There are many blood vessels in the body. Arteries are those that carry blood away from the heart. Most of them deliver blood full of oxygen to the tissues, organs, and other blood vessels serviced by the circulatory system. They also remove waste products, maintain pH levels, and circulate proteins. Each artery has three layers and a lumen, the opening through which blood travels.
The layers of the arteries are tunicae. The outermost layer is the tunica externa, though some researchers refer to it as the tunica adventitia. This layer consists primarily of collagen, which anchors the arteries to nearby organs for stability. The layer also has bundles of the protein elastin, which form elastic fibers. In addition, the tunica externa contains the vasa vasorum, a network of small blood vessels that supply the walls of bigger blood vessels. Elastic arteries such as the aorta receive blood and nourishment from the vasa vasorum.
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