The walls of vessels are composed of variable amounts of connective tissue cells and fibres, while the walls of arteries, arterioles and large veins are composed of smooth muscle cells. In the walls of the heart and of some large veins, the contractile cells are striated. The lumina of all blood and lymphatic vessels are lined with a simple squamous epithelium or endothelium.

The histological structure of the walls of arteries and veins and of the walls of arterioles and venules are sufficiently characteristic to permit their identification and classification. However, the composition or arrangement of their elements in their walls varies not only in vessels of various sizes but also as a function of their locations in the body. For example, large pulmonary arteries are structurally different from large distributing arteries of the same calibre elsewhere in the body.

The walls of arteries and arterioles present three distinct layers, or tunicae: the intima, the media and the adventitia. The adventitia often merges with the dense or loose connective tissue surrounding the vessels. In the walls of veins or venules equivalent tunicae are generally difficult to identify, or their boundaries are blurred.

The purpose of the present chapter is to illustrate the histological features of the various vessels, from the largest to the smallest, using a variety of staining methods to offer useful criteria for identifying vessels in transverse, oblique or longitudinal sections. It should be emphasized that, owing to the structural variability of the various vessels, familiarity with their histology can be acquired only through examination of numerous sections with the microscope.