( It ( ( is ( ( ( ( ( ( ( a ( `` ( leftover '' ) ) ) ( after ( ( the body ) ( absorbs ( what ( nutrients ( it ( needs ( from ( the ( intestines Brainstorm ) ) ) ) ) ) ) ) ) ) ) : ) ( ( the relationships ) ( between ( ( ( the ( excretory system ) ) and ) ( the ( respiratory system ) ) ) ) ) ) , ) and ) ( the ( digestive system ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (NP (DT a) (`` ``) (NN leftover) ('' '')) (SBAR (IN after) (S (NP (DT the) (NN body)) (VP (VBZ absorbs) (SBAR (WHNP (WP what)) (S (VP (VBZ nutrients) (SBAR (S (NP (PRP it)) (VP (VBZ needs) (PP (IN from) (NP (DT the) (NNP intestines) (NNP Brainstorm))))))))))))) (: :) (NP (NP (DT the) (NNS relationships)) (PP (IN between) (NP (NP (DT the) (JJ excretory) (NN system)) (CC and) (NP (DT the) (JJ respiratory) (NN system))))) (, ,) (CC and) (NP (DT the) (JJ digestive) (NN system)))) (. .)))
It is a “leftover” after the body absorbs what nutrients it needs from the intestines  Brainstorm: the relationships between the excretory system and the respiratory system, and the digestive system.
(ROOT (S (NP (NP (DT A) (NN relationship)) (PP (IN between) (NP (CD two) (JJ human) (NN body) (NNS systems)))) (VP (MD could) (VP (VB be) (NP (NP (DT the) (JJ digestive) (NN system)) (SBAR (S (VP (VBZ absorbs) (NP (NP (NNS nutrients)) (SBAR (WHNP (WDT which)) (S (VP (VBP are) (VP (VBN transported) (PP (IN by) (NP (DT the) (JJ circulatory) (NN system)))))))))))))) (. .)))