Vasoconstriction and vasodilation are very important mechanisms. They are, for instance, a key aspect of thermoregulation, the ability of the body to control its internal temperature. When you are cold, your body will constrict surface blood vessels to reduce the amount of blood going to the skin, which in turn slows heat loss. When you are hot, the reverse happens; more blood flows to the skin, where it can act as a radiator to help cool you down.
Vasoconstriction/dilation is also an important aspect of the fight or flight response. In this instance, stress or fear causes blood to be shunted toward muscles so one can better respond to a challenging situation.
Blood vessels have some smooth muscle tissue in their walls. This muscle makes it possible for the body to change the diameter of the blood vessel by tightening or loosening the muscle cells, which controls the blood flow through the vessel. Blood vessel control of this sort is accomplished by a variety of factors. Rising levels of carbon dioxide in the blood cause vasodilation, as can noradrenalin, histamine, niacin, and prostaglandins, to name just a few. Antidiuretic hormone, insulin, epinephrine, and low oxygen levels are examples of chemicals that can cause vasoconstriction.