How does the body decrease the blood vessel radius?

The correct answer is through vasoconstriction. The body decrease the blood vessel radius by vasoconstriction. Vasoconstriction is the contraction of the vessel which narrows the blood vessels. This process makes the blood pressure increase and decrease the blood vessel. 

Ihaven't done that yet, so no idea,but this is not the right place to ask

Molecules called lipids have long hydrocarbon chains that determine the way they act. they can be fats, oils, or hormones, and even exist in our cell membranes