Autacoids are biological factors which act like local hormones have a brief duration and act near their site of synthesis. The effects of autacoids are primarily local though large quantities can be produced and moved into circulation. Autacoids may thus have systemic effects by being transported via the circulation. These regulating molecules are also metabolized locally. Some autacoids are chiefly characterized by the effect they have on specific tissues such as smooth muscle. With respect to vascular smooth muscle there exist both vasoconstrictor and vasodilator autacoids. Vasodilator autacoids are released during periods of exercise. Their main effect is seen in the skin where they facilitate heat loss.