Introduction and Key Concepts for the Circulatory System

The circulatory system includes the cardiovascular system and the lymphatic vascular system. The cardiovascular system includes the heart and the arterial, capillary, and venous systems. Blood is transported from the heart through the arterial system to the capillaries, where exchange of gases, nutrients, and other substances takes place. Blood is carried back to the heart by the venous system. Blood flows through two routes:

(1) The systemic circulation system transports oxygenated blood from the heart to the capillaries in the tissues and organs of the body and then collects and carries the blood back to the heart (Fig. 9-1). (2) The pulmonary circulation system transports deoxygenated blood from the heart to the capillaries of the lungs. After gas exchange, blood is carried back to the heart (see Fig. 9-1). The lymphatic vascular system consists of lymphatic capillaries, lymphatic vessels, and lymphatic ducts. This system collects lymph (excess tissue fluid) from the tissues of all organs (except the nervous system, bone marrow, and hard tissues) by lymphatic capillaries and then transports it through lymphatic vessels to the lymphatic ducts, which eventually empty the lymph into the venous system. The collected lymph passes through lymph organs, where it is filtered, and lymphocytes are exposed to antigens. Lymphopoiesis and the immune response occur here (Fig. 9-19A,B).

The Cardiovascular System

The Heart

The heart contains four chambers: the left and right atria and the left and right ventricles. The atria receive blood flow discharged from the venous system, whereas the ventricles pump blood into the arterial system (Fig. 9-1). The wall of the heart is composed of three layers: endocardium (innermost layer), myocardium (middle layer), and epicardium (outermost layer).

(1) Endocardium consists of endothelium, subendothelial

connective tissue, and subendocardium (Purkinje fibers, small coronary blood vessels, and nerve fibers). (2) Myocardium, the thickest layer of the heart, contains an abundance of cardiac muscle cells (Fig. 9-3A,B). Cardiac muscle contracts producing heart beats, which are generated and regulated by the heart conductive system including the sinoatrial (SA) node, the atrioventricular (AV) node, the AV bundle, and Purkinje fibers

(Fig. 9-2). (3) Epicardium is covered by mesothelium and contains fibrous connective tissue, nerves, coronary vessels, and adipose tissue (Fig. 9-3C).

Types of Blood Vessels

THE ARTERIAL SYSTEM is composed of large (conducting) arteries, medium (distributing) arteries, small arteries, and arterioles. The arterial system conducts blood (under higher pressure than veins) from the ventricles to the capillary networks. The walls of arteries can be generally divided into three layers: tunica intima, tunica media, and tunica adventitia (Figs. 9-5 and 9-6).

Large arteries are also called elastic arteries because of the large quantity of elastic material in their walls (Fig. 9-7A–C). They have a thick tunica media with numerous elastic membranes. The internal and external elastic laminae are hard to distinguish from the nearby elastic membranes. Large arteries conduct blood from the ventricles into the medium arteries. Rich elastic materials in large arteries enable the vessels to recoil to accommodate pressure changes and maintain a continuous flow of blood during ventricular diastole (relaxation).

Medium arteries are also called muscular arteries because of their thick tunica media, which contains circularly arranged multiple layers of smooth muscle cells in a distinct sheath (Figs. 9-8 to 9-9A). Internal and external elastic laminae are easy to distinguish from nearby tissues.

Small arteries and arterioles are smaller diameter vessels. The walls of small arteries contain two to six layers of smooth muscle cells (Figs. 9-9B and 9-10A,B). Arterioles are the smallest components of the arterial system, with only one or two layers of smooth muscle cells (Figs. 9-10A,C, and 9-11). They control the blood flow into the capillaries.

THE CAPILLARY SYSTEM contains continuous capillaries, which have a continuous basal lamina and complete endothelial cells. These structures allow only a very limited amount of materials to pass through the capillary walls (Fig. 9-13A,B).

