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In the alveoli, oxygen from the air is absorbed into the blood. Carbon dioxide, a waste product of metabolism, travels from the blood to the alveoli, where it can be exhaled. Between the alveoli is a thin layer of cells called the interstitium, which contains blood vessels and cells that help support the alveoli.
Inside this layer is the tunica media, or media, which is made up of smooth muscle cells and elastic tissue. The innermost layer, which is in direct contact with the flow of blood is the tunica intima.This layer is made up of mainly endothelial cells.eins: The thick, outer-most layer of a vein is made of collagen, wrapped in bands of smooth muscle while the interior is lined with epithelial cells called intima. Most veins have one-way flaps called venous valves that prevent blood from flowing back and pooling in the lower extremities due to the effects of gravity.