( ( ( ( ( ( contractions ( of ( the heart ) ) ) ( generate ( blood pressure ) ) ) , ) and ) ( ( heart valves ) ( prevent ( backflow ( of ( blood ( in ( the ( circulatory system ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (NNS contractions)) (PP (IN of) (NP (DT the) (NN heart)))) (VP (VBP generate) (NP (NN blood) (NN pressure)))) (, ,) (CC and) (S (NP (NN heart) (NNS valves)) (VP (VBP prevent) (NP (NP (NN backflow)) (PP (IN of) (NP (NP (NN blood)) (PP (IN in) (NP (DT the) (JJ circulatory) (NN system)))))))) (. .)))
contractions of the heart generate blood pressure, and heart valves prevent backflow of blood in the circulatory system.
(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))))))) (. .)))