Bradykinin can form anywhere in the circulatory system. It arises from plasma or tissue fluids and is a very powerful arteriolar dilator. It also increases capillary permeability which can lead to peripheral edema.
Serotonin is present in the intestines and platelets. It can have either a vasodilating or vasoconstricting effect depending on the conditions or the area of circulation.
Histamine can be found in virtually every tissue of the body and is derived from mast cells in the tissues and basophils in the blood. It is a powerful vasodilator of arterioles and also increases capillary porosity, allowing leakage of both fluids and plasma proteins into the surrounding tissues causing local edema. It is responsible for allergic reactions. Prostaglandins (primarily vasodilators) are also found in almost every tissue of the body.