What does VEINS stand for? 

veins

Veins are blood vessels in the circulatory system of humans and most other animals that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal circulations which carry oxygenated blood to the heart. In the systemic circulation arteries carry oxygenated blood away from the heart, and veins return deoxygenated blood to the heart. There are three sizes of veins, large, medium, and small. Smaller veins called venules, and the smallest the post-capillary venules are microscopic, and they make up the veins of the microcirculation. Veins have less smooth muscle, and connective tissue than arteries, and are often closer to the skin. Because of the thinner walls in veins they are able to expand and can hold more blood. At any time, nearly 70% of the total volume of blood in the human body is in the veins. In medium and large sized veins the flow of blood is maintained by one-way (unidirectional) venous valves to prevent backflow.

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