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Arteries and veins are both parts of the biological circulatory system, and both perform the function of transporting blood through the various parts of the body, most importantly the heart, lungs, and brain.
Arteries carry blood away from the heart, pumping it through the lungs where it picks up oxygen. The arteries continue throughout the body, delivering their blood to the capillaries where the oxygen load is discharged. Arteries are thicker than veins because they have to withstand high internal pressure; they also contain smooth muscle, which is used to contract the arteries and keep the blood moving. Arterial blood becomes bright red when removed from the arteries because of its high oxygen content.
Veins carry blood away from the extremities of the body and back to the heart, where it is pumped back into the arteries for re-oxygenation. Veins are connected to arteries in a closed system, meaning that the blood circulates and is always contained. Veins are differentiated from arteries in several ways; they are closer to the skin and thinner, since they don't need to contract for blood flow. However, this also means that there is less pressure than in arteries; to keep blood from flowing backwards, veins contain one-way valves. Venous blood is dark red because of its low oxygen content.
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