Chapter 21:  Blood Vessels and Circulation


The smallest arterial branches are called arterioles. From the arterioles, blood moves into the capillaries , where diffusion occurs between blood and interstitial fluid. From the capillaries, blood enters small venules, which unite to form larger veins that return blood to the heart. Two major arteries are connected to the heart, one associated with each ventricle.


Several hundred million tiny arterioles provide blood to more than 10 billion capillaries. These capillaries, barely the diameter of a single red blood cell, form extensive, branching networks. If all the capillaries in your body were placed end to end, they would circle the globe with a combined length of over 25,000 miles. The vital functions of the cardiovascular system depend entirely on events at the capillary level: All chemical and gaseous exchange between blood and interstitial fluid takes place across capillary walls


The Anatomy of Blood Vessels

The pressures experienced by vessels vary with distance from the heart, and their structures reflect this fact. Moreover, arteries, veins, and capillaries differ in function, and these functional differences, as always, involve particular structural adaptations


 A Comparison of a Typical Artery and a Typical Vein



The tunica intima, or tunica interna , is the innermost layer of a blood vessel. In arteries, the outer margin of the tunica intima contains a thick layer of elastic fibers called the internal elastic membrane .

The tunica media , the middle layer, contains concentric sheets of smooth muscle tissue in a framework of loose connective tissue. When these smooth muscles contract, the vessel decreases in diameter; when they relax, the diameter increases.

The tunica externa, or tunica adventitia, is the outermost layer of a blood vessel and forms a connective tissue sheath. The connective–tissue fibers of the tunica externa typically blend into those of adjacent tissues, stabilizing and anchoring the blood vessel.

The walls of large vessels contain small arteries and veins that supply the smooth muscle cells and fibroblasts of the tunica media and tunica externa. These blood vessels are called the vasa vasorum ("vessels of vessels").


Differences Between Arteries and Veins
Arteries and veins servicing the same region typically lie side by side.

In general, the walls of arteries are thicker than those of veins.

When not opposed by blood pressure, the elastic fibers in the arterial walls recoil, constricting the lumen.

The endothelial lining of an artery cannot contract, so when an artery constricts, its endothelium is thrown into folds that give arterial sections a pleated appearance. The lining of a vein lacks these folds.

The thicker walls of the arteries can be felt if the vessels are compressed.

Arteries usually retain their cylindrical shape, whereas veins often collapse.

Arteries are more resilient: When stretched, they keep their shape and elongate, and when released, they snap back. A small vein cannot tolerate as much distortion without collapsing or tearing.

Veins typically contain valves –internal structures that prevent the backflow of blood toward the capillaries


When stimulated, arterial smooth muscles contract and thereby constrict the artery–a process called vasoconstriction . Relaxation of these smooth muscles increases the diameter of the lumen–a process called vasodilation . Vasoconstriction and vasodilation affect (1) the afterload on the heart, (2) peripheral blood pressure, and (3) capillary blood flow.


Elastic arteries , or conducting arteries , are large vessels with a diameter up to 2.5 cm (1 in.). These vessels transport large volumes of blood away from the heart. The pulmonary trunk and aorta, as well as their major arterial branches (the pulmonary, common carotid, subclavian , and common iliac arteries ), are elastic arteries.

Muscular arteries , also known as medium–sized arteries or distribution arteries , distribute blood to the body's skeletal muscle and internal organs. Most of the vessels of the arterial system are muscular arteries. The external carotid arteries of the neck, the brachial arteries of the arms, the mesenteric arteries of the abdomen, and the femoral arteries of the thighs are examples of muscular arteries.

Arterioles are considerably smaller than muscular arteries. The tunica media of the smallest arterioles contains scattered smooth muscle cells that do not form a complete layer.

Focal calcification is the gradual degeneration of smooth muscle in the tunica media and the subsequent deposition of calcium salts. Typically, the process involves arteries of the limbs and genital organs. Some focal calcification occurs as part of the aging process, and it may develop in association with atherosclerosis (see below). Rapid and severe calcification may occur as a complication of diabetes mellitus.

Atherosclerosis is associated with damage to the endothelial lining and the formation of lipid deposits in the tunica media. Atherosclerosis is the most common form of arteriosclerosis.


