The diagram shown here illustrates the process of exudation, aided by endothelial cell contraction and vasodilation, which typically is most pronounced in venules. Chemical mediators producing endothelial contraction include: histamine, leukotrienes, bradykinin, platelet activating factor, and the C3a and C5a components from complement activation. Mediators of this process over a longer term include tumor necrosis factor and interleukin-1. Chemical mediators that promote vasodilation include: histamine, prostaglandins, and nitric oxide.