( ( ( The presence ) ( of ( valves ( in ( the veins ) ) ) ) ) ( ( prevents ( ( the backflow ) ( of ( blood ( once ( drawn ( toward ( the heart ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN presence)) (PP (IN of) (NP (NP (NNS valves)) (PP (IN in) (NP (DT the) (NNS veins)))))) (VP (VBZ prevents) (NP (NP (DT the) (NN backflow)) (PP (IN of) (NP (NP (NN blood)) (VP (ADVP (RB once)) (VBN drawn) (PP (IN toward) (NP (DT the) (NN heart)))))))) (. .)))
The presence of valves in the veins prevents the backflow of blood once drawn toward 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))))))) (. .)))