( It ( ( becomes ( more ( difficult ( for ( the ( heart ( to function ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ becomes) (ADJP (RBR more) (JJ difficult) (PP (IN for) (NP (DT the) (NN heart) (S (VP (TO to) (VP (VB function)))))))) (. .)))
It becomes more difficult for the heart to function.
(ROOT (S (PP (IN In) (NP (NNS humans))) (, ,) (NP (DT the) (NN heart)) (VP (VBZ is) (NP (DT the) (JJ first) (NN organ) (S (VP (TO to) (VP (VP (VB form)) (CC and) (VP (VB become) (ADJP (JJ functional)))))))) (. .)))