( ( ( These valves ) ( ( open and ) close ) ) ( ( depending ( on ( ( the phase ) ( of ( the heart ) ) ) ) ) ( ( beat ( to ( allow ( the ( blood ( ( ( to ( ( flow ( from ( ( one chamber ) ( of ( the heart ) ) ) ) ) ( to another ) ) ) and ) ( to ( prevent ( ( the backflow ) ( of blood ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (NP (DT These) (NNS valves)) (ADJP (JJ open) (CC and) (JJ close))) (PP (VBG depending) (PP (IN on) (NP (NP (DT the) (NN phase)) (PP (IN of) (NP (DT the) (NN heart)))))) (VP (VBD beat) (S (VP (TO to) (VP (VB allow) (NP (DT the) (NN blood) (S (VP (VP (TO to) (VP (VB flow) (PP (IN from) (NP (NP (CD one) (NN chamber)) (PP (IN of) (NP (DT the) (NN heart))))) (PP (TO to) (NP (DT another))))) (CC and) (VP (TO to) (VP (VB prevent) (NP (NP (DT the) (NN backflow)) (PP (IN of) (NP (NN blood))))))))))))) (. .)))
These valves open and close depending on the phase of the heart beat to allow the blood to flow from one chamber of the heart  to another and to prevent the backflow of blood.
(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))))))) (. .)))