Fenestrated capillaries have a continuous basal lamina and fenestrated endothelial cells (perforated by small pores). A greater range of substances can, therefore, pass through the capillary walls (Fig. 9-14A,B).

Discontinuous (sinusoidal) capillaries have a discontinuous (or missing) basal lamina and incomplete endothelial cells

(perforated by large pores), which allow proteins and other materials, even cells, to pass through the capillary walls freely (Fig. 9-15A,B).

THE VENOUS SYSTEM is composed of venules and small, medium, and large veins. Veins collect blood (under lower pressure than arteries) from capillaries and transport it to the heart. Veins have larger, flat lumens and thinner walls than their companion arteries (Figs. 9-5 and 9-6).

Venules and small veins have very thin walls and few valves. Venules drain exchanged blood from the capillaries to the small

veins. Venules are the primary sites of many inflammatory reactions (Fig. 9-16A–C).

Medium veins have various diameters, and their structure varies based on their size and location. Segments of medium veins in some locations have a thick tunica adventitia with a few longitudinal smooth muscle bundles (Fig. 9-17A–C). Valves are prominent in medium veins, with an abundance in the extremities to prevent blood from flowing backwards.

Large veins have a thick tunica adventitia with numerous longitudinal smooth muscle bundles, which help to force blood to flow toward the heart (Fig. 9-18A–C). There are some large valves in the large veins.

The Lymphatic Vascular

System

Lymphatic Vessels

The lymphatic vessels have a structure similar to small veins (Fig. 9-19A,B). They have a large lumen, and valves are plentiful in all sizes of lymphatic vessels.

veins Medium

160 UNIT 3 ■ Organ Systems

A

Myocardium

Endocardium

Myocardium

Figure 9-3A. Layers of the heart wall: endocardium, ventricle. H&E, 68

The wall of the heart is much thicker than the wall of the large vessels and is composed of three basic layers, as are the blood vessels: endocardium (equivalent to the tunica intima), myocardium (equivalent to the tunica media), and epicardium (equivalent to the tunica adventitia). The endocardium is the innermost layer of the heart wall, which lines the lumen of the heart. This layer consists of endothelium (simple squamous epithelium), subendothelial connective tissue, and subendocardium. The subendocardium is in contact with the cardiac muscle and contains small coronary blood vessels, nerves, and Purkinje fibers in certain areas (Fig. 9-4A). Some adipose cells are also present within the connective tissue. The endocardium provides a smooth lining for the four chambers of the heart and provides a covering for the AV valves.

Figure 9-3B. Layers of the heart wall: myocardium, ventricle. H&E, 272; insets 786

Myocardium is the thickest layer of the heart wall and makes up the bulk of the heart. It consists of cardiac muscle cells that are arranged in branching columns. The ends of the cardiac muscle cells are connected to each other by intercalated disks. The inset shows cardiac muscles with their characteristic striations and an intercalated disk (Fig. 9-4A). These muscles contract to pump blood out of the ventricles of the heart and distribute blood to the tissues and organs of the body. Myocardium of the left ventricle wall is the thickest because of the fact that it must pump the blood a great distance and overcome the high pressure and resistance of the systemic circulation. In general, the atria have thinner walls than the ventricles. Myocardium of the right atrium is the thinnest because of the relatively low pressure and resistance of the blood circulation.

C

Myocardium

Epicardium (visceral layer)

Lumens of the

blood vessels

Figure 9-3C. Layers of the heart wall: epicardium, ventricle.

H&E, 68

Epicardium surrounds the heart. It is a layer of connective tissue that contains nerves, blood vessels, and adipocytes. The inner surface of the epicardium is connected with cardiac muscle, and the outer surface is covered by mesothelium (see Fig. 3-2) that faces the pericardial cavity. Mesothelium secretes a fluid known as pericardial fluid, which provides lubrication and reduces friction between the epicardium (visceral pericardium) and the parietal pericardium during the movements caused by heart contraction. Epicardium covers and protects the heart and allows small blood vessels and nerves to pass through to provide nutrients and nerve innervation.

