( ( After ( ( the blood ) ( has ( passed ( through ( ( the ( small ( capillary network ) ) ) ( in ( the lungs ) ) ) ) ) ) ) ) ( , ( it ( ( returns ( to ( ( ( the ( heart 's ) ) ( left atrium ) ) ( via ( the ( pulmonary veins ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN After) (S (NP (DT the) (NN blood)) (VP (VBZ has) (VP (VBN passed) (PP (IN through) (NP (NP (DT the) (JJ small) (NN capillary) (NN network)) (PP (IN in) (NP (DT the) (NNS lungs))))))))) (, ,) (NP (PRP it)) (VP (VBZ returns) (PP (TO to) (NP (NP (NP (DT the) (NN heart) (POS 's)) (JJ left) (NN atrium)) (PP (IN via) (NP (DT the) (JJ pulmonary) (NNS veins)))))) (. .)))
After the blood has passed through the small capillary network in the lungs, it returns to the heart's left atrium via the pulmonary veins.
(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)))) (. .)))