tunica externa

(noun)

The outermost layer of a blood vessel.

Related Terms

Examples of tunica externa in the following topics:

  • Blood Vessel Structure

    • The tunica intima is surrounded by a thin membrane comprised of elastic fibers running parallel to the vessel.
    • Surrounding the tunica intima is the tunica media, comprised of smooth muscle cells and elastic and connective tissues arranged circularly around the vessel.
    • The outermost layer is the tunica externa or tunica adventitia, composed entirely of connective fibers and surrounded by an external elastic lamina which functions to anchor vessels with surrounding tissues.
    • The tunica externa is often thicker in veins to prevent collapse of the blood vessel and provide protection from damage since veins may be superficially located.
    • This diagram of the artery wall indicates the smooth muscle, external elastic membrane, endothelium, internal elastic membrane, tunica externa, tunica media, and tunica intima.

  • Arteries, Veins, and Capillaries

    • The inner, tunica intima is a smooth, inner lining of endothelial cells that are in contact with the red blood cells.
    • Veins and arteries both have two further tunics that surround the endothelium: the middle, tunica media is composed of smooth muscle, while the outer tunica externa is connective tissue (collagen and elastic fibers).
    • Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner tunica intima.
    • Capillaries consist of a single layer of epithelial cells, the endothelium tunic (tunica intima).

  • Artery Function

    • As with veins, arteries are comprised of three layers: the tunicae intima, media, and externa.
    • In arteries, the tunica media, which contains smooth muscle cells and elastic tissue, is thicker than that of veins so it can modulate vessel caliber and thus control and maintain blood pressure.
    • Arteries of the systemic circulation can be subdivided into muscular or elastic types according to the the relative compositions of elastic and muscle tissue in their tunica media.

  • Elastic Arteries

    • An elastic or conducting artery has a large number of collagen and elastin filaments in the tunica media.
    • Elastic arteries contain larger numbers of collagen and elastin filaments in their tunica media than muscular arteries do, giving them the ability to stretch in response to each pulse.
    • Due to position as the first part of the systemic circulatory system closest to the heart and the resultant high pressures it will experience, the aorta is perhaps the most elastic artery, featuring an incredibly thick tunica media rich in elastic filaments.
    • Arterial wall layers including the tunica intima and the tunica media.
    • In elastic arteries, the tunica media is rich with elastic and connective tissue.

  • Muscularis

    • The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer.
    • The thickness of muscularis externa varies in each part of the tract.
    • In the colon, for example, the muscularis externa is much thicker because the feces are large and heavy, and require more force to push along.
    • Alone among the GI tract, the stomach has a third layer of muscularis externa.

  • Submucosa

    • The submucosa lies under the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer, the muscularis externa.
    • The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and muscularis externa.
    • It contains Meissner's plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.

  • Testes

    • Under a tough membranous shell, the tunica albuginea contains very fine coiled tubes called seminiferous tubules.
    • Tunica albuginea, 2.
    • Tunica vaginalis (parietal lamina), 11b.
    • Tunica vaginalis (visceral lamina), and 12.
    • Cavity of tunica vaginalis.

  • Microscopic Anatomy of the Stomach

    • The submucosa lies under this and consists of fibrous connective tissue, separating the mucosa from the next layer, the muscularis externa.
    • The muscularis externa is made up of three layers of smooth muscle.

  • Blood Flow in the Skin

    • The skin contains a network of small blood vessels containing muscle fibers in their tunica media.

  • Overview of Hemostasis

    • Blood vessel with an erythrocyte (red blood cell) within its lumen, endothelial cells forming its tunica intima or inner layer, and pericytes forming its tunica adventitia (outer layer).
    • Microvessel showing an erythrocyte (E), a tunica intima of endothelial cells, and a tunica adventitia of pericytes.