Pericardial effusion refers to excess fluid in the pericardial cavity due to inflammation of the pericardium (pericarditis); hemopericardium is a condition in which blood is trapped in the pericardial cavity. In either case, compression of the thinwalled atria and vena cava can result in cardiac tamponade and failure of circulation.

Figure 9-4A. Purkinje fibers and intercalated disks, ventricle.

H&E, 136; inset (right) 198, inset (left) 229

Purkinje fibers (impulse-conducting fibers) are large, modified cardiac muscle cells, which are part of the heart conducting system. They are terminal branches of the AV bundle branches (Fig. 9-2), located in subendocardial connective tissue. Purkinje fibers often appear large in size and cluster as groups. Each cell has only one or two nuclei and palestaining cytoplasm because it has fewer myofibrils than regular cardiac muscle cells. Purkinje fibers work together with other impulse-conducting structures to regulate the heartbeats by conveying impulses to neighboring cardiac muscle cells (Fig. 9-2). Intercalated disks are specialized junctional complexes that contain fascia adherens, desmosomes (macula adherens), and gap junctions, which provide connection and communication between the cardiac muscle cells. Intercalated disks bind cardiac muscle cells together in an entire unit to prevent muscle cells being pulled apart during contraction. They also provide ion exchange through gap junctions, allowing electrical impulses to pass from one cell to another.

Spongiosa

Fibrosa

Tricuspid

valve

Papillary

muscle

Figure 9-4B. Cardiac valves, aortic valve. H&E, 134

There are four valves in the heart: two AV valves (mitral and tricuspid valves) in the chambers and two semilunar valves (aortic and pulmonary valves) in the arteries leaving the heart. Heart valves consist of connective tissues and both surfaces are covered by endothelium. They are composed of three layers: (1) Spongiosa consists of loosely arranged collagen and elastic fibers and the surface is covered by the endothelium. This layer is continuous with the atrial or blood vessel side. (2) Fibrosa is the core of the heart valve, which contains dense irregular connective tissue. (3) Ventricularis is a dense connective tissue layer with many elastic and collagen fibers. The surface of the ventricularis is covered by endothelium. The heart valves open and close to allow the blood to flow through the openings and to prevent the backflow of blood. Shown is an example of the aortic valve. The aortic valve has three cusps and lies between the left ventricle and the aorta. The tricuspid and mitral valves are anchored to the ventricle wall by chordae tendineae, which are attached to papillary muscles.

Common heart valve diseases include calcific aortic valve disease, valvitis, and rheumatic heart disease. These diseases can lead to aortic stenosis, aortic regurgitation, embolism, and mitral valve stenosis.

CLINICAL CORRELATIONS

Remaining normal cardiac muscle cells

Necrotic cardiac muscle

C

Figure 9-4C. Myocardial Infarction. H&E, 68

Myocardial infarction (MI), also known as a heart attack, occurs when the blood supply to part of the heart is completely or partially blocked because of atherosclerosis or the rupture of an atherosclerotic plaque and the formation of a blood clot. Symptoms range from characteristic chest discomfort (angina) and shortness of breath to sudden death. Coronary artery disease (CAD) is the most common underlying cause of this medical emergency. Atherosclerotic plaques, which consist of lipids, fibroblasts, collagen, and white blood cells (especially macrophages), are found in the wall of an affected coronary artery. The plaques cause luminal narrowing or occlusion (Fig. 9-12A). The histologic appearance of MI depends on the age of the infarct. Early changes include edema and hypereosinophilia, followed by infiltration by neutrophils, coagulation necrosis, phagocytosis of dead cells by macrophages, formation of granulation tissue, and, ultimately, the formation of a dense collagenous scar.

