General Characteristics of Blood Vessels

1. Blood Vessel Circuits

Blood is carried through the body via blood vessels. There are three general categories of blood vessels in the body: arteries, capillaries, and veins. An artery is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries branch into tiny capillaries, where nutrients and wastes are exchanged. Capillaries then combine with one another to form a vein, a blood vessel that returns blood to the heart.

Arteries, capillaries, and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit. Systemic arteries provide blood rich in oxygen to the body’s tissues. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation.

terms to know

Artery

A blood vessel that conducts blood away from the heart.

Capillary

A blood vessel where physical exchange occurs between the blood and the surrounding tissue.

Vein

The blood vessel that conducts blood toward the heart.

Systemic Circuit

The blood vessels that transport blood to and from virtually all of the tissues of the body.

Pulmonary Circuit

The blood vessels that transport blood to and from the lungs for gas exchange.

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2. Shared Structures

Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries have thicker walls than veins because they are closer to the heart and receive blood that is surging at a far greater pressure. 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.

key concept

By the time blood has passed through capillaries and entered veins, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, 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.

Both arteries and veins have the same three distinct tissue layers, called tunics (tunica, coat) From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa. The table below compares and contrasts the tunics of the arteries and veins.

Tunics	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 in the tunica media 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 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 Vasa vasorum present	Normally the thickest layer in veins Collagenous and smooth fibers predominate Some smooth muscle fibers Vasa vasorum present

term to know

Lumen

The interior of a tubular structure such as a blood vessel or a portion of the digestive tract through which blood, chyme, or other substances travel.

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3. Tunica Interna

The tunica interna (also called the tunica intima) is composed of epithelial and connective tissue layers. Composing the tunica interna is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagenous fibers beneath is one of the primary causes of clot formation.

Next to the endothelium is the basement membrane (also known as the basal lamina) that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it.

did you know

Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow.

In larger arteries, there is also a thick, distinct layer of elastic fibers known as the internal elastic membrane (also called the internal elastic lamina) at the boundary with the tunica media. Like the other components of the tunica interna, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow the exchange of materials between the tunics. The internal elastic membrane is not apparent in veins.

Under the microscope, the lumen and the entire tunica interna of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls.

Term	Pronunciation	Audio File
Tunica interna	tu·ni·ca in·ter·na

terms to know

Tunica Interna

The innermost lining or tunic of a blood vessel.

Internal Elastic Membrane

The membrane, which is composed of elastic fibers, and that separates the tunica interna from the tunica media; seen in larger arteries.

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4. Tunica Media

The tunica media is the substantial middle layer of the vessel wall. It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is made up primarily of elastic fibers. Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. In contrast, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.

Together, neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration. Regulation of both blood flow and blood pressure will be discussed in detail later.

The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also bind the tunica media to the inner and outer tunics. Along with the collagenous fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. 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 structure is not usually seen in smaller arteries, nor is it seen in veins.

Term	Pronunciation	Audio File
Tunica media	tu·ni·ca me·di·a
Vasoconstriction	va·so·con·stric·tion
Vasodilation	va·so·di·la·tion

terms to know

Tunica Media

The middle layer or tunic of a blood vessel.

Vasoconstriction

The constriction of the smooth muscle of a blood vessel, resulting in a decreased vascular diameter.

Vasodilation

The relaxation of the smooth muscle in the wall of a blood vessel, resulting in an increased vascular diameter.

External Elastic Membrane

The membrane composed of elastic fibers that separates the tunica media from the tunica externa; seen in larger arteries.

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5. Tunica Externa

The outer tunic, the tunica externa (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibers. Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries.

The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position. 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. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow.

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 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.

Term	Pronunciation	Audio File
Tunica externa	tu·ni·ca ex·ter·na
Vasa vasorum	vasa va·so·rum

terms to know

Tunica Externa

The outermost layer or tunic of a blood vessel.

Vasa Vasorum

The 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.