( These ( ( are ( valves ( at ( ( the ( two exits ) ) ( of ( ( the heart ) ( that ( ( prevent backflow ) ( into ( the ventricles ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT These)) (VP (VBP are) (NP (NP (NNS valves)) (PP (IN at) (NP (NP (DT the) (CD two) (NNS exits)) (PP (IN of) (NP (NP (DT the) (NN heart)) (SBAR (WHNP (WDT that)) (S (VP (VBP prevent) (NP (NN backflow)) (PP (IN into) (NP (DT the) (NNS ventricles)))))))))))) (. .)))
These are valves at the two exits of the heart that prevent backflow into the ventricles.
(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))))))) (. .)))