Structure of Blood Vessels

Blood vessel walls consist of three distinct tunics. The tunica interna is the innermost layer containing the endothelium, a thin squamous epithelium forming the vessel lumen. The tunica media is the thickest layer, rich in circularly arranged smooth muscle cells and elastin sheets. The tunica externa is the outermost layer infiltrated with nerves and lymphatic vessels.

Smooth muscle cells in the tunica media can contract or relax to control vessel diameter. Contraction causes vasoconstriction, narrowing the lumen and reducing blood flow. Relaxation causes vasodilation, widening the lumen and increasing blood flow. This pivotal process regulates the rate of blood flow and blood pressure within vessels, controlled by nervi vasorum and hormonal factors.

The endothelium is a specialized layer of flattened simple squamous epithelial cells lining the innermost surface of blood vessels. Beyond merely demarcating the vessel wall, it regulates capillary exchange and blood flow alteration. The endothelium produces endothelins, local chemicals that constrict vessel walls and elevate blood pressure. Disruption of this lining exposes collagen fibers, significantly contributing to blood clot formation.

Vasa vasorum are tiny vessels within blood vessel walls that supply nutrients and remove waste from vessel cells. Large vessel walls are too thick for direct nutrient exchange from circulating blood. In arteries, vasa vasorum are positioned in outer layers to prevent collapse from high pressure. In veins, lower pressure allows vasa vasorum to be situated nearer the lumen, facilitating more efficient nourishment delivery.

Arteries have thicker walls with more prominent tunica media due to high-pressure blood from the heart, and smaller lumens to maintain blood pressure. Veins have thinner walls, larger lumens, and lower pressure. Arteries contain internal and external elastic laminae, layers absent in veins. Many veins contain valves directing blood toward the heart, counteracting gravity and low pressure in limbs.

The basement membrane, or basal lamina, lies beneath the endothelium and links it to underlying connective tissue. This membrane ensures both strength and flexibility while allowing material passage between the vessel lumen and surrounding tissues. It provides structural support to the endothelium and contributes to the selective permeability of blood vessels.

The tunica externa is the outermost layer composed primarily of collagenous fibers with some elastic fibers. In veins, the tunica externa is the thickest tunic and may surpass the tunica media in some larger arteries. This layer merges with surrounding connective tissue, stabilizing the vessel's position. The tunica externa provides structural support and anchors blood vessels within surrounding tissues.