 The central adrenomedullary vein that passes through the adrenal medulla and its tributaries have an unusual tunica media. It contains several longitudinally oriented bundles of smooth muscle cells that vary in size and apperance (Fig 13.29). These irregularly arranged smooth muscle bundles (also called muscle cushions) extend into larger tributaries of the central adrenomedullary vein. This unique eccentric arrangement of smooth muscle bundles results in the irregularity in the thickness of the vascular wall. In areas where muscle bundles are absent, cells of the adrenal medulla or sometimes adrenal cortex are separated from the lumen of the vein only by a thin layer of the tunica intima (see Fig. 13.29). Contraction of the longitudinally arranged smooth muscles in the tunica media enhances the efflux of hormones from the adrenal medulla into the circulation.
Veins in certain other locations (e.g., retina, placenta, trabeculae of the spleen) also have atypical walls and are discussed in the chapters that describe these organs.
Lymphatic vessels convey fluids from the tissues to the bloodstream.
In addition to blood vessels, another set of vessels circulates fluid called lymph through most parts of the body. These lymph-carrying vessels serve as adjuncts to the blood vessels. Unlike the blood vessels, which convey blood to and from tissues, the lymphatic vessels are unidirectional, conveying fluid
FIGURE 13.26 • Photomicrograph of a large vein. This photomicrograph shows the three tunics in a section through the wall of the portal vein stained with H&E. The tunica intima consists of endothelium and a thin subendothelial layer of connective tissue containing a few smooth muscle cells. The tunica media contains relatively thin layer of circularly arranged smooth muscle cells. Tunica adventitia is the thickest layer of this vessel. It contains a thick layer of longitudinally arranged smooth muscle bundles (seen here in cross section) separated by collagen and elastic fibers. Note a layer of connective tissue containing coarse collagen and elastic fibers that separates longitudinal bundles of smooth muscle in the tunica adventitia from a layer of smooth muscles of the tunica media. 240. (Courtesy of Dr. Donald
J. Lowrie Jr., University of Cincinnati College of Medicine.) only from tissues. The smallest lymphatic vessels are called lymphatic capillaries. They are especially numerous in the loose connective tissues under the epithelium of the skin and mucous membranes. The lymphatic capillaries begin as “blind-ended” tubes in the microcapillary beds (see Fig. 13.23). Lymphatic capillaries converge into increasingly larger vessels called lymphatic vessels. They ultimately unite to form two main channels that empty into the blood vascular system by draining into the large veins in the base of the neck. Lymph enter the vascular system at the junctions of the internal jugular and subclavian veins. The largest lymphatic vessel, draining most of the body and emptying into the veins on the left side, is the thoracic duct. The other main channel is the right lymphatic trunk.
Lymphatic capillaries are more permeable than blood capillaries and collect excess protein-rich tissue fluid.