( ( Blood vessels ) ( ( are ( part ( of ( ( ( the ( circulatory system ) ) , ) ( ( ( the ( `` ( highway ( '' system ) ) ) ) ( of ( the ( human body ) ) ) ) ( that ( ( transports materials ) ( to ( all ( of ( its cells ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Blood) (NNS vessels)) (VP (VBP are) (NP (NP (NN part)) (PP (IN of) (NP (NP (DT the) (JJ circulatory) (NN system)) (, ,) (NP (NP (DT the) (`` ``) (NN highway) ('' '') (NN system)) (PP (IN of) (NP (DT the) (JJ human) (NN body))) (SBAR (WHNP (WDT that)) (S (VP (VBZ transports) (NP (NNS materials)) (PP (TO to) (NP (NP (DT all)) (PP (IN of) (NP (PRP$ its) (NNS cells))))))))))))) (. .)))
Blood vessels are part of the circulatory system, the "highway" system of the human body that transports  materials to all of its cells.
(ROOT (S (NP (NP (DT The) (JJ main) (NN function)) (PP (IN of) (NP (DT the) (JJ circulatory) (NN system)))) (VP (VBZ is) (S (VP (TO to) (VP (VB transport) (NP (NNS materials)))))) (. .)))