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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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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.


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.


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