( It ( ( ( enters ( the ( right atrium ) ) ) ( at ( ( ( the ( lower right ) ) , ) ( ( back side ) ( of ( the heart ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ enters) (NP (DT the) (JJ right) (NN atrium)) (PP (IN at) (NP (NP (DT the) (JJR lower) (NN right)) (, ,) (NP (NP (JJ back) (NN side)) (PP (IN of) (NP (DT the) (NN heart))))))) (. .)))
It enters the right atrium at the lower right, back side of the heart.
(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)))) (. .)))