Connective tissue

Tunica adventitia

Fibroblast

Longitudinal smooth muscle

Connective Tunica

tissue adventitia

Fibroblast

Figure 9-7A. A representation of a large (elastic) artery.

Large arteries (elastic arteries) are also referred to as conducting arteries, because of their function in conducting blood flow out of the heart. There are abundant elastic laminae (thick bundles of elastic fibers) in the tunica media of large arteries, which enable the walls to recoil to resist pressure as well as to maintain arterial pressure during diastole when the ventricles are relaxed and pressure is not generated by the heart. Small vessels located in the tunica adventitia are called vasa vasorum; they supply the tissue of the artery wall with oxygen and nutrients when they are far from the lumen and diffusion of nutrients is difficult. The aorta, pulmonary arteries, and their main branches are good examples of elastic arteries.

B

Elastic laminae

Tunica

adventitia

Tunica

media

Tunica

intima

Figure 9-7B. Large (elastic) artery, carotid artery. Elastic stain, 68; inset 242

This is an example of a large artery (elastic artery) from a portion of the carotid artery. The carotid artery generally has the characteristics of elastic arteries. The tunica media of the vessel wall has multiple layers of elastic lamina that are well organized in concentric fashion. These elastic laminae are sheets of fenestrated elastic material produced by smooth muscle cells in the tunica media. These cells are interspersed between the elastic laminae (see Fig. 4-9B). The elastic laminae in this specimen are revealed as parallel wavy black sheets by a special elastic stain; smooth muscle cells are not visible here with this type of stain. The tunica adventitia of an elastic artery consists of loosely arranged connective tissue fibers. These fibers are mainly collagen fibers, with a small number of elastic fibers that are produced by fibroblasts.

C

Elastic laminae

Vasa vasorum

Smooth muscle cells

Figure 9-7C. Large (elastic) artery, aorta. H&E, 68; insets 198

An example of a large artery (aorta) is shown. The elastic laminae are pink and are more difficult to distinguish with routine H&E stain than they are with elastic stains. The dark nuclei in the tunica media belong to smooth muscle cells (left inset), which produce the elastic membrane. There are some vasa vasorum deep in the tunica adventitia (right inset), which provide oxygen and nutrients for the tunica media and tunica adventitia of the large artery wall. Nerves belonging to the autonomic nervous system may occasionally be found in the tunica adventitia (see Fig. 9-7A).

Tunica adventitia

Internal elastic

lamina

Smooth muscle

cells

Lumen of the medium artery

Figure 9-8B. Medium artery (small muscular artery).

H&E, 68; inset 207

The size of medium arteries (muscular arteries) varies depending upon the location. This is an example of a small segment, which has 10 to 12 layers of smooth muscle cells in the tunica media. Smooth muscle cells are arranged in a circular orientation around the lumen, are connected to each other by gap junctions, and are surrounded by a network of extracellular matrix (see Fig. 6-13). This arrangement allows the smooth muscle cells to function as one unit. When smooth muscle contracts, the lumen size decreases and blood pressure increases, thereby keeping blood moving forward from medium arteries to small arteries. Smooth muscle cells in the tunica media are the targets of various neural and endocrine substances. Both the IEL and EEL are prominent, appearing red-pink with H&E stain.

Tunica adventitia

Figure 9-8C. Medium artery (large muscular artery).

H&E, 68; inset 218

This is an example of a large segment of a medium artery, which has about 20 to 40 layers of smooth muscle cells. The tunica intima is composed of endothelium, subendothelial connective tissue, and an IEL. The thin subendothelial connective tissue layer may become thicker with age and in atherosclerosis. The IEL is a wavy pink sheet forming the boundary between the tunica intima and tunica media. In general, the tunica media is the thickest layer, composed of smooth muscle cells. The tunica adventitia is composed of a layer of dense irregular connective tissue. Vasa vasorum and nerve fibers can be found in this layer but are not as prominent as in elastic arteries.

