( ( ( ( A decrease ) ( in ( the diameter ) ) ) ( of ( blood vessels ) ) ) . )
(ROOT (NP (NP (DT A) (NN decrease)) (PP (IN in) (NP (DT the) (NN diameter))) (PP (IN of) (NP (NN blood) (NNS vessels))) (. .)))
A decrease in the diameter of blood vessels.
(ROOT (S (NP (NNP Blood)) (VP (VBP flow) (VP (VB decrease) (SBAR (WHADVP (WRB when)) (S (NP (NN blood) (NNS vessels)) (VP (VBP constrict)))))) (. .)))