LM of a venule and arteriole in transverse section. The venule has a thin wall and a relatively larger lumen than the arteriole. The lumen of each vessel holds many erythrocytes, but the venule lumen also has many white blood cells, a feature often seen in sections of venules. Venules have thin walls and are thus the main site of migration of leukocytes from the bloodstream to tissues. Via contraction, smooth muscle in arterioles regulates pressure in the arterial system. 450×. H&E.