14.4: Structure and Function of Blood Vessels

By the end of this section, you will be able to:

• 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
• Compare and contrast the three types of capillaries 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
• Compare and contrast veins and venules on the basis of structure, location, and function
• Discuss factors affecting blood flow in the venous system

Blood is carried through the body via blood vessels. An artery is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, vessels called arterioles, further branch into tiny capillaries, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form venules, small blood vessels that carry blood to a vein, a larger blood vessel that returns blood to the heart.

Blood flow is the movement of blood through a vessel, tissue, or organ. The slowing or blocking of blood flow is called resistance. Blood pressure is the force that blood exerts upon the walls of the blood vessels or chambers of the heart. The components of blood pressure include systolic pressure, which results from ventricular contraction, and diastolic pressure, which results from ventricular relaxation. Pulse, the expansion and recoiling of an artery, reflects the heartbeat. In the arterial system, vasodilation and vasoconstriction of the arterioles is a significant factor in systemic blood pressure: slight vasodilation greatly decreases resistance and increases flow, whereas slight vasoconstriction greatly increases resistance and decreases flow. In the arterial system, as resistance increases, blood pressure increases and flow decreases. In the venous system, constriction increases blood pressure as it does in arteries; the increasing pressure helps to return blood to the heart. In addition, constriction causes the vessel lumen to become more rounded, decreasing resistance and increasing blood flow.

Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit (Figure \(\PageIndex{1}\)). The systemic circuit begins in the left atrium of the heart and ends in the venae cavae and coronary sinus that drain into the right atrium. Systemic arteries provide blood rich in oxygen to the body’s tissues; this blood is often referred to as oxygenated blood. 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; this blood is often referred to as deoxygenated blood. In contrast, the pulmonary circuit begins in the right atrium and ends in the pulmonary veins that drain into the left atrium. Pulmonary 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. Although arteries and veins differ structurally and functionally, they share certain features.

Figure \(\PageIndex{1}\): Cardiovascular Circulation in Pulmonary and Systemic Circuits. The pulmonary circuit moves blood from the heart to the lungs and back to the heart. The systemic circuit moves blood from the heart to the body and returns it to the heart. Within the lungs and the body regions are capillary beds where oxygen and carbon dioxide are exchanged. (Image Credit: "Systemic and Pulmonary Circuits" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Open Learning Initiative.)

Structural Characteristics of Blood Vessels

Blood Vessel Tunics

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 \(\PageIndex{2}\)). 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.

Figure \(\PageIndex{2}\): Structure of Blood Vessels. The walls of arteries and veins share the same general features, but the muscular layer of arteries is much thicker because of the higher pressure of the blood that flows through them. (Image Credit: "Tunics of Arteries and Veins" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Scientific Animations.)

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.

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). From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa (see Figure \(\PageIndex{2}\). Table \(\PageIndex{1}\) compares and contrasts the tunics of the arteries and veins.

Table \(\PageIndex{1}\): Comparison of Tunics in Arteries and Veins

Tunic (Layer)	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) Nervi vasorum and vasa vasorum present External elastic membrane present in larger vessels	Normally thinner than the tunica externa Smooth muscle cells with elastic and collagen fibers Nervi vasorum and vasa vasorum present External elastic membrane absent
Tunica Externa	Normally thinner than the tunica media in all but the largest arteries Collagen and elastic fibers Nervi vasorum and vasa vasorum present	Normally the thickest layer in veins Collagen and smooth fibers predominate Some smooth muscle fibers Nervi vasorum and vasa vasorum present

Arterial Structure

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 (Figure \(\PageIndex{3}\)). 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 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 gives it a low resistance and enables it to accept a large volume of blood from the heart which is conducted to smaller branches within regions of the body.

Figure \(\PageIndex{3}\): Elastic Artery. Elastic arteries have elastic fibers throughout their tunica media. (Image Credit: "Elastic Artery" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Scientific Animations.)

Farther from the heart, where the surge of blood has dampened, the percentage of elastic fibers in an artery’s tunica media decreases and becomes restricted to two thin bands, one on each side of the tunic. The layer between the tunica intima and tunica media is the internal elastic lamina while the layer between the tunica media and tunica externa is the external elastic lamina. The artery at this point is described as a muscular artery (Figure \(\PageIndex{4}\)). 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 which controls blood flow to individual organs. 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.

Figure \(\PageIndex{4}\): Muscular Artery. Muscular arteries have elastic fibers in two elastic laminae, one on each side of the tunica media. (Image Credit: "Muscular Artery" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Scientific Animations.)

