( There ( ( are ( ( ( ( ( four valves ) , ) ( composed ( of ( ( dense ( connective tissue ) ) ( covered ( by endothelium ) ) ) ) ) ) , ) ( which ( ( prevent ( backflow ( of ( blood ( in ( ( the ( heart 's ) ) ( blood path ) ) ) ) ) ) ) ( by ( ( ( opening and ) closing ) ( ( relative ( to pressure ) ) changes ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBP are) (NP (NP (CD four) (NNS valves)) (, ,) (VP (VBN composed) (PP (IN of) (NP (NP (JJ dense) (NN connective) (NN tissue)) (VP (VBN covered) (PP (IN by) (NP (NN endothelium))))))) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBP prevent) (NP (NP (NN backflow)) (PP (IN of) (NP (NP (NN blood)) (PP (IN in) (NP (NP (DT the) (NN heart) (POS 's)) (NN blood) (NN path)))))) (PP (IN by) (S (VP (VBG opening) (CC and) (VBG closing) (NP (ADJP (JJ relative) (PP (TO to) (NP (NN pressure)))) (NNS changes)))))))))) (. .)))
There are four valves, composed of dense connective tissue covered by endothelium, which prevent backflow of blood in the heart's blood path by opening and closing relative to pressure changes.
(ROOT (S (NP (NNP Heart) (NNS valves)) (VP (VBP prevent) (NP (NP (DT the) (NN backflow)) (PP (IN of) (NP (NN blood)))) (PP (IN from) (S (VP (VBG happening) (PP (IN in) (NP (DT the) (NN heart))))))) (. .)))