, ▁blood ▁pressure ▁is ▁related ▁to ▁the ▁wall ▁t ension ▁of ▁the ▁vessel ▁according ▁to ▁the ▁Young – L ap lace ▁equation ▁( ass uming ▁that ▁the ▁thick ness ▁of ▁the ▁vessel ▁wall ▁is ▁very ▁small ▁as ▁compared ▁to ▁the ▁diameter ▁of ▁the ▁l umen ): ▁ ▁where ▁ ▁P ▁is ▁the ▁blood ▁pressure ▁ ▁t ▁is ▁the ▁wall ▁thick ness ▁ ▁r ▁is ▁the ▁inside ▁radius ▁of ▁the ▁cyl inder . ▁▁ ▁is ▁the ▁cyl inder ▁stress ▁or ▁" ho op ▁stress ". ▁For ▁the ▁thin - w alled ▁assumption ▁to ▁be ▁valid ▁the ▁vessel ▁must ▁have ▁a ▁wall ▁thick ness ▁of ▁no ▁more ▁than ▁about ▁one - t enth ▁( o ften ▁c ited ▁as ▁one ▁tw enti eth ) ▁of ▁its ▁radius . ▁ ▁The ▁cyl inder ▁stress , ▁in ▁turn , ▁is ▁the ▁average ▁force ▁ex ert ed ▁circum fer entially ▁( per pend icular ▁both ▁to ▁the ▁axis ▁and ▁to ▁the ▁radius ▁of ▁the ▁object ) ▁in ▁the ▁cyl inder ▁wall , ▁and ▁can ▁be ▁described ▁as : ▁ ▁where : ▁ ▁F ▁is ▁the ▁force ▁ex ert ed ▁circum fer entially ▁on ▁an ▁area ▁of ▁the ▁cyl inder ▁wall ▁that ▁has ▁the ▁following ▁two ▁lengths ▁as ▁sides : ▁ ▁t ▁is ▁the ▁radial ▁thick ness ▁of ▁the ▁cyl inder ▁ ▁l ▁is ▁the ▁ax ial ▁length ▁of ▁the ▁cyl inder ▁ ▁Str ess ▁When ▁force ▁is ▁applied ▁to ▁a ▁material ▁it ▁starts ▁to ▁de form ▁or ▁move . ▁As ▁the ▁force ▁needed ▁to ▁de form ▁a ▁material