17.1: Shared Structures
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 17.1 ). 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.
Comparison of Tunics in Arteries and Veins
| Arteries | Veins | |
| General Appearance |
Thick Walls with small lumens
|
Thin walls with large lumens
Generally appear flattened |
| Tunica Intima |
Endothelium usually appears wavy due to constriction of smooth muscle
Internal elastic membrane presents in larger vessels |
Endolithelium 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" External elastic membrane present in larger vessels |
Normally thinner than the tunica externa
Smooth muscle cells and collagenous fibers predeominate Esternal elastic membrane absent |
| Tunica externa |
Normally thinner than the tunica media in all but the largest arteries
Collagenous and elastic fibers Nervi vasorum and vasa vasorum present |
Normally the thickest layer in veins
Collagenous and smooth fibers predominate Some smooth muscle fibers Nervi vasoum and vasa vasorum present |