( ( With ( too ( little oxygen ) ) ) ( ( ( the ( blood vessels ) ) ( leading ( to ( ( the lungs ) ( -LRB- ( ( the ( pulmonary arteries ) ) -RRB- ) ) ) ) ) ) ( ( become ( narrow ( so ( ( the heart ) ( must ( ( work harder ) ( to ( ( get blood ) ( to ( the lungs ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN With) (NP (RB too) (JJ little) (NN oxygen))) (NP (NP (DT the) (NN blood) (NNS vessels)) (VP (VBG leading) (PP (TO to) (NP (NP (DT the) (NNS lungs)) (PRN (-LRB- -LRB-) (NP (DT the) (JJ pulmonary) (NNS arteries)) (-RRB- -RRB-)))))) (VP (VBP become) (ADJP (JJ narrow) (SBAR (IN so) (S (NP (DT the) (NN heart)) (VP (MD must) (VP (VB work) (ADVP (RBR harder)) (S (VP (TO to) (VP (VB get) (NP (NN blood)) (PP (TO to) (NP (DT the) (NNS lungs)))))))))))) (. .)))
With too little oxygen the blood vessels leading to the lungs (the pulmonary arteries) become narrow so the heart must work harder to get blood to the lungs.
(ROOT (S (SBAR (IN After) (S (NP (NP (DT the) (NN blood)) (PP (IN in) (NP (DT the) (JJ pulmonary) (NNS capillaries)))) (VP (VBZ becomes) (VP (VBN saturated) (PP (IN with) (NP (NN oxygen))))))) (, ,) (NP (PRP it)) (VP (VBZ leaves) (NP (NP (DT the) (NNS lungs) (CC and) (NNS travels)) (PP (TO to) (NP (DT the) (NN heart))))) (. .)))