TEM of human venule seen in longitudinal section. A venule is a very small blood vessel in the microcirculation that allows blood to return from the capillary beds to drain into the larger blood vessels, the veins. Venules range from 7 to 50?m in diameter. Veins contain approximately 70% of total blood volume, 25% of which is contained in the venules. Many venules unite to form a vein. Magnification @ 7000x. 8x10 print.