These holes may be closed by a thin diaphragm. These vessels also have pinocytotic vesicles. These vessels typically have a larger lumen than the preceding types and show wide gaps between adjacent endothelial cells. The tunics in the walls of veins are not as clearly defined as in the walls of arteries. The walls of veins are thinner and more distensible than arteries of the same diameter and the lumens of veins are often partially collapsed.
Many veins have valves that allow blood flow in one direction only.
Valves are formed from folds of endothelium supported by small amounts of connective tissue fibers. Postcapillary venules . Postcapillary venules are distinguished from capillaries by their large diameter and are the principal sites of leukocyte emigration from the vasculature. Fluid is lost from the circulation in these vessels also. Tunica intima .