Medium veins in the range of 1–9 mm in diameter have a well – developed intima, a media consisting of connective tissue and loosely organized smooth muscle, and an adventitia (usually the thickest layer) composed of collagen bundles, elastic fibers, and smooth muscle cells oriented along the longitudinal axis of the vessel. Venous valves are sheet—like outfoldings of endothelium and underlying connective tissue that form flaps to permit uni—di rectional flow of blood.
Large veins, such as the external iliac, hepatic portal, and vena cavae, are the major conduits of return toward the heart. The intima is similar to that of medium veins. Although a network of elastic fibers may occur at the boundary between the intima andmedia, a typical internal elastic membrane as seen in arteries is not present. A tunica media may or may not be present. If pre sent, smooth muscle cells are most often circularly arranged. The ad—ventitia is the thickest layer of the wall and consists of elastic fibers and longitudinal bundles of collagen. In the vena cava, this layer also contains well—developed bundles of longitudinally oriented smooth muscle.
New words
vein – вена
low—pressure – низкое давление
collagenous – коллагеновый
intima – интима
reflux – рефлюкс
inflammation – воспаление
longitudinal – продольный
flaps – створки
iliac – подвздошный
hepatic – печеночный
The heart is a muscular organ, composed primarily of cardiac muscle tissue, which contracts rhythmically to pump blood throughout the body. Structure of the heart wall: the walls of the heart are constructed in layers that are similar to those of the major blood vessels.
Endocardium is the innermost layer of the heart and is lined with endothelium. Veins, nerves, and components of the impulse conduc—ting system are present in the suben—docardial connective tissue layer.
Myocardium is composed of branching, anastomotic cardiac myocytes attached to one another by intercalated disks. Most of these cells are involved in the pumping function of the heart; others are specialized for the control of rhythmicity (impulse conducting system) or secretion (myocardial endocrine cells).
Epicardium is a serous membrane that forms the visceral lining of the pericardium. Its external mesothelium is supported by a loose connective tissue subepicardial layer.
Cardiac skeleton is composed mainly of dense connective tissue and consists of the annuli fibrosi, the trigonum fibrosum, and the septum membranaceum.
Cardiac valves are composed of dense fibrous tissue covered by endothelium. Unidirectional flow is maintained from the.