<< Chapter < Page Chapter >> Page >

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.

The walls of arteries and veins are largely composed of living cells and their products (including collagenous and elastic fibers); the cells require nourishment and produce waste. Since blood passes through the larger vessels relatively quickly, there is limited opportunity for blood in the lumen of the vessel to provide nourishment to or remove waste from the vessel’s cells. Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. Larger arteries and veins contain small blood vessels within their walls known as the vasa vasorum    —literally “vessels of the vessel”—to provide them with this critical exchange. Since the pressure within arteries is relatively high, the vasa vasorum must function in the outer layers of the vessel (see [link] ) or the pressure exerted by the blood passing through the vessel would collapse it, preventing any exchange from occurring. The lower pressure within veins allows the vasa vasorum to be located closer to the lumen. The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since its location makes it more difficult to nourish the cells of the arteries and remove waste products. There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle. These minute nerves are known as the nervi vasorum.

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; the term tunic is also used for some modern garments. From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa (see [link] ). [link] compares and contrasts the tunics of the arteries and veins.

Comparison of Tunics in Arteries and Veins
Arteries Veins
General appearance Thick walls with small lumens
Generally appear rounded
Thin walls with large lumens
Generally appear flattened
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)
External elastic membrane present in larger vessels
Normally thinner than the tunica externa
Smooth muscle cells and collagenous fibers predominate
Nervi vasorum and vasa vasorum present
External 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 vasorum and vasa vasorum present

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, 101-321-va - vertebrate form and function ii. OpenStax CNX. Jul 22, 2015 Download for free at https://legacy.cnx.org/content/col11850/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the '101-321-va - vertebrate form and function ii' conversation and receive update notifications?

Ask