Inner Coat (Tunica intima)
The deepest layer of the vessel, in contact with blood or lymph. Its structure always includes two main layers:
- Endothelium — a continuous sheet of squamous cells resting on a basal lamina. It lines the vessel from the inside.
- Subendothelial layer — formed by loose connective tissue. Its main function is to cushion pressure surges, which is why it is best developed in large arteries.
In addition to the mandatory layers, the tunica intima may contain specific components: elastic elements in arteries or smooth muscle cells in selected veins and lymphatic vessels. Furthermore, the inner coat forms the valve apparatus (in lymphatic vessels and half of all veins), which prevents the backflow of fluid.
Middle Coat (Tunica media)
This coat lacks a clear division into sublayers and functions as a single unit. It is primarily composed of smooth myocytes, arranged mostly in a circular orientation, and an extracellular matrix (elastic and collagen fibers, proteoglycans).
A notable feature of the middle coat is the dual role of its smooth muscle cells:
- Contractile function — cells regulate the diameter of the vessel lumen.
- Biosynthetic function — myocytes synthesize extracellular matrix components themselves (supported by a well-developed granular endoplasmic reticulum).
Outer Coat (Tunica externa)
Also known as the adventitia (adventitia). It is constructed from loose connective tissue containing collagen and elastic fibers, along with fibroblasts and adipocytes (fat cells).
In the outer coat of large vessels, longitudinal bundles of myocytes may be found. Most importantly, this layer houses neural elements, lymphatic capillaries, and vasa vasorum (vessels of the vessel) that nourish the wall.
Blood Supply to the Vascular Wall
Nutrition of the wall varies depending on the vessel type:
- In arteries: oxygen-rich blood flows through the lumen. Therefore, the inner coat and part of the middle coat receive nutrients via diffusion directly from the lumen. Vasa vasorum are primarily located in the outer coat (with the exception of the aorta, where wall thickness causes them to penetrate into the middle coat). Venous drainage is directed outward.
- In veins: venous blood is oxygen-poor, so diffusion is insufficient. Consequently, vasa vasorum penetrate all three layers of the venous wall, and the venous ends of capillaries empty directly into the vein's lumen.