C

Damage to endothelial cells can lead to various types of cardiovascular diseases, such as atherosclerosis and arteriosclerosis (see Synopsis 9-2).

Internal elastic lamina

Collagen

fibers

Figure 9-9A. Medium artery. TEM, 1,000 left; 8,700 above

In the wall of the healthy medium artery on the left, the layer of squamous endothelial cells adheres closely to the prominent, highly corrugated, internal elastic membrane. The external elastic membrane is also corrugated but thinner. The thick tunica media is composed of many layers of closely apposed smooth muscle cells. In the higher magnification view on the right, the dense bodies and basal laminae that are characteristic of smooth muscle cells are readily apparent. The amorphous appearance of the internal elastic membrane is typical of elastic membranes in transmission electron micrographs.

B

Smooth muscle

Figure 9-9B. Small artery. SEM,

1,300

In this scanning electron micrograph of a small artery, the endothelial cells are extremely squamous, and they are so closely apposed to the internal elastic membrane that the interface between

the two layers is indistinguishable.

Erythrocytes The smooth muscle cells of the tunica media are also closely apposed to each other so that boundaries between cells are difficult to discern. The ground substance of the tunica adventitia and the surrounding connective tissue has been removed during specimen processing so that only some bundles of collagen fibers remain.

Figure 9-10A. A representation of a small artery and an arteriole.

Small arteries (left) have the general structure of muscular arteries but with a smaller diameter and no EEL. The tunica media usually contains about three to six layers of smooth muscle cells. An IEL is usually present but not prominent. Small arteries help control and modulate blood pressure. Arterioles (right) are the smallest arteries, leading blood flow into capillary beds. They play an important role in regulating blood pressure and controlling the blood flow entering capillaries. Arterioles have a very small diameter and narrow lumen. There are only one to two layers of smooth muscle cells in the arteriolar walls, and the tunica adventitia is barely visible. Both an IEL and a subendothelial layer are often absent.

Small arteries and arterioles are sometimes referred to as resistance vessels because of their function in reducing and stabilizing blood pressure before blood flows into the capillary network.

Internal elastic lamina

Small arteries

B

Figure 9-10B. Small arteries, small intestine (ileum).

H&E, 136; inset 408

An example of a small artery in the tissue of the small intestine is shown. Small arteries can be found in various types of tissues and organs where oxygen, nutrients, and other materials must be exchanged. Small arteries have a thin tunica intima and tunica adventitia. The tunica media, the most obvious layer in small arteries, contains three to six layers of circularly arranged smooth muscle cells. An IEL is often present. Smooth muscle is innervated by the autonomic nervous system and regulates blood pressure by causing vasoconstriction (sympathetic nervous system) and vasodilation (parasympathetic nervous system).

Lumen of

venule

C

Arteriole

Endothelial cell

Smooth muscle cell

Figure 9-10C. Arterioles, tongue. H&E, 680; inset

1,020

Arterioles have a very small, round lumen (<0.5 mm in diameter) and one or two layers of smooth muscle cells in the arteriolar wall. Arterioles are the last segment of the arterial system. They connect small arteries to the capillary network. The endothelium of these small vessels is able to sense changes in blood pressure, blood flow, and oxygen tension and to respond to these changes by releasing signals, such as endothelin (vasoconstrictor) and nitric oxide (vasodilator). These signals regulate the tone of adjacent smooth muscle cells. Changes in muscle tone control blood flow into the capillaries. Arterioles can be structurally distinct depending on their location. For example, arterioles in the lung are not structured identically to those in the kidney. A venule accompanying an arteriole is shown here. Venules have very thin walls consisting mainly of endothelium. Smooth muscle cells are not prominent in venules.

