Basal vascular tone is different among organs. Vascular tone is determined by many different competing vasoconstrictor and vasodilator influences acting upon the blood vessel. These influences can be separated into extrinsic factors that originate from outside of the organ or tissue where the blood vessel is located, and intrinsic factors that originate from the vessel itself or the surrounding tissue. Extrinsic factors primarily serve the function of regulating arterial blood pressure, while intrinsic mechanisms are concerned with local blood flow regulation within an organ. Vascular tone at any given instant is determined by the balance of competing vasoconstrictor and vasodilator influences.
Endothelial factors can either increase or decrease tone.
Metabolic by-products or hypoxia generally decrease tone. The mechanisms by which the above influences either constrict or relax blood vessels involve a variety of signal transduction mechanisms that ultimately influence the interaction between actin and myosin in the smooth muscle.