Basic Structure of a Blood Vessel

Three tunics of blood vessel

The three structural layers of a generalized blood vessel from innermost to outermost are the tunica intima, tunica media and tunica adventitia. Modification of this basic design account for the five types of blood vessels and the structural and functional differences among the various vessel types. The tunica intima forms the inner lining of blood vessel and direct contact with the blood as it flows through the lumen of the vessel. Its innermost layer is endothelium which continuous with endocardial lining of the heart. The internal elastic lamina is the outermost part of tunica intima and facilitates diffusion of materials through the tunica intima to thicker tunica media. Besides that, tunica media is a muscular and connective tissue layer that displays the greatest variation among vessel types. The primary role of smooth muscle in tunica media is to regulate the diameter of lumen. Furthermore, tunica adventitia consists of elastic and collagen fibers. The tunica adventitia helps anchor the vessels to surrounding tissues. The small vessels that supply blood to tissues of the vessel are called vasa vasorum.  

Tunica Interna (Intima)

The tunica interna (intima) forms the inner lining of a blood vessel and is in direct contact with the blood as it flows through the lumen, or interior opening, of the vessel. Although this layer has multiple parts, these tissues components contribute minimally to the thickness of the vessel  wall. Its innermost layer is a simple squamous epithelium, called endothelium, which is continuous with the endocardial cells were regarded as little more than a passive barrier between the blood and the remainder of the vessel wall. it is now known that endothelial cells are active participants in a variety of vessel-related activities, including physically influencing blood flow, secreting locally acting chemical mediators that influence the contractile state of the vessel;s overlying smooth muscle, and  assisting with capillary permeability.

The second components of the tunica interna is a basement membrane deep to the endothelium. It provides a physical support base for the the epithelial layer. Its framework of collagen fibers affords the basal lamina significant tensile strength yet its properties also provide resilience fro stretching and recoil. The basal lamina anchors the endothelium to the underlying connective tissues while also regulating molecular movement. it appears to play an important role in guiding cell movements during tissues repair of blood vessel wall. The outermost part of the tunica interna, which forms the boundary between the tunica interna and tunica media,  is the internal elastic lamina. The internal elastic lamina is a thin sheet of elastic fibers with a variable number of window-like openings that give it the look of Swiss cheese. These openings facilitate diffusion of materials through the tunica interna to the thicker tunica media.

Tunica Media

The tunica media is a muscular and connective tissues layer that displays the greatest variation among the different vessel types. In most vessels, it is a relatively thick layer comprised mainly of smooth muscle cells and substantial amounts of elastic fibers. The primary role of the smooth muscle cells, which extend circularly around the lumen like a ring encirles your finger, is to regulate the diameter of the luman, As you will learn in more detail shortly, the rate of blood flow through different parts of the body is regulated by the vessels. Furthermore, the extents of smooth muscle contraction in particular vessel types is crucial in the regulation of blood pressure.

In addition to regulating blood flow and blood pressure, smooth muscle contract when vessels are damaged to help limit loss of blood through the injured vessel and smooth muscle cells help produce the elastic fibers within the tunica media that allow the vessels to stretch and recoil under the applied pressure of the blood.

The tunica media is the most variable of the tunics. 

Tunica Adventitia

The outer covering of a blood vessel, the tunica adventitia, consists of elastic and collagen fibers. Separating the tunica adventitia from the tunica media is a network of elastic fibers, the external adventitia contains numerous nerves and , especially in larger vessels, tiny blood vessels that supply the tissues of the vessel wall. These small vessels that supply blood to the tissues of the vessel is called vasa vasorum, or vessels to the vessels. They care easily seen on large vessels such as the aorta. In addition to the important role of supplying the vessel wall with nerves and self-vessels, the tunica adventitia helps anchor the vessels to surrounding tissues.

Source: http://www.cayuga-cc.edu/people/web_pages/greer/biol204/vessels1/vessels1.html