Blood vessel, light micrograph. Typical small venule with a very thin endothelium wall and vessel diameter of only 20-40 micrometres. Small venules are identified by the absence of an external layer of smooth muscle that accompanies larger venules to facilitate limited contraction. Erythrocytes (pink) are membrane-bound deformable structures filled with haemoglobin. They are about 7 micrometres in diameter but can deform in passing through the smallest capillaries of 3-4 micrometres. The supporting tissues are fibroblasts, extracellular matrix and collagen. Paraffin section, Masson's trichrome stain. Magnification: x 380 when width printed at 10cm.