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Figure 9-8A. A representation of a medium artery

(muscular or distributing artery).

The medium arteries are also called muscular arteries, refer-

ring to the fact that the arterial wall is dominated by smooth

muscle. There are about 6 to 40 layers of smooth muscle cells

that form a distinct sheath in the tunica media. In general,

the tunica media is much thicker than the tunica adventitia.

Vasoconstriction can be caused by smooth muscle contrac-

tion that is controlled by the sympathetic nervous system.

Internal and external elastic laminae are obvious in medium

arteries. The IEL separates the tunica intima from the tunica

media; the external elastic lamina (EEL) separates the tunica

media from the tunica adventitia. The tunica adventitia is

composed of connective tissues (ﬁ broblasts and connective

tissue ﬁ bers). Vasa vasorum may also be present. Medium

arteries, also sometimes called distributing arteries, distribute

blood to the small arteries in various organs of the body.

D. Cui /T. Yang

Tunica

adventitia

Tunica

media

Tunica

intima

Connective tissue

Fibroblast

Smooth

muscle cell

External elastic

lamina

Internal elastic

lamina

Vasa vasorum

Nerve

Smooth

muscle

cells

Smooth

muscle

cells

Tunica

intima

Tunica

media

Tunica

adventitia

Vein

Lumen of the

medium artery

Internal elastic

lamina

Internal elastic

lamina

External

elastic lamina

External

elastic lamina

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 keep-

ing 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.

Internal

elastic

lamina

Internal

elastic

lamina

Endothelium

Subendothelium

Tunica

intima

Tunica

media

Tunica

adventitia

Endothelium

Subendothelium

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 ﬁ bers can be found in this layer but are not as

prominent as in elastic arteries.

Damage to endothelial cells can lead to various types

of cardiovascular diseases, such as atherosclerosis and

arteriosclerosis (see Synopsis 9-2).

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Erythocytes

Lumen

Endothelium

Erythrocytes

Lumen

Endothelium

External

elastic lamina

External

elastic lamina

Tunica

media

Tunica

media

Internal

elastic lamina

Internal

elastic lamina

Endothelial

cell junction

Endothelial

cell junction

Collagen

fibers

Collagen

fibers

Tunica

media

Dense

bodies

Basal lamina

Tunica

media

Dense

bodies

Basal lamina

Internal

elastic

lamina

Internal

elastic

lamina

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.

The smooth muscle cells of the tunica

media are also closely apposed to each

other so that boundaries between cells

are difﬁ cult to discern. The ground

substance of the tunica adventitia and

the surrounding connective tissue has

been removed during specimen pro-

cessing so that only some bundles of

collagen ﬁ bers remain.

Smooth muscle

Erythrocytes

Collagen

fiber

bundle

Collagen

fiber

bundle

