High blood pressure stimulates the sympathetic nerve fibers to produce norepinephrine, which causes vasoconstriction.

Vasoconstriction of the afferent arterioles decreases GFR, thereby restoring blood volume and pressure to normal.

Low blood pressure causes the granular cells of the juxtaglomerular complex to release renin.

Renin activates Angiotensin II, which promotes vasoconstriction and hence raises blood pressure.