Recent evidence indicates that many forms of atherosclerosis are associated with either (1) low levels of apolipoprotein–E ( ApoE ), a transport protein whose lipids are quickly absorbed by body tissues, or (2) high levels of lipoprotein(a) , a low–density lipoprotein ( LDL ) that is absorbed at a much slower rate


Figure 21-3  A Plaque Blocking an Artery.
(a) A section of a coronary artery narrowed by plaque formation. (b) A cross–sectional view of a large plaque




Elderly individuals–especially elderly men–are most likely to develop atherosclerotic plaques. Evidence suggests that estrogens may slow plaque formation; this may account for the lower incidence of CAD, myocardial infarctions (MIs), and strokes in women. After menopause, when estrogen production declines, the risk of CAD, MIs, and strokes in women increases markedly.


Arterioles in most tissues vasodilate when oxygen levels are low and vasoconstrict under sympathetic stimulation. More pressure is required to push blood through a constricted vessel than through a dilated one. The force opposing blood flow is called resistance (R) , and arterioles are therefore called resistance vessels .

Occasionally, local arterial pressure exceeds the capacity of the elastic components of the tunics, producing an aneurysm, or bulge in the weakened wall of an artery. The most dangerous aneurysms are those involving arteries of the brain, where they cause strokes, or of the aorta, where a ruptured aneurysm will cause fatal bleeding in a matter of minutes.


Aneurysms most commonly occur in individuals with arteriosclerosis . Over time, arteriosclerosis causes vessel walls to become less elastic, and a weak spot can develop.



Capillaries are the only blood vessels whose walls permit exchange between the blood and the surrounding interstitial fluids.


Because capillary walls are thin, the diffusion distances are small, so exchange can occur quickly. In addition, blood flows through capillaries relatively slowly, allowing sufficient time for the diffusion or active transport of materials across the capillary walls.


The average diameter of a capillary is very close to that of a single red blood cell. There are two major types of capillaries: (1) continuous capillaries and (2) fenestrated capillaries .

Capillary Structure.
(a) A continuous capillary, showing routes for the diffusion of water and solutes. (b) A fenestrated capillary, showing the pores that facilitate diffusion across the endothelial lining.




In a small continuous capillary, a single endothelial cell may wrap all the way around the lumen, just as your hand wraps around a small glass.

Continuous capillaries are located in all tissues except epithelia and cartilage. Continuous capillaries permit the diffusion of water, small solutes, and lipid–soluble materials into the surrounding interstitial fluid, but prevent the loss of blood cells and plasma proteins.

The endothelial cells in specialized continuous capillaries throughout most of the central nervous system and in the thymus are bound together by tight junctions. These capillaries have very restricted permeability characteristics. We discussed one example, the capillaries responsible for the blood–brain barrier.

Fenestrated Capillaries  are capillaries that contain "windows," or pores, that span the endothelial lining . The pores permit the rapid exchange of water and solutes as large as small peptides between plasma and interstitial fluid. Examples of fenestrated capillaries noted in earlier chapters include the choroid plexus of the brain and the blood vessels in a variety of endocrine organs, such as the hypothalamus and the pituitary, pineal, and thyroid glands.


 Sinusoids resemble fenestrated capillaries that are flattened and irregular. In contrast to fenestrated capillaries, sinusoids commonly have gaps between adjacent endothelial cells

Blood moves through sinusoids relatively slowly, maximizing the time available for exchange across the sinusoidal walls. Sinusoids occur in the liver, bone marrow, spleen, and many endocrine organs, such as the pituitary and adrenal glands. At liver sinusoids, plasma proteins secreted by the liver cells enter the bloodstream. Along sinusoids of the liver, spleen, and bone marrow, phagocytic cells monitor the passing blood, engulfing damaged red blood cells, pathogens, and cellular debris.

Capillary Beds
Capillaries do not function as individual units but as part of an interconnected network called a capillary bed , or capillary plexus



A single arteriole generally gives rise to dozens of capillaries that empty into several venules , the smallest vessels of the venous system. The entrance to each capillary is guarded by a band of smooth muscle called a precapillary sphincter . Contraction of the smooth muscle cells constricts and narrows the diameter of the capillary entrance, thereby reducing the flow of blood. Relaxation of the sphincter dilates the opening, allowing blood to enter the capillary faster.

The capillary bed contains several relatively direct connections between arterioles and venules. The wall in the initial part of such a passageway possesses smooth muscles capable of changing its diameter. This segment is called a metarteriole.


A capillary bed may receive blood from more than one artery. The multiple arteries, called collaterals , enter the region and fuse before giving rise to arterioles. The fusion of two collateral arteries that supply a capillary bed is an example of an arterial anastomosis . The interconnections between the anterior and posterior interventricular arteries of the heart are arterial anastomoses


Arteriovenous anastomoses are direct connections between arterioles and venules. When an arteriovenous anastomosis is dilated, blood will bypass the capillary bed and flow directly into the venous circulation.