CLINICAL CORRELATIONS

Figure 9-12A. Coronary Artery Atherosclerosis. H&E, 21 Coronary artery atherosclerosis, also known as CAD, is characterized by the presence of atherosclerotic changes within the walls of the coronary arteries (see Fig. 3-3C). These changes include accumulation of lipid, formation of atherosclerotic plaques, and thickening of arterial tunica intima. Eventually, normal blood flow to the heart muscle is impaired or obstructed. Rupture of an atherosclerotic plaque promotes the formation of a blood clot, or thrombus, which may occlude the lumen of the coronary artery. Angina (cardiac chest pain), even MI (heart muscle death) (see Fig. 9-4C), can develop when blood flow is reduced below a certain level. CAD is a progressive disease process that generally begins in childhood and manifests clinically in mid-to-late adulthood. Lifestyle changes to lower blood cholesterol and control hypertension and diabetes are important in preventing the disease and reducing the severe consequences.

Diminished lumen

of the small artery

Disrupted internal elastic lamina

Figure 9-12B. Polyarteritis Nodosa (Vasculitis), Small Artery in Intestine. H&E, 71

Polyarteritis nodosa is a multisystem necrotizing vasculitis of small and medium-sized muscular arteries in which involvement of renal and visceral arteries is characteristic. Classic polyarteritis nodosa spares the pulmonary arteries. The cause of the disease is not clear, but it is associated with hepatitis B virus infection in 20% to 30% of cases. Symptoms may be nonspecific, making it difficult to diagnose. Fever, weight loss, and malaise are present in more than 50% of cases. Patients may present with fever, headache, weakness, abdominal pain, and myalgias. Histologically, affected arteries are characterized by segmental transmural inflammation. Internal and external elastic laminae are disrupted, which may lead to aneurysms and bleeding. Polymorphonuclear leukocytes and mononuclear leukocytes are visible in the cellular infiltrate. Steroids are the major drugs used to control disease progression.

Muscle fibers

Arteriole

Capillaries

A Endothelial cells

Figure 9-13A. Continuous capillaries, skeletal muscle.

H&E, 272; inset 870

Capillaries, the smallest vessels (5–10 μm in diameter) in the blood circulatory system, connect arterioles to venules. Capillaries contain one layer of endothelial cells with a basal lamina sometimes wrapped by pericytes. According to the structure of the endothelial cells and the continuity of the basal lamina, capillaries can be classified into three types: (1) continuous capillaries, (2) fenestrated capillaries, and (3) discontinuous (sinusoidal) capillaries. Continuous capillaries have complete endothelial cells (no pores), interrupted only by intercellular clefts between the cells, and a continuous basal lamina with no breaks in the endothelial barrier between the vascular compartment and the surrounding tissues. Such structures in the continuous capillaries allow very limited amounts of materials to pass through their walls. Continuous capillaries can be found in muscles, connective tissues, nervous tissues, lungs, exocrine glands, and the thymus.

Podocyte

Glomerular

capillaries

Fenestrated capillaries have endothelial cells that are perforated by small pores, often bridged by diaphragms (the glomerulus of the kidney is an exception). The basal laminae beneath the endothelial cells are continuous. This type of capillary allows a greater range of substances to pass freely through the capillary wall. Examples of fenestrated capillaries are the capillary beds in the intestine (where nutrients are absorbed), the glomerulus of the kidney (where blood is filtered and urine is formed), and the capillaries in the endocrine organs, such as in the pituitary gland and endocrine pancreas (where hormones are secreted and released into the capillaries). The glomerular capillaries in the kidney are shown here.

The Venous System

Figure 9-16A. A representation of a venule and small vein.

