁on ▁the ▁hem od ynamic ▁conditions . ▁Therefore , ▁this ▁situation ▁can ▁lead ▁to ▁a ther os cl eros is ▁disease . ▁ ▁Cap ac it ance ▁▁ ▁Ve ins ▁are ▁described ▁as ▁the ▁" cap ac it ance ▁vessels " ▁of ▁the ▁body ▁because ▁over ▁ 7 0 % ▁of ▁the ▁blood ▁volume ▁res ides ▁in ▁the ▁ven ous ▁system . ▁Ve ins ▁are ▁more ▁comp li ant ▁than ▁ar