An artery is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart. However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. This type of artery is known as an elastic artery (image). Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump (the stroke volume) and maintain adequate pressure and flow. Artery walls would have to become even thicker in response to this increased pressure. The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system. 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.

Farther from the heart, where the surge of blood has dampened, the percentage of elastic fibers in an artery’s tunica intima decreases and the amount of smooth muscle in its tunica media increases. The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0.1 mm to 10 mm. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction. In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important.

Notice that although the distinctions between elastic and muscular arteries are important, there is no “line of demarcation” where an elastic artery suddenly becomes muscular. Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. For this reason, a muscular artery is also known as a distributing artery.

Blood relatively high in oxygen concentration is returned from the pulmonary circuit to the left atrium via the four pulmonary veins. From the left atrium, blood moves into the left ventricle, which pumps blood into the aorta. The aorta and its branches—the systemic arteries—send blood to virtually every organ of the body (image).

The external iliac artery exits the body cavity and enters the femoral region of the lower leg (Figure). As it passes through the body wall, it is renamed the femoral artery. It gives off several smaller branches as well as the lateral deep femoral artery that in turn gives rise to a lateral circumflex artery. These arteries supply blood to the deep muscles of the thigh as well as ventral and lateral regions of the integument. The femoral artery also gives rise to the genicular artery, which provides blood to the region of the knee. As the femoral artery passes posterior to the knee near the popliteal fossa, it is called the popliteal artery. The popliteal artery branches into the anterior and posterior tibial arteries.

The anterior tibial artery is located between the tibia and fibula, and supplies blood to the muscles and integument of the anterior tibial region. Upon reaching the tarsal region, it becomes the dorsalis pedis artery, which branches repeatedly and provides blood to the tarsal and dorsal regions of the foot. The posterior tibial artery provides blood to the muscles and integument on the posterior surface of the tibial region. The fibular or peroneal artery branches from the posterior tibial artery. It bifurcates and becomes the medial plantar artery and lateral plantar artery, providing blood to the plantar surfaces. There is an anastomosis with the dorsalis pedis artery, and the medial and lateral plantar arteries form two arches called the dorsal arch (also called the arcuate arch) and the plantar arch, which provide blood to the remainder of the foot and toes. Figure shows the distribution of the major systemic arteries in the lower limb. Table summarizes the major systemic arteries discussed in the text.

As the subclavian artery exits the thorax into the axillary region, it is renamed the axillary artery. Although it does branch and supply blood to the region near the head of the humerus (via the humeral circumflex arteries), the majority of the vessel continues into the upper arm, or brachium, and becomes the brachial artery (Figure). The brachial artery supplies blood to much of the brachial region and divides at the elbow into several smaller branches, including the deep brachial arteries, which provide blood to the posterior surface of the arm, and the ulnar collateral arteries, which supply blood to the region of the elbow. As the brachial artery approaches the coronoid fossa, it bifurcates into the radial and ulnar arteries, which continue into the forearm, or antebrachium. The radial artery and ulnar artery parallel their namesake bones, giving off smaller branches until they reach the wrist, or carpal region. At this level, they fuse to form the superficial and deep palmar arches that supply blood to the hand, as well as the digital arteries that supply blood to the digits. Figure shows the distribution of systemic arteries from the heart into the upper limb. Table summarizes the arteries serving the upper limbs.

Arteries carry blood away from the heart. Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs. Systemic arteries transport oxygenated blood from the left ventricle to the body tissues. Blood is pumped from the ventricles into large elastic arteries that branch repeatedly into smaller and smaller arteries until the branching results in microscopic arteries called arterioles. The arterioles play a key role in regulating blood flow into the tissue capillaries. About 10 percent of the total blood volume is in the systemic arterial system at any given time.

The wall of an artery consists of three layers. The innermost layer, the tunica intima (also called tunica interna), is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers. The middle layer, the tunica media, is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure. The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel.

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 (image). 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.

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 image) 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 image). image compares and contrasts the tunics of the arteries and veins.