The venous system is composed of venules, small veins, medium veins, and large veins. These vessels carry blood from capillaries in the tissues back to the heart. Small veins and venules are very similar in structure and are sometimes difficult to distinguish from one another. In general, small veins have larger lumens and more visible smooth muscle cells than venules. Venules, the smallest veins, receive blood from the capillary network. Venules have small lumens and very thin walls, with only a single layer of endothelium and a small amount of underlying connective tissue (similar to large capillaries). They may have a few scattered smooth muscle cells in the vessel walls. Venules gradually increase in size to form small veins. They can be categorized as postcapillary venules (10–30 μm in diameter), collecting venules

(30–50 μm) and muscular venules (50–100 μm). Postcapillary venules connect directly to capillaries and drain blood into the collecting venules. Muscular venules often accompany arterioles.

B

Blood cells

Venule

Smooth muscle cells

Figure 9-16B. Venule and arteriole, colon. H&E, 680

This venule (muscular venule) and companion arteriole were taken from the colon of the large intestine. Venules have larger lumens than arterioles. In histological sections, they often have blood cells remaining inside the lumen. Venules are the smallest veins; they drain blood from capillaries into the small veins. They are also involved in exchange of metabolites with tissues and allow blood cells to migrate from the vessels into the tissues (diapedesis). The postcapillary venules are the primary sites involved in the inflammatory response; they allow leukocytes (white blood cells) to migrate into the affected tissues in cases of inflammation or infection. Note that the companion artery has a small, round lumen and a single layer of smooth muscle cells. There are fewer or no blood cells in the lumen of an arteriole specimen, because of the higher pressure and stronger contraction of the arteriole wall.

C

Smooth muscle nucleus

Figure 9-16C. Small vein, colon. H&E, 272; inset 527

Small veins have one to three layers of isolated smooth muscle cells in the walls. The tunica adventitia of small veins is a little thicker than that of venules. Small veins have a thinner wall and less distinguishable smooth muscle layers than do small arteries (also see Fig. 9-10B). Small veins gradually increase in size to become medium veins. Microvessels include arterioles, capillaries, and venules and play a key role in the response to inflammation. During infection or tissue injury, immune cells (such as mast cells and basophils) release histamine and heparin. In response, the endothelium of microvessels allows fluid leakage (edema) and relaxation of smooth muscle cells, leading to vessel dilation (redness and heat).

Exudation is the escape of blood fluid and protein from the lumen of the vessel into the interstitial tissue when the cell-to- cell junctions of the endothelium loosen. Transudation is movement, due to hydrostatic imbalance, of the blood plasma fluid from the lumen to the interstitial tissue, with large protein and cells filtered out by the normal endothelial wall.

Figure 9-17A. A representation of a medium vein.

In general, the tunica media of veins is thinner than the tunica adventitia, and smooth muscle cells in this layer are not as well organized as in arteries. The tunica adventitia of medium veins is more prominent and much thicker than the tunica media. Medium veins have various sizes and structural features depending on their location. The tunica intima consists of an endothelium and a thin subendothelial layer. The tunica media contains a few layers of circularly arranged smooth muscle cells, which usually do not form a distinct sheet. In some areas, large-sized medium veins have a structure similar to that of large veins. Their tunica intima is thinner, and the tunica adventitia contains fewer bundles of longitudinal smooth muscle than large veins. However, this characteristic is not prominent in small segments of the medium veins. Valves are prominent in medium veins, especially in the extremities of the body.

Tunica media

Tunica adventitia

Smooth muscle bundles

Longitudinal smooth muscle bundles

Figure 9-17B. Medium vein, large segment of medium vein. H&E, 68; inset 182

This sample of a large segment of medium vein has a similar structure to a large vein but has fewer longitudinal smooth muscle bundles in the tunica adventitia (Fig. 9-18B). The tunica intima is thinner than in the large veins, and the tunica media contains several layers of circular smooth muscle. The tunica adventitia is the thickest and the most prominent layer in most medium veins. There are some longitudinal smooth muscle bundles (inset) and connective tissue in the tunica adventitia. In the extremities, such as the legs and arms, there are many valves at varying intervals in the innermost layer of veins. They prevent blood backflow against gravity. Veins carry blood toward the heart under lower pressure than arteries and have a great capacity for housing blood (about 65% of blood is in the venous system); for this reason, they are sometimes called capacitance vessels.

