A vasoconstrictor, also called vasopressor, is any substance that causes the layer of smooth muscle in the blood vessels to contract, resulting in a shortening of the diameter of the blood vessel. This causes a rise in vascular resistance or the amount of energy it takes for blood to move through the blood vessels, and an increase in blood pressure. A vasoconstrictor may be made endogenously, or naturally within the body, such as with antidiuretic hormone (ADH) and adrenaline. A vasoconstrictor can also be made exogenously, or outside the body, and be taken as a drug, such as caffeine, pseudoephedrine, amphetamines, and antihistamines. In a medical setting, such drugs are used as decongestants, agents to raise blood pressure, and agents to stem blood flow to a certain area.
The purpose of an endogenous vasoconstrictor is to help preserve homeostasis, the body’s balancing act that keeps all of its processes within a set of safe parameters. Vasopressors achieve this by helping thermoregulation, or maintenance of normal body temperature, and by preventing hypotension. Hypotension, or low blood pressure, occurs as a result of too much vasodilation, or opening of the blood vessels, hormonal upsets, anemia, or lack of sufficient red blood cells, side effects from medicines, and heart conditions.
The body commonly releases vasopressors when it is undergoing orthostatic hypotension, a condition in which blood pools in the lower extremities while sitting or lying down, causing a drop in blood pressure towards the head. This causes the head rush that some people experience when standing up. The body uses vasoconstrictors to push the blood back up through the blood vessels towards the heart and head.
The body may also release a vasoconstrictor when the outside temperature is cold and the body wants to retain heat. Because animals lose heat as blood travels to the extremities, vasopressors restrict blood flow to places like the fingers, toes, and nose to keep as much of the body’s warmth as possible. Sometimes the body overreacts to the cold, causing excessive vasoconstriction and whiteness in the hands or feet. This is called Raynaud’s phenomenon.
When the body is unable to prevent hypotension, doctors may prescribe an exogenous vasoconstrictor to raise the blood pressure. They may also use vasopressors to restrict blood flow to a local area. Many anesthetics, for example, include a vasoconstrictor to narrow the blood vessel at the site of the injection, allowing the drug more time to enter the slowed bloodstream. Vasopressors may also be used to control hemorrhaging, or excessive bleeding. In decongestants and antihistamines, the drug works by tightening the blood vessel, thereby impeding the blood’s ability to induce inflammation.
Vasoconstriction is also part of the fight or flight response, a physiological response to stress started by the sympathetic nervous system. During this response, the nervous system triggers the release of chemicals, including vasoconstrictor hormones, which cause the body to shake, the bladder to relax, the face to alternately blush and drain of color, the muscles to be reactive, and the pupils to dilate, amongst other signs of excitation. This rush from vasoconstriction is sometimes prescribed and sometimes sought after through recreational drugs, such as cocaine, or ecstasy.