http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25, Thick walls with small lumens; Generally appear rounded, Thin walls with large lumens; Generally appear flattened, Endothelium usually appears wavy due to constriction of smooth muscle; Internal elastic membrane present in larger vessels, Endothelium appears smooth; Internal elastic membrane absent, 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, 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, Present most commonly in limbs and in veins inferior to the heart, Compare and contrast the three tunics that make up the walls of most blood vessels, Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function, Describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains, Explain the structure and function of venous valves in the large veins of the extremities, Large venous networks (liver, bone marrow, and integument) 21%. Those in the brain are part of the blood-brain barrier. By the end of this section, you will be able to: Blood is carried through the body via blood vessels.

Figure 6.

The tunica externa in veins also contains groups of smooth muscle fibers. Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others.

The 2.5-mile channel meanders through some of nature's most spectacular forested scenery. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy.

Postcapillary venules join multiple capillaries exiting from a capillary bed.

This profession often overlaps with cardiovascular technology, which would also include treatments involving the heart. It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. Thoroughfares are used by a variety of traffic, such as cars on roads and highways. A thoroughfare channel connects the metarteriole to a venule.
arteriole: (also, resistance vessel) very small artery that leads to a capillary, arteriovenous anastomosis: short vessel connecting an arteriole directly to a venule and bypassing the capillary beds, artery: blood vessel that conducts blood away from the heart; may be a conducting or distributing vessel, capacitance: ability of a vein to distend and store blood, capillary: smallest of blood vessels where physical exchange occurs between the blood and tissue cells surrounded by interstitial fluid, capillary bed: network of 10–100 capillaries connecting arterioles to venules, continuous capillary: most common type of capillary, found in virtually all tissues except epithelia and cartilage; contains very small gaps in the endothelial lining that permit exchange, elastic artery: (also, conducting artery) artery with abundant elastic fibers located closer to the heart, which maintains the pressure gradient and conducts blood to smaller branches, external elastic membrane: membrane composed of elastic fibers that separates the tunica media from the tunica externa; seen in larger arteries, fenestrated capillary: type of capillary with pores or fenestrations in the endothelium that allow for rapid passage of certain small materials, internal elastic membrane: membrane composed of elastic fibers that separates the tunica intima from the tunica media; seen in larger arteries, lumen: interior of a tubular structure such as a blood vessel or a portion of the alimentary canal through which blood, chyme, or other substances travel, metarteriole: short vessel arising from a terminal arteriole that branches to supply a capillary bed, microcirculation: blood flow through the capillaries, muscular artery: (also, distributing artery) artery with abundant smooth muscle in the tunica media that branches to distribute blood to the arteriole network, nervi vasorum: small nerve fibers found in arteries and veins that trigger contraction of the smooth muscle in their walls, perfusion: distribution of blood into the capillaries so the tissues can be supplied, precapillary sphincters: circular rings of smooth muscle that surround the entrance to a capillary and regulate blood flow into that capillary, sinusoid capillary: rarest type of capillary, which has extremely large intercellular gaps in the basement membrane in addition to clefts and fenestrations; found in areas such as the bone marrow and liver where passage of large molecules occurs, thoroughfare channel: continuation of the metarteriole that enables blood to bypass a capillary bed and flow directly into a venule, creating a vascular shunt, tunica externa: (also, tunica adventitia) outermost layer or tunic of a vessel (except capillaries), tunica intima: (also, tunica interna) innermost lining or tunic of a vessel, tunica media: middle layer or tunic of a vessel (except capillaries), vasa vasorum: small blood vessels located within the walls or tunics of larger vessels that supply nourishment to and remove wastes from the cells of the vessels, vascular shunt: continuation of the metarteriole and thoroughfare channel that allows blood to bypass the capillary beds to flow directly from the arterial to the venous circulation, vasoconstriction: constriction of the smooth muscle of a blood vessel, resulting in a decreased vascular diameter, vasodilation: relaxation of the smooth muscle in the wall of a blood vessel, resulting in an increased vascular diameter, vasomotion: irregular, pulsating flow of blood through capillaries and related structures, vein: blood vessel that conducts blood toward the heart, venous reserve: volume of blood contained within systemic veins in the integument, bone marrow, and liver that can be returned to the heart for circulation, if needed, venule: small vessel leading from the capillaries to veins.

Their ability to hold this much blood is due to their high capacitance, that is, their capacity to distend (expand) readily to store a high volume of blood, even at a low pressure. The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagenous fibers to provide additional strength. Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. 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. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance. Their function is critical: If all of the capillary beds in the body were to open simultaneously, they would collectively hold every drop of blood in the body and there would be none in the arteries, arterioles, venules, veins, or the heart itself.

Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid.

Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells. A thoroughfare channel connects the metarteriole to a venule. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. On water a thoroughfare may refer to a strait, channel or waterway. There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle.

Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively.

Hormones and local chemicals also control blood vessels.

During sleep or rest periods, vessels in both areas are largely closed; they open only occasionally to allow oxygen and nutrient supplies to travel to the tissues to maintain basic life processes.

If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure.

The outer tunic, the tunica externa (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibers.

In turn, muscular arteries branch to distribute blood to the vast network of arterioles. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart.

The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle. An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches.
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 2).

In other words, thoroughfare may refer to the legal right to use a particular way. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane (also called the external elastic lamina), which also appears wavy in slides.

This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. They may also assist physicians in treating disorders involving the arteries and veins.

The three major types of capillaries: continuous, fenestrated, and sinusoid. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. This structure is not usually seen in smaller arteries, nor is it seen in veins.

Figure 5. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins.

Why?