( ( This ( oxygen-rich blood ) ) ( ( ( ( ( returns ( from ( the lungs ) ) ) ( to ( the heart ) ) ) and ) ( enters ( the ( left atrium ) ) ) ) . ) )
(ROOT (S (NP (DT This) (JJ oxygen-rich) (NN blood)) (VP (VP (VBZ returns) (PP (IN from) (NP (DT the) (NNS lungs))) (PP (TO to) (NP (DT the) (NN heart)))) (CC and) (VP (VBZ enters) (NP (DT the) (JJ left) (NN atrium)))) (. .)))
This oxygen-rich blood returns from the lungs to the heart and enters the left atrium .
(ROOT (S (NP (NP (NNP Blood)) (PP (IN from) (NP (DT the) (NNS lungs)))) (VP (VBZ enters) (PP (IN into) (NP (DT the) (JJ left) (NN atrium)))) (. .)))