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 – окруженный тонкой стеной
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
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.