Английский язык для медиков
Шрифт:
The tunica adventitia is a longitudinally oriented collection of collagenous bundles and delicate elastic fibers with associated fibroblasts. Large blood vessels have their own blood supply (vasa vasorum), which consists of small vessels that branch profusely in the walls of larger arteries and veins. Muscular distributing arteries are medium—sized vessels that are characterized by their predominance of circularly arranged smooth muscle cells in the media interspersed with a few elastin compo nents. Up to 40 layers of smooth muscle may occur. Both internal and external elastic limiting membranes are clearly demonstrated. The intima is thinner than that of the large arteries.
Arterioles are the smallest components of the arterial tree. Generally, any artery less than 0,5 mm in diameter is considered to be a small artery or arteriole. A suben—dothelial layer and the inter nal elastic membrane may be present in the largest of these vessels but are absent in the smaller ones. The media is composed of sev eral smooth muscle cell layers, and the adventitia is poorly devel oped. An external elastic membrane is absent.
New words
endothelium –
media – средняя
arteries – артерии
to be classified – классифицированный
according – соответственно
their – их
size – размер
appearance – вид
tunica – оболочка
major – главный
elastic – эластичный
conducting – проведение
arteries – артерии
to include – включать
aorta – аорта
branches – ветви
up to – до
layers – слои
smooth – гладкий
may – может
infima – внутренняя полость артерии
24. Capillaries
Capillaries are thin—walled, narrow—diameter, low—pressure vessels that generally permit easy diffusion across their walls. Most capillar ies have a cross—sectional diameter of 7 —12 mm. They are composed of a simple layer of endothelium, which is the lining of the entire vas cular system, and an underlying basal lamina. They are attached to the surrounding tissues by a delicate reticu—lum of collagen. Associated with these vessels at various points along their length are specialized cells called pe—ricytes. These cells, enclosed within their own basal lamina, which is continuous with that of the endothelium, contain contractile proteins and thus may be involved in the control of capillary dynamics. They may also serve as stem cells at times of vascular repair. Capillaries are generally divided into three types, according to the structure of their endothelial cell walls.
Continuous (muscular, somatic) capillaries are formed by a single uninterrupted layer of endothelial cells rolled up into the shape of a tube and can be found in locations such as connective tissue, muscle, and nerve.
Fenestrated (visceral) capillaries are characterized by the presence of pores in the endothelial cell wall. The pores are covered by a thin diaphragm (except in the glome—ruli of the kidney) and are usually encountered in tissues where rapid substance interchange occurs (e. g., kidney, intestine, endocrine glands).
Sinusoidal capillaries can be found in the liver, hemato—poietic and lymphopoietic organs, and in certain endocrine glands. These tubes with discontinuous endothelial walls have a larger diame ter than other capillaries (up to 40 mm), exhibit irregular cross—sec tional profiles, have more tortuous paths, and often lack a con tinuous basal lamina. Cells with phagocytic activity (macropha—ges) are present within, or just subjacent to, the en—dothelium.
New words
capillaries – капилляры
to thin—walled –
narrow—diameter – узкий диаметр
low—pressure – низкое давление
that – тот
generally – главным образом
permit – разрешение easy – легкий
diffusion – распространение
cross—sectional – поперечный
to be composed – быть сложным
simple – простой
endothelium – эндотелий
lining – выравнивание
entire – весь
vas cular – сосудистый
underlying – лежащий в основе
basal – основной
lamina – тонкая пластинка
25. Veins
Veins are low—pressure vessels that have larger lumina and thinner walls than arteries. In general, veins have more collagenous connec tive tissue and less muscle and elastic tissue than their arterial coun terparts. Although the walls of veins usually exhibit the three layers, they are much less distinct than those of the arter ies. Unlike arteries, veins contain one—way valves composed of exten sions of the intima that prevent reflux of blood away from the heart. Veins can be divided into small veins or venules, medium veins, and large veins.
Venules are the smallest veins, ranging in diameter from approxi mately 15–20 mm (post—capillary venules) up to 1–2 mm (small veins). The walls of the smaller of these are structurally and func tionally like those of the capillaries; they consist of an endothelium surrounded by delicate collagen fibers and some pericytes. In those vessels of increased diameter, circularly arranged smooth muscle cells occur surrounding the intima layer, but unlike in the small arteries, these cells are loosely woven and widely spaced. Venules are important in inflammation because their endothelial cells are sensitive to hista—mine released by local mast cells. This causes endotheli—al cells to contract and separate from each other, exposing a naked basement membrane. Neutrophils stick to the exposed collagen and extravasate (i. e., move out into the connective tissue). Histamine also causes local arterioles to relax, affect ing a rise in venous pressure and increased leaking of fluid. This produces the classic signs of inflammation: redness, heat, and swelling.
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.