( ( Some capillaries ) ( ( have ( ( ( precapillary sphincters ) ( -LRB- ( ( ( a ring ) ( of ( smooth muscle ) ) ) -RRB- ) ) ) ( that ( help ( regulate ( blood flow ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT Some) (NNS capillaries)) (VP (VBP have) (NP (NP (JJ precapillary) (NNS sphincters)) (PRN (-LRB- -LRB-) (NP (NP (DT a) (NN ring)) (PP (IN of) (NP (JJ smooth) (NN muscle)))) (-RRB- -RRB-)) (SBAR (WHNP (WDT that)) (S (VP (VBP help) (VP (VB regulate) (NP (NN blood) (NN flow)))))))) (. .)))
Some capillaries have precapillary sphincters (a ring of smooth muscle) that help regulate blood flow.
(ROOT (S (NP (JJ Smooth) (NN muscle)) (VP (VBZ regulates) (NP (NP (NN air) (NN flow)) (PP (IN in) (NP (NNS lungs))))) (. .)))
Smooth muscle regulates air flow in lungs.