Tunica Intima

The tunica intima (also called the tunica interna) is composed of epithelial and connective tissue layers. Lining the tunica intima 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. 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. The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension (high blood pressure) and cardiovascular disease.

Next to the endothelium is the basement membrane, or 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. 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.

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 intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen.

Under the microscope, the lumen and the entire tunica intima 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.

Tunica Media

The tunica media is the substantial middle layer of the vessel wall (see image). 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 primarily made up of elastic fibers, most of which are arranged in circular sheets. 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. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop. Both vasoconstriction and vasodilation are regulated in part by small vascular nerves, known as nervi vasorum, or “nerves of the vessel,” that run within the walls of blood vessels. These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction, depending upon the nature of the neurotransmitter and receptors located on the target cell. Parasympathetic stimulation does trigger vasodilation as well as erection during sexual arousal in the external genitalia of both sexes. Nervous control over vessels tends to be more generalized than the specific targeting of individual blood vessels. Local controls, discussed later, account for this phenomenon. (Seek additional content for more information on these dynamic aspects of the autonomic nervous system.) Hormones and local chemicals also control blood vessels. Together, these 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 is discussed in detail later in this chapter.

The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also binds 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.

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.

An arteriole is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin (see image).

With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure. Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow.

Healthy arteries are flexible and have little or no plaque buildup. They allow blood to flow freely and can constrict or dilate in response to changes in blood pressure and your body's varying needs for blood supply. But in atherosclerosis, plaques accumulate and arterial walls swell and become thick and stiff. Arteries clogged by atherosclerosis are susceptible to partial or complete blockage by debris or blood clots. The result may be a transient ischemic attack (TIA) or an ischemic stroke.

Transient Ischemic Attacks (TIAs)

Although strokes strike suddenly and without warning, there is one possible indication that a person might suffer a stroke in the future: a transient ischemic attack (TIA), or "ministroke." TIAs are caused by a brief interruption of blood supply to part of the brain, usually due to a blood clot or small debris that temporarily block an artery. The symptoms of a TIA resemble those of ischemic stroke, except that the symptoms resolve within 1 hour, and often no permanent damage is done. One out of three people who have had one or more TIAs will have a stroke at a later date.

Ischemic Stroke

When one of the large arteries that supply the brain is blocked, some people have few or no symptoms, whereas others have a massive ischemic stroke. This is because some people are born with larger collateral arteries than others. Collateral arteries run between other arteries, providing extra connections. When one artery is blocked, blood can run through a collateral artery instead. But small collateral arteries may not be able to deliver enough blood to the affected area, and a stroke is the result.

What Is Plaque?

Arterial plaque is chiefly composed of accumulated macrophages (white blood cells that are part of the body's inflammatory reaction), cholesterol crystals and other lipids, calcium, smooth muscle cells, and fibrous connective tissue. Plaques build up over a period of years, progressively narrowing the interior of the artery and causing ischemia in the tissues. Blood pressure increases because of the narrowed opening of the vessel. Weakened blood vessels stiffen in response to inflammation and to the increased blood pressure.

Plaques may rupture, and the body attempt to "heal" the rupture by forming a clot around the break. These clots may stay in place, cutting off blood flow to nearby tissue, or they may break off and travel through the arteries, potentially causing a stroke.

Atherosclerosis

Atherosclerosis, also called hardening of the arteries, is the leading cause of death in the U.S. and in most of the developed world. By 2020, atherosclerosis is expected to be the leading cause of death worldwide. It doesn't have to be. Atherosclerosis is largely caused by having an unhealthy lifestyle. It develops through eating too many fatty foods, smoking, being overweight, not getting enough exercise, or not controlling diabetes and high blood pressure. It's both preventable and treatable.

Atherosclerosis can occur in any of the arteries of the body. When the two carotid arteries, which run up along either side of the front of your neck, develop atherosclerosis, it's termed carotid artery disease. When the arteries that supply blood to the heart, the coronary arteries, develop atherosclerosis, the condition is called coronary artery disease (CAD) or coronary heart disease (CHD).