Although blood normally flows from arterioles to venules at a constant rate, the flow within each capillary is quite variable. Each precapillary sphincter alternately contracts and relaxes, perhaps a dozen times per minute.

The net effect is that blood may reach the venules by one route now and by a different route later. The cycling of contraction and relaxation of smooth muscles that changes blood flow through capillary beds is called vasomotion .

Vasomotion is controlled locally by changes in the concentrations of chemicals and dissolved gases in the interstitial fluids.


 When you are at rest, blood flows through roughly 25 percent of the vessels within a typical capillary bed in your body. Your cardiovascular system does not contain enough blood to maintain adequate blood flow to all the capillaries in all the capillary beds in your body at the same time.


 Venous walls need not be as thick as arterial walls because the blood
pressure in veins is lower than that in the arteries. Veins are classified on the basis of their size. Even though their walls are thinner, veins are in general, larger in diameter than their corresponding arteries.

Venules , which collect blood from capillary beds, are the smallest venous vessels. They vary widely in size and character.
Medium–sized veins range from 2 to 9 mm in internal diameter, comparable in size to muscular arteries. In these veins, the tunica media is thin and contains relatively few smooth muscle cells. The thickest layer of a medium–sized vein is the tunica externa, which contains longitudinal bundles of elastic and collagen fibers.

Large veins include the superior and inferior venae cavae and their tributaries within the abdominopelvic and thoracic cavities. All the tunica layers are present in large veins.


Venous Valves
 Blood pressure in a peripheral venule is only about 10 percent of that in the ascending aorta, and pressures continue to fall along the venous system.

The blood pressure in venules and medium–sized veins is so low that it cannot oppose the force of gravity. In the limbs, veins of this size contain valves, folds of the tunica intima that project from the vessel wall and point in the direction of blood flow. These valves act like the valves in the heart in that they permit blood flow in one direction only.



As long as the valves function normally, any movement that distorts or compresses a vein will push blood toward the heart. This effect improves venous return. The mechanism is particularly important when you are standing, because blood returning from your feet must overcome the pull of gravity to ascend to the heart. When you lie down, venous valves have much less impact on venous return, because your heart and major vessels are at the same level.

If the walls of the veins near the valves weaken or become stretched and distorted, the valves may not work properly. Blood then pools in the veins, and the vessels become grossly distended. The effects range from mild discomfort and a cosmetic problem, as in superficial varicose veins in the thighs and legs, to painful distortion of adjacent tissues, as in hemorrhoids.



The Distribution of Blood
The total blood volume is unevenly distributed among arteries, veins, and capillaries



The heart, arteries, and capillaries normally contain 30–35 percent of the blood volume (roughly 1.5 liters of whole blood), and the venous system contains the rest (65–70 percent, or about 3.5 liters). Of the blood in the venous system, roughly one–third (about a liter) is circulating within the liver, bone marrow, and skin. These organs have extensive venous networks that at any moment contain large volumes of blood.

For a given rise in blood pressure, a typical vein will stretch about eight times as much as a corresponding artery. The capacitance of a blood vessel is the relationship between the volume of blood it contains and the blood pressure. If the vessel behaves like a child's balloon, expanding easily with little pressure, it has high capacitance. If it behaves more like a truck tire, expanding only when large pressures are applied, it has low capacitance. Veins expand easily, so they are called capacitance vessels . Because veins have high capacitance, large changes in blood volume have little effect on arterial blood pressure. If the blood volume rises or falls, the elastic walls stretch or recoil, changing the volume of blood in the venous system.

If serious hemorrhaging occurs, the vasomotor centers of the medulla oblongata stimulate sympathetic nerves that innervate smooth muscle cells in the walls of medium–sized veins. This activity has two major effects:

1. Systemic Veins Constrict . This process, called venoconstriction, reduces the amount of blood within the venous system and increases the volume within the arterial system and capillaries. Venoconstriction can keep the blood volume within the arteries and capillaries at near–normal levels despite a significant blood loss.

2. The Constriction of Veins in the Liver, Skin, and Lungs Redistributes a Significant Proportion of the Total Blood Volume . As a result, blood flow to delicate organs, such as the brain, and to active skeletal muscles can be increased or maintained after a blood loss. The amount of blood that can be shifted from veins in the liver, skin, and lungs to the general circulation, called the venous reserve , is normally about 20 percent of the total blood volume.