Following infection
or injury, vasodilators are released that increase venule permeability.
Constriction of the endothelial cells of the venules allows for diapedesis ( extravasation), during which defense white blood cells such as neutrophils and monocytes leave the blood and enter the tissue around capillary beds where they are chemotactically attracted to the infection site. In addition, plasma leaves the bloodstream and enters the tissue delivering defense chemicals such as antibodies, complement proteins, and clotting factors.
