Blood Vessel Tunics

Both artery and vein walls have three layers, called tunics (too'nik; tunica = coat). The tunics surround the lumen (loo'men), or inside space, of the vessel through which blood flows. These tunics are the tunica intima, tunica media, and tunica externa (figure 23.1).



The innermost layer of a blood vessel wall is the tunica intima (too'ni-ka in-ti'ma; intimus = inmost), or tunica interna. It is composed of an endothelium (a simple squamous epithelium lining the blood vessel lumen) and a subendothelial layer made up of a thin layer of areolar connective tissue.



The tunica media (me'de-a; medius = middle) is the middle layer of the vessel wall. It is composed of circularly arranged layers of smooth muscle cells. Sympathetic innervation causes the smooth muscle to contract, resulting in vasoconstriction (va'so-kon-strik'shun), or narrowing of the blood vessel lumen. When the fibers relax, vasodilation (va'so-dl-la'shun), or widening of the blood vessel lumen, results.



The tunica externa (eks-ter'na; externe = outside), or funica adventitia, is the outermost layer of the blood vessel wall. It is composed of areolar connective tissue that contains elastic and collagen fibers. The tunica externa helps anchor the vessel to other structures. Very large blood vessels require their own blood supply to the tunica externa in the form of a network of small arteries called the vasa vasorum (va-sa va-sor'um; vessels of vessels). The vasa vasorum extend through the tunica externa.



In arteries, the thickest layer is the tunica media, while veins have a thicker tunica externa. The lumen in an artery is narrower than in its companion vein of the same size (figure 23.2). Further, arteries tend to have more elastic and collagen fibers in all their tunics, which means that artery walls remain open (patent), can spring back to shape, and can withstand changes in blood pressure. In contrast, vein walls tend to collapse if there is no blood in them. Table 23.1 compares the characteristics of arteries and veins.



Finally, capillaries contain only the tunica intima, but this layer consists of a basement membrane and endothelium only. Intercellular clefts are the thin spaces between adjacent cells in the




Figure 23.1



Walls of an Artery, a Capillary, and a Vein. Both arteries and veins have a tunica intima, tunica media, and tunica externa. However, an artery has a thicker tunica media and a relatively smaller lumen, while a vein’s thickest layer is the tunica externa, and it has a larger lumen. Some veins also have values. Capillaries typically have only a tunica intima, but they do not have a subendothelial layer—just the endothelium and a basement membrane.



Capillary wall. Having only the tunica intima, without connective tissue and muscle layers, allows for rapid gas and nutrient exchange between the blood and the tissues.

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