Histamine and Bradykinin
Histamine does not normally play a significant role in regulating blood flow and microvascular function. However, during tissue injury, inflammation and allergic responses, mast cells within the tissue can release large amounts of histamine that can have pronounced vascular effects. Histamine, binding to vascular H1and H2 receptors, causes arteriolar vasodilation, venous constriction in some vascular beds, and increased capillary permeability. These effects increase local blood flow and cause tissue edema.
The actions of bradykinin are like histamine. Bradykinin binding to vascular bradykinin receptors is a potent stimulator of nitric oxide formation by vascular endothelium. It also stimulates prostacyclin formation.