Name the following:

The smallest common blood vessels formed by the union of capillaries.

A venule is the smallest common blood vessels formed by the union capillaries. A venule is an extremely small vein, generally 8–100 micrometers in diameter. Post capillary venules join multiple capillaries exiting from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa. Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid.