General Structure of Blood Vessels

VASCULAR SYSTEM

The cardiovascular system is concerned with the transport of blood and lymph through the body. It may be divided into four major components: the heart, the macrocirculation, the microcirculationand the lymph vascular system.

Essentially, the macrocirculation comprises all vessels, both arteries and veins, that would be visible to the eye. The vessels of the macrocirculation supply and drain a network of fine vessels interposed between them, the capillaries. This network is also called the capillary bed. Water and other components of the blood plasma which exude from the blood vessels form the interstitial fluid, which is returned to the circulation by the lymph vascular system.

General Structure of Blood Vessels

You have already seen blood vessels of various sizes and types in preparations available in other lab sessions, and you should be aware that the histological appearances of vessels of different sizes (arterioles vs. arteries) and different types (arteries vs. veins) are different from each other. These differences are the result of quantitative variations of a common structural pattern that can be seen in all blood vessels with the exception of capillaries, i.e. the division of the walls of the blood vessels into three layers or tunics.

The tunica intima delimits the vessel wall towards the lumen of the vessel and comprises its endothelial lining (typically simple, squamous) and associated connective tissue. Beneath the connective tissue, we find the internal elastic lamina, which delimits the tunica intima from the tunica media. The tunica media is formed by a layer of circumferential smooth muscle and variable amounts of connective tissue. A second layer of elastic fibers, the external elastic lamina, is located beneath the smooth muscle. It delimits the tunica media from the tunica adventitia, which consist mainly of connective tissue fibres. The tunica adventitia blends with the connective tissue surrounding the vessel. The definition of the outer limit of the tunica adventitia is therefore somewhat arbitrary.