( ( The ( brain stem ) ) ( ( monitors ( ( the level ) ( of ( ( carbon dioxide ) ( in ( the blood ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN brain) (NN stem)) (VP (VBZ monitors) (NP (NP (DT the) (NN level)) (PP (IN of) (NP (NP (NN carbon) (NN dioxide)) (PP (IN in) (NP (DT the) (NN blood))))))) (. .)))
The brain stem monitors the level of  carbon dioxide in the blood.
(ROOT (S (NP (DT The) (NN brain)) (VP (VBP stem) (S (NP (NP (NN monitor)) (PRN (-LRB- -LRB-) (PRP s) (-RRB- -RRB-))) (NP (NP (DT the) (NN level)) (PP (IN of) (NP (NP (NN carbon) (NN dioxide)) (PP (IN in) (NP (DT the) (NN blood)))))))) (. .)))