Symptoms of Atherosclerosis

Because atherosclerosis develops slowly, there may be no symptoms until an artery is so clogged that it can't deliver adequate blood to the organs or tissues and they become ischemic. Plaques can form in any of the arteries of the body. If atherosclerosis cuts off blood flow to the

• coronary arteries, symptoms can include chest pain (angina), shortness of breath, and fatigue.
• brain, stroke-like symptoms may occur, such as sudden numbness or weakness in the arms and legs, difficulty speaking, or drooping facial muscles.
• arms and legs, walking may become painful.

Complications

Atherosclerosis can result in many complications, ranging from the painful to the fatal.

• Angina

Partially clogged coronary arteries make it difficult for oxygenated blood to reach the heart muscle tissue, resulting in chest pain, fatigue, or shortness of breath.

• Heart Attack

Eventually a plaque may rupture. Platelets in the bloodstream sense the plaque rupture and attempt to "heal" the injury by forming a blood clot, or thrombus. If the thrombus remains in place and completely blocks the artery, the tissue that artery supplies blood to will be deprived of oxygen and will die. If a coronary artery is blocked, the result will be myocardial infarction-a heart attack. One-third of all heart attacks are fatal. Even if they are nonfatal, they cause permanent damage to the heart.

• Stroke

If the thrombus breaks away and travels to the brain, it can cause a stroke. A thrombus that breaks off and travels through the bloodstream is called an embolus. If the embolus becomes lodged in an artery in your brain, an ischemic stroke, in which blood supply is cut off to portions of your brain, can be the result.

Symptoms of stroke vary according to the location of the blockage or bleeding in your brain. For instance, if there is a blockage in the area of the brain that controls movement in the left arm, there will be weakness or complete loss of movement in the left arm. Strokes generally affect only one side of the body because they usually occur on only one side of the brain.

• Arrhythmias

Irregular heart beats, termed arrhythmias, occur when the electrical impulses that coordinate your heartbeat don't function properly. Heart disease is a major cause of arrhythmia, due to lack of blood flow and consequent heart tissue damage.

• Carotid Artery Disease

The carotid arteries run along either side of the neck and carry blood to the brain. They can become clogged, causing stroke or transient ischemic attack (temporary clotting of a blood vessel in the brain).

• Peripheral Artery Disease

The arteries in the arms and legs become narrowed and circulatory problems develop. People with peripheral artery disease may be less sensitive to heat and cold, increasing the risk of burns or frostbite. In extreme cases, lack of circulation can cause tissue death.

• Aneurysm

Blood vessels weakened by atherosclerosis may develop aneurysms, bulges in the arterial wall. There are usually no symptoms. If the aneurysm ruptures and creates internal bleeding, the results can be life threatening.

Over the years, if we don't take care of our bodies, our blood vessels can start to lose their resiliency and plaque can start to build up in our arteries. Plaque is composed of cholesterol, inflammatory (immune) cells, calcium, and other substances that flow through our bloodstreams. Plaque buildup occurs if we eat high-fat diets, don't get enough exercise, are overweight or diets too high in refined carbohydrates, smoke, or have other unhealthy habits.

Plaques may rupture and form a clot, termed a thrombus. If the thrombus breaks off and travels through the bloodstream, it's called an embolus. Thrombi and emboli may completely block blood supply through an artery, causing heart attack or stroke.

Compounding the problem of plaque buildup, the narrowing of the arteries increases the blood pressure inside them. This prompts the arteries to stiffen their walls as a defense against the increased blood pressure. The presence of inflammatory cells also causes the vessel walls to thicken with collagen. This narrowing and stiffening of the arteries is called atherosclerosis, and it's the leading cause of death in most of the developed world for both women and men.

Plaque buildup can even begin in youth. Studies have found that teenagers can develop well-established fatty streaks (the precursors to plaque) in their coronary artery walls, and that even children as young as 10 can have the artery-narrowing plaque that may lead to heart attacks and strokes. Children with high blood cholesterol are likely to remain at risk of elevated blood cholesterol as they grow older.