( ( ( ( ( ( The valves ) ( open ( to ( ( allow blood ) ( to ( flow through ) ) ) ) ) ) , ) and ) ( they ( close ( to ( prevent ( ( the backflow ) ( of ( blood ( -LRB- ( ( ( the flow ) ( of ( ( blood ( back ( into ( the chamber ) ) ) ) ( that ( ( the blood ) ( just ( came from ) ) ) ) ) ) ) -RRB- ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT The) (NNS valves)) (VP (VBP open) (S (VP (TO to) (VP (VB allow) (NP (NN blood)) (S (VP (TO to) (VP (VB flow) (PRT (RP through)))))))))) (, ,) (CC and) (S (NP (PRP they)) (VP (VBP close) (S (VP (TO to) (VP (VB prevent) (NP (NP (DT the) (NN backflow)) (PP (IN of) (NP (NP (NN blood)) (PRN (-LRB- -LRB-) (NP (NP (DT the) (NN flow)) (PP (IN of) (NP (NP (NN blood)) (ADVP (RB back) (PP (IN into) (NP (DT the) (NN chamber)))) (SBAR (IN that) (S (NP (DT the) (NN blood)) (ADVP (RB just)) (VP (VBD came) (PP (IN from)))))))) (-RRB- -RRB-)))))))))) (. .)))
The valves open to allow blood to flow through, and they close to prevent the backflow of blood (the flow of blood back into the chamber that the blood just came from).
(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))))))) (. .)))