A vasoconstrictor is a substance that causes blood vessels to narrow, which can increase blood pressure. This process occurs when the smooth muscles in the walls of blood vessels contract. Vasoconstrictors can be naturally occurring in the body, such as hormones like norepinephrine and angiotensin II, or they can be introduced through medications. Vasoconstriction plays a crucial role in regulating blood flow and maintaining blood pressure during various physiological conditions. It can also be a response to cold temperatures or stress, helping to redirect blood to vital organs. Understanding vasoconstrictors is important in fields like cardiology and pharmacology.