( ( These valves ) ( ( ( allow forward ) ( ( ( ( flow and ) prevent ) ( backflow ( of blood ) ) ) ( moving ( through ( the heart ) ) ) ) ) . ) )
(ROOT (S (NP (DT These) (NNS valves)) (VP (VBP allow) (ADVP (RB forward)) (VP (VB flow) (CC and) (VB prevent) (NP (NP (NN backflow)) (PP (IN of) (NP (NN blood)))) (S (VP (VBG moving) (PP (IN through) (NP (DT the) (NN heart))))))) (. .)))
These valves allow forward flow and prevent backflow of blood moving through the heart.
(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))))))) (. .)))