Structure of Blood Vessels

The walls of blood vessels, whether arteries or veins, have three distinct layers or tunics.

Tunica externa is the outermost layer infiltrated with several nerves and lymphatic vessels.

In larger blood vessels, the tunica externa also has a network of tiny vessels called vasa vasorum that supply blood to the cells of blood vessel walls.

The middle layer, tunica media, is the thickest layer, rich in circularly arranged smooth muscle cells and elastin sheets.

These smooth muscle cells can either contract, resulting in vasoconstriction, or relax, leading to vasodilation. This pivotal process regulates the rate of blood flow and blood pressure within the vessels.

Tunica interna is the innermost layer. It consists of the endothelium, a thin layer of flattened simple squamous epithelial cells that form the lumen of blood vessels.

The endothelium is attached to the underlying connective tissue by a basement membrane.

While the walls of arteries and veins share most components, there are two additional layers, the internal and external elastic lamina, present only in arteries