( ( Backflow ( of ( blood ( in ( the heart ) ) ) ) ) ( ( is ( prevented ( by ( ( four valves ) ( made ( of ( flaps ( of ( connective tissue ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NNP Backflow)) (PP (IN of) (NP (NP (NN blood)) (PP (IN in) (NP (DT the) (NN heart)))))) (VP (VBZ is) (VP (VBN prevented) (PP (IN by) (NP (NP (CD four) (NNS valves)) (VP (VBN made) (PP (IN of) (NP (NP (NNS flaps)) (PP (IN of) (NP (JJ connective) (NN tissue)))))))))) (. .)))
Backflow of blood in the heart is prevented by four valves made of flaps of connective tissue.
(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))))))) (. .)))