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vessels that transport blood from organs and tissues to the heart. Veins of the systemic circulation collect blood saturated with carbon dioxide, metabolites, hormones produced by the endocrine glands, and other substances from organs and parts of the body and transport it to the right atrium. Veins of the pulmonary circulation return oxygen-enriched blood from the lungs to the left atrium. Food absorbed in the gastrointestinal tract is transferred to the general bloodstream through the portal vein system. The veins originate in the capillary network where venules are formed, and on coalescing the venules form veins. The venous wall is much thinner and more plastic than the arterial wall, and its musculature is relatively weakly developed. Blood pressure is very low in the veins and below atmospheric pressure in the large ones. The flow of blood in the veins is greatly affected by the respiratory movements of the chest (suction) and movements of the diaphragm, while the deep veins of the extremities are affected by muscular contractions. The venous valves along the bloodstream allow blood to flow only to the heart. The numerous nerve endings in the walls of certain large veins (superior and inferior venae cavae, portal vein, and others) take part in regulating blood circulation.
IU. I. DENISOV-NIKOL’SKII
(also, nerves), in plants, the fibrous vascular bundles in leaf blades that perform conducting and mechanical functions. Large veins are formed by several of these bundles and often include strands of mechanical tissues and parenchyma. They are usually in the form of ribs on the lower side (sometimes on the upper side) of a leaf blade. Veins are also located in the petals and sepals of flowers, in fruits, and in plant stems. Veins in insects are hollow, tubular, hardened thickenings in the wing membrane. After the adult insect passes through the pupal or nymphal stages, the veins fill with hemolymph. This creates pressure that straightens the wing. There are both longitudinal and transverse veins, which maintain the stability and aerodynamic properties of the wing.