Blood vessels
This drawing of an artery illustrates the layers comprising blood vessels. Tunica intima is the innermost layer facing the blood, tunica media is the middle layer and tunica adventitia, the outermost tunic, is continuous with the connective tissue that surrounds it. The three tunics of blood vessels are continuous with analogous layers of the heart.
Tunica intima >
The tunica intima is lined by an inner layer of specialized simple squamous epithelium termed endothelium. Underlying the endothelium is a thin connective tissue layer. Beneath the connective tissue is the internal elastic lamina, consisting of a single, broad, fenestrated elastic sheet. This lamina is lacking in the smallest arteries and all but the larger veins.
- Internal elastic lamina
The tunica intima is lined by an inner layer of specialized simple squamous epithelium termed endothelium. Underlying the endothelium is a thin connective tissue layer. Beneath the connective tissue is the internal elastic lamina, consisting of a single, broad, fenestrated elastic sheet. This lamina is lacking in the smallest arteries and all but the larger veins.
Tunica media >
The tunica media is the middle layer and contains circularly arranged smooth muscle cells. These cells are the source of the connective tissue components of tunica media, including fenestrated elastic sheets in elastic arteries.
External elastic lamina >
The external elastic lamina is only present in medium or muscular arteries and consists of a layer of fenestrated elastic sheets at the junction of the tunica media and tunica adventitia. This layer is also referred to as external elastic lamellae.
Tunica adventitia >
Tunica adventitia is the outermost tunic of blood vessels. This layer is composed of connective tissue, however, in the largest veins, it also contains longitudinally-oriented smooth muscle cells. The connective tissue of tunica adventitia blends imperceptibly with surrounding connective tissue.
