Venule. Coloured transmission electron micrograph (TEM). Venules are small veins that transport deoxygenated blood from the capillary beds to the veins. Endothelial cells line the entire circulatory system. Smooth muscle cells and connective tissue surround the endothelium. Red blood cells and three neutrophils are present in the lumen of the venule. Magnification: x1000 when printed 10 centimetres wide.