Lumen of the medium artery

Nerve

fibers Venule Arteriole

Figure 9-17C. Medium vein, small segment of medium vein. H&E, 68

Veins accompanying companion arteries are often found in the tissues and organs. Structurally, medium arteries have relatively smaller lumens, thicker walls, and more prominent IEL than medium veins. The IEL is usually absent in medium veins. In this example, a small segment of a medium vein with a companion medium artery, the vein has a larger lumen and a thinner wall than a companion artery. This medium vein does not show the prominent tunica adventitia or the longitudinal smooth muscle bundles usually seen in medium veins.

A

Figure 9-18A. A representation of a large vein.

The vena cava and pulmonary veins are the largest veins in the venous system. They connect directly to the heart. The tunica adventitia is the most prominent and distinct layer in large veins; it contains numerous longitudinally arranged smooth muscle bundles and some connective tissues (mostly collagen fibers). The vasa vasorum are also present in the tunica adventitia; they provide blood and nutrient supply to the wall of the large vein. The tunica media contains a few layers of smooth muscle cells that are loosely arranged and do not form a distinct sheath. Contraction of the circularly arranged smooth muscle cells in the tunica media and the longitudinally arranged smooth muscle bundles in the tunica adventitia help to push blood toward the heart.

B Tunica

Vasa vasorum

Figure 9-18B. Large vein, vena cava. H&E, 68; insets 185

The tunica intima of the vena cava consists of an endothelial layer, a thick subendothelial layer (connective tissue), and an IEL. In general, the IEL is not well developed in veins. Although it is not commonly seen in veins, it may still be present in some large veins. The tunica media contains a few layers of loosely organized smooth muscle cells; the tunica media is much thinner compared to its tunica adventitia. There are densely arranged smooth muscle bundles in the tunica adventitia, which give the large vein a distinct appearance. There is a thin layer of connective tissue (mainly collagen fibers) between the circular smooth muscle and the longitudinal smooth muscle bundles in the tunica adventitia (Fig. 9-18C). A vasa vasorum is shown on the right, and a cross section of longitudinal smooth muscle bundles is shown on the left.

A

Figure 9-19A. Small lymphatic vessels, lymph node. H&E,

Valve

Lymphatic

vessels

Figure 9-19B. Large lymphatic vessels, lymph node. H&E,

136; inset 422

Large lymphatic vessels have thicker walls than small lymphatic vessels. Large lymphatic vessels are composed of connective tissues and multiple layers of smooth muscle cells. Contraction of smooth muscle cells helps to move the lymph forward. Large lymphatic vessels are structurally similar to small veins, except they have larger lumens and prominent valves. Valves are present in all sizes of lymphatic vessels. They prevent lymph from flowing backward. Lymphatic vessels are often distinguished by lumens that contain clusters of lymphocytes and coagulated plasma. Lymphatic vessels can be found in most of the tissues of the body but not in the central nervous system, the bone marrow, or the hard tissues.

CLINICAL CORRELATION

Lymphatic vessels

Lymphatic vessel

C

Figure 9-19C. Lymphangioma, Skin. H&E, 44

Lymphangioma is a congenital malformation of the lymphatic system that involves the skin and subcutaneous tissues. It occurs most often in the head and the neck. It is characterized by multiple clusters of translucent vesicles that contain lymph fluid varying from clear to pink to dark red. These vesicles are separated from the normal network of lymphatic vessels but have communications with the superficial lymph vesicles through vertical and dilated lymph channels. Skin lesions range from minute vesicles to cystic and cavernous spaces. Clinically, it appears as a raised, soft, spongy, and pinkish-white lesion. Surgical excision may be chosen if vital structures are involved or for cosmetic correction, but it has a high recurrence rate after surgery.