How does the structure of the veins differ from the structure of the arteries?

The walls of the veins are also three-layered, but the thickness of the layers is much less than that of the arteries. There are two types of veins: smooth muscle cells are absent in the walls of non-muscular veins, they are tightly adhered to the walls of organs and are found in the retina, meninges, spleen; in the walls of muscle-type veins there are smooth muscle cells. The blood flow velocity in the veins is less than in the arteries. Most veins have valves on the inner lining that prevent blood from flowing back.

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