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 that deliver blood to capillaries within specific organs and tissues. For this reason, a muscular artery is also known as a distributing artery.

Arteriole Structure

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 as arterioles are supported and held in place by their positioning within organs and tissues (see Figure \(\PageIndex{5}\)). At the distal end of arterioles the tunica externa may be absent.

Figure \(\PageIndex{5}\): Arteriole. Arterioles have a much thinner wall and almost no tunica externa. (Image Credit: "Arteriole" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Scientific Animations.)

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 to capillary beds as well as regulation of systemic blood pressure.

DISORDERS OF THE...

Cardiovascular System: Arteriosclerosis

Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed. This helps promote blood flow. In arteriosclerosis, compliance is reduced, and pressure and resistance within the vessel increase. This is a leading cause of hypertension and coronary heart disease, as it causes the heart to work harder to generate a pressure great enough to overcome the resistance.

Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors. Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured—which means that hypertension can promote arteriosclerosis, as well as result from it.

Recall that tissue injury causes inflammation. As inflammation spreads into the artery wall, it weakens and scars it, leaving it stiff (sclerotic). As a result, compliance is reduced. Moreover, circulating triglycerides and cholesterol can seep between the damaged lining cells and become trapped within the artery wall, where they are frequently joined by leukocytes, calcium, and cellular debris. Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow. The term for this condition, atherosclerosis (athero- = “porridge”) describes the mealy deposits (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): Atherosclerosis. (a) Atherosclerosis can result from plaques formed by the buildup of fatty, calcified deposits in an artery. (b) Plaques can also take other forms, as shown in this micrograph of a coronary artery that has a buildup of connective tissue within the artery wall. LM × 40. (Image credit: "Atherosclerosis" by OpenStax is licensed under CC BY 3.0; Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Sometimes a plaque can rupture, causing microscopic tears in the artery wall that allow blood to leak into the tissue on the other side. When this happens, platelets rush to the site to clot the blood. This clot can further obstruct the artery and—if it occurs in a coronary or cerebral artery—cause a sudden heart attack or stroke. Alternatively, plaque can break off and travel through the bloodstream as an embolus until it blocks a more distant, smaller artery.

Even without total blockage, vessel narrowing leads to ischemia—reduced blood flow—to the tissue region “downstream” of the narrowed vessel. Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues. Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function.

A major risk factor for both arteriosclerosis and atherosclerosis is advanced age, as the conditions tend to progress over time. Arteriosclerosis is normally defined as the more generalized loss of compliance, “hardening of the arteries,” whereas atherosclerosis is a more specific term for the build-up of plaque in the walls of the vessel and is a specific type of arteriosclerosis. There is also a distinct genetic component, and pre-existing hypertension and/or diabetes also greatly increase the risk. However, obesity, poor nutrition, lack of physical activity, and tobacco use all are major risk factors.

Treatment includes lifestyle changes, such as weight loss, smoking cessation, regular exercise, and adoption of a diet low in sodium and saturated fats. Medications to reduce cholesterol and blood pressure may be prescribed. For blocked coronary arteries, surgery is warranted. In angioplasty, a catheter is inserted into the vessel at the point of narrowing, and a second catheter with a balloon-like tip is inflated to widen the opening. To prevent subsequent collapse of the vessel, a small mesh tube called a stent is often inserted. In an endarterectomy, plaque is surgically removed from the walls of a vessel. This operation is typically performed on the carotid arteries of the neck, which are a prime source of oxygenated blood for the brain. In a coronary bypass procedure, a non-vital superficial vessel from another part of the body (often the great saphenous vein) or a synthetic vessel is inserted to create a path around the blocked area of a coronary artery.

Capillary Structure

A capillary is a microscopic channel that supplies blood to the tissues, through a process called perfusion. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5–10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation.

The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. Some variation in wall structure is seen depending on the size of the capillary. In a large capillary, several endothelial cells bordering each other may line the lumen, while in a small capillary, there may be only a single cell layer that wraps around to contact itself.

For capillaries to function, their walls must be leaky, or permeable, allowing some substances to pass through. There are three major types of capillaries, which differ according to their degree of permeability: continuous, fenestrated, and sinusoid capillaries.

Venous System

The venous system is the system of veins in the systemic and pulmonary circulations that return blood from the tissues to the heart. In the systemic circuit the venous return is of deoxygenated blood from the organs and tissues of the body. All of the systemic veins are tributaries of the two largest veins, the superior and inferior vena cavae, that empty the oxygen-depleted blood into the right atrium of the heart. In the pulmonary circuit the pulmonary veins return oxygenated blood from the lungs to the heart.

The whole of the venous system is a large volume, low pressure system. The thin walls of the veins and their larger cross-sectional area (total luminal diameter) allow them to hold a large volume of blood, giving them the term of capacitance vessels. Veins do not have a consistent pressure generator equivalent to the heart that propels the blood in a single direction. While the blood in some veins can rely on gravity to cause movement, most work against gravity and their structure is specialized to prevent backflow and to allow for fluctuations in blood volume.

Veins vary in size from the smallest post-capillary venules, to larger venules, small veins, medium veins, and large veins. The thickness of the walls of the veins varies according to their location – in the legs the vein walls are much thicker than those in the arms - as does the number of valves present.

Venules

A venule is an extremely small vein, generally 8–100 micrometers in diameter. Postcapillary venules drain multiple capillaries exiting from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin or non-existent middle layer with a few smooth muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a thin tunica externa (Figure \(\PageIndex{7}\)). Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze between adjacent cells to enter the tissue fluid.

Figure \(\PageIndex{7}\): Venule. The wall of venules is also thin, with the tunica externa being thicker than the tunica media. (Image Credit: "Venule" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Scientific Animations.)

Veins

A vein is a larger blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens (see Figure \(\PageIndex{8}\)). Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity (Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{8}\): Vein with Valve. When pressure distal to the valve is higher than the pressure proximal to the valve the valve cusps will open and allow blood to flow toward the heart. When blood begins to flow in reverse, it will build up in the cusp pockets and cause the valve to close. This prevents further backflow. (Image Credit: "Vein with Valve" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by Scientific Animations.)

Figure \(\PageIndex{9}\): Small Artery and Vein. Small arteries and veins clearly demonstrate the histological differences between the two vessel types. Small veins have a thicker tunica externa and a very thin tunic media whereas small arteries have a thick tunica media and a relatively thinner tunica externa. (Image credit: “Artery and Vein Histological Comparison" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, slide provided by the Regents of the University of Michigan Medical School © 2022.)

Skeletal Muscle Pump

The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed (atrial diastole). Second, two physiologic “pumps” increase pressure in the venous system. The use of the term “pump” implies a physical device that speeds flow. These physiological pumps are less obvious.

In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the skeletal muscle pump (Figure \(\PageIndex{10}\)), helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart. As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods. Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart. Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness.

(Figure \(\PageIndex{10}\)): Venous blood flow due to muscle contraction. Some veins are located in between muscles (e.g. in the lower extremity). When the muscles contract the pressure increases. Therefore valves will open so that blood will flow towards the heart. (Image Credit: "KnowledgeWorks - Drawing Venous blood flow due to muscle contraction - English labels" by KnowledgeWorks Global Ltd., license: CC BY)

DISORDERS OF THE...

Cardiovascular System: Edema and Varicose Veins

Despite the presence of valves and the contributions of other anatomical and physiological adaptations we will cover shortly, over the course of a day, some blood will inevitably pool, especially in the lower limbs, due to the pull of gravity. Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. The presence of excess tissue fluid around the cells leads to a condition called edema.

Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet (like most health professionals). However, clinical edema goes beyond normal swelling and requires medical treatment. Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated.

Figure \(\PageIndex{11}\): Varicose Veins. Varicose veins are commonly found in the superficial veins of the lower limbs. Defective valves cause localized pooling of blood in the veins that can lead to distention and deformity of the vessel appearing as lumps under the surface of the skin. (Image credits: "Left Calf and Varicose Veins" by Thomas Kriese is licensed under CC BY 2.0; "Varicose Veins and Valves" by BlueLink is licensed under CC BY-NC 4.0 with notification of the original authors.)

Edema may be accompanied by varicose veins, especially in the superficial veins of the legs (Figure \(\PageIndex{11}\)). 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. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy. More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing. Without treatment, they tend to grow worse over time. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. Severe cases may require conventional surgery to remove the damaged vessels. As there are typically redundant circulation patterns, that is, anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation. There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot.

Table \(\PageIndex{2}\): Comparison of General Features of Arteries and Veins

	Arteries	Veins
Direction of blood flow	Conducts blood away from the heart	Conducts blood toward the heart
General appearance	Rounded	Irregular, often collapsed
Pressure	High	Low
Wall thickness	Thick	Thin
Relative oxygen concentration	Higher in systemic arteries Lower in pulmonary arteries	Lower in systemic veins Higher in pulmonary veins
Valves	Not present	Present most commonly in limbs and in veins inferior to the heart

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology