Some common effectors that cause vasoldilation and vasoconstriction include heat, medications, antidiuretic hormones and nitric oxide according to Midlands Technical College. When heat, a drop or rise in blood pressure or hormonal changes occur, patients' vessels may dilate or constrict thanks to impulses from the brain stem. The vasomotor center in the brain stem, which is made up of sympathetic neurons in the medulla, changes the diameter of blood vessels.
External factors influence the impulses sent to the vasomotor center, and then the vasomotor center takes control by adjusting the vascular system's blood vessels accordingly. In some cases, the system may increase or decrease heart rate as well as constrict blood vessels. In others, the actual volume of blood may be increased or decreased.