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blood vessel that returns blood to the heartheart,
muscular organ that pumps blood to all parts of the body. The rhythmic beating of the heart is a ceaseless activity, lasting from before birth to the end of life. Anatomy and Function

The human heart is a pear-shaped structure about the size of a fist.
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. Except for the pulmonary vein, which carries oxygenated blood from the lungs to the heart, veins carry deoxygenated blood. The oxygen-depleted blood passes from the capillariescapillary
, microscopic blood vessel, smallest unit of the circulatory system. Capillaries form a network of tiny tubes throughout the body, connecting arterioles (smallest arteries) and venules (smallest veins).
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 to the venules (small veins). The venules feed into larger veins, which eventually merge into the superior and inferior vena cavae, large vessels that consolidate the blood flow from the head, neck, and arms and from the trunk and legs, respectively (see also circulatory systemcirculatory system,
group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the lungs, and the
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). The vena cavae direct the blood back into the heart. The walls of a vein are formed of three layers like the walls of an artery. However, these layers are thinner and less muscular and collapse when empty. With such notable exceptions as the portal system, most veins contain valves, formed by pouches in their inner coats, that keep the blood from flowing backward. Valves are most numerous in the veins of the extremities, and are absent in the smallest veins. Veins are subject to inflammation, dilatation or enlargement (as in a varicose veinvaricose vein,
superficial vessel that is abnormally lengthened, twisted, or dilated, seen most often on the legs and thighs. Varicose veins develop spontaneously, and are usually attributed to a hereditary weakness of the vein; the valves in the vein that keep the blood
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), rupture, and blockage by blood clots (thrombosisthrombosis
, obstruction of an artery or vein by a blood clot (thrombus). Arterial thrombosis is generally more serious because the supply of oxygen and nutrition to an area of the body is halted.
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The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a mineral body filling a fissure in rock. Simple veins are single mineralized fissures; complex veins are clusters of interwoven fissures or of zones of fracturing or schist formation. According to their morphological details veins are called lenticular, chambered, saddle-reef, ladder, or feather. Veins that cut across the layers of enclosing rock are called intersecting veins; those that lie in conformity with the stratification and dip of the enclosing rock are called stratified. The length of veins of mineral products varies from 1 m or less to 200 km—for example, the veins of gold ore in California. In terms of dip some veins taper off close to the earth’s surface, and others, for example, the vein of the Kolar deposit in India, are worked at a depth of more than 3 km. A vein has a geological and a working thickness, the minimum thickness for exploiting the vein deposit. Depending on the value of the constituent minerals, the working thickness of a vein may vary from several centimeters to dozens of meters.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A relatively thin-walled blood vessel that carries blood from capillaries to the heart in vertebrates.
One of the vascular bundles in a leaf.
A mineral deposit in tabular or shell-like form filling a fracture in a host rock.
(invertebrate zoology)
One of the thick, stiff ribs providing support for the wing of an insect.
A venous sinus in invertebrates.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. any of the tubular vessels that convey oxygen-depleted blood to the heart
2. any of the hollow branching tubes that form the supporting framework of an insect's wing
3. any of the vascular strands of a leaf
4. a clearly defined mass of ore, mineral, etc., filling a fault or fracture, often with a tabular or sheetlike shape
5. a natural underground watercourse
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
2--From the geologic studies on the Magnesite veins and veinlets on north of Naein and Ashin it can be inferred that the ultrabasic rocks, especially Harzburgites have tolerated a tectonic phase after their formation and as a result of performance of this tectonic forces a series of shear fractures_and fractures have been developed in them so that they have been suitable points of penetration of hot carbon dioxide rich solutions; such solutions, through absorbing the Magnesium ions from the mentioned rocks and the subsequent cooling and sedimentation near the ground surface have formed Magnesite veins and veinlets.
An about 8 mm thick veinlet crosscuts the metamorphic foliation and is inclined at about 30[degrees] to the drill-axis.
The tunnel having been turned again to the south, its end had reached a point 12+ meters below the famous "Red Pocket." Swoboda decided to turn again to the north, and soon they were entering a region of yellow wulfenite veinlets coursing through the powdery black manganese oxide and fractured siliceous rock.
It also occurs as fracture fillings and the largest of these veinlets is 0.4 cm wide by 1.4 cm long.
It seemed logical to favor the direction in which the veinlet was enlarging, so we shifted our tunnel direction to the right to almost due north to follow its lead.
Weed (1912) also reported the mineral from the Gagnon mine, as veinlets flanking the big veins, and in a number of veinlet occurrences in the East Ridge area.
The eastern Robino northsouth shear sector had previously returned 37.6 g/t gold in association with quartz-pyrite veinlets in the shear and 2 channels of some 5 g/t gold over 1 m each.
The Sparrowhawk mineralized zone comprises silicified magmatic-hydrothermal breccias with quartz-magnetite-chalcopyrite-chlorite matrix cut by quartz-calcite-chalcopyrite-pyrite-magnetite-hematite veinlets. Silicified intrusive rocks with quartz-hematite-pyrite-chalcopyrite stringers are also present (S850058).