Blood Vessel Tunics
Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure (Figure (PageIndex{2})). Each type of vessel has a lumen—a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins.
By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity.
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Both arteries and veins have the same three distinct tissue layers, called tunics (from the Latin term tunica, for the garments first worn by ancient Romans). From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa (see Figure (PageIndex{2})). Table (PageIndex{1}) compares and contrasts the tunics of the arteries and veins.
Table (PageIndex{1}): Comparison of Tunics in Arteries and Veins Tunic (Layer) Arteries Veins General Appearance
- Thick walls with small lumens
- Generally appear rounded
- Thin walls with large lumens
- Generally appear flattened
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Tunica Intima
- Endothelium usually appears wavy due to constriction of smooth muscle
- Internal elastic membrane present in larger vessels
- Endothelium appears smooth
- Internal elastic membrane absent
Tunica Media
- Normally the thickest layer in arteries
- Smooth muscle cells and elastic fibers predominate (the proportions of these vary with distance from the heart)
- Nervi vasorum and vasa vasorum present
- External elastic membrane present in larger vessels
- Normally thinner than the tunica externa
- Smooth muscle cells with elastic and collagen fibers
- Nervi vasorum and vasa vasorum present
- External elastic membrane absent
Tunica Externa
- Normally thinner than the tunica media in all but the largest arteries
- Collagen and elastic fibers
- Nervi vasorum and vasa vasorum present
- Normally the thickest layer in veins
- Collagen and smooth fibers predominate
- Some smooth muscle fibers
- Nervi vasorum and vasa vasorum present
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