3. Tunica adventitia: entirely made of connective tissue. It also contains nerves that supply the muscular layer, as well as nutrient capillaries (vasa vasorum) in the larger blood vessels.

• 中膜层（最厚层）：环形分布的弹性纤维、结缔组织、多糖物质，其第二和三层由另一称为外弹性膜的厚弹性带分隔。中膜（尤其是动脉）富含血管平滑肌，血管吕径由血管平滑肌控制。