As the name implies, vasoconstrictor drugs contract the smooth muscle in blood vessels, which causes the vessels to constrict. Constriction of arterial (resistance) vessels increases systemic vascular resistance, which leads to an increase in arterial blood pressure because mean arterial pressure is determined by the product of systemic vascular resistance and cardiac output.. Constriction of venous (capacitance) vessels increases venous blood pressure and increases cardiac preload and cardiac output by the Frank-Starling mechanism, which increases arterial pressure. Because vasoconstrictor drugs increase arterial pressure, they comprise a functional group of drugs known as pressor drugs.