( Veins ( also ( ( have ( valves ( to ( prevent ( ( the backflow ) ( of blood ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (NNS Veins)) (ADVP (RB also)) (VP (VBP have) (NP (NNS valves) (S (VP (TO to) (VP (VB prevent) (NP (NP (DT the) (NN backflow)) (PP (IN of) (NP (NN blood))))))))) (. .)))
Veins also have valves 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))))))) (. .)))