TABLE Characteristics of Blood Vessels 13.1 Arteries Vessel Diameter Tunica intima (inner layer) Tunica media (middle layer) Tunica adventitia (outer layer) Large artery (elastic artery) >10 mm Endothelium Connective tissue Smooth muscle Smooth muscle Elastic lamellae Connective tissue Elastic fibers Thinner than tunica media Medium artery (muscular artery) 2–10 mm Endothelium Connective tissue Smooth muscle Prominent internal elastic membrane Smooth muscle Collagen fibers Relatively little elastic tissue Connective tissue Some elastic fibers Thinner than tunica media Small artery 0.1–2 mm Endothelium Connective tissue Smooth muscle Internal elastic membrane Smooth muscle (8–10 cell layers) Collagen fibers Connective tissue Some elastic fibers Thinner than tunica media Arteriole 10–100 µm Endothelium Connective tissue Smooth muscle Smooth muscle (1–2 cell layers) Thin, ill-defined sheath of connective tissue Capillary 4–10 µm Endothelium None None Veins Vessel Diameter Tunica intima (inner layer) Tunica media (middle layer) Tunica adventitia (outer layer) Postcapillary venule 10–50 µm Endothelium Pericytes None None Muscular venule 50–100 µm Endothelium Smooth muscle (1–2 cell layers) Connective tissue Some elastic fibers Thicker than tunica media Small vein 0.1–1 mm Endothelium Connective tissue Smooth muscle (2–3 layers) Smooth muscle (2–3 layers continuous with tunica intima) Connective tissue Some elastic fibers Thicker than tunica media Medium vein 1–10 mm Endothelium Connective tissue Smooth muscle Internal elastic membrane in some cases Smooth muscle Collagen fibers Connective tissue Some elastic fibers Thicker than tunica media Large vein 10 mm Endothelium Connective tissue Smooth muscle Smooth muscle (2–15 layers) Cardiac muscle near heart Collagen fibers Connective tissue Some elastic fibers, longitudinal smooth muscles Much thicker than tunica media transforming growth factor  (TGF-). Endothelial cells function in the conversion of angiotensin I to angiotensin II in the renin–angiotensin system that controls blood pressure, as well as in the inactivation or conversion of a several compounds conveyed in the blood (norepinephrine, thrombin, prostaglandins, bradykinin, and serotonin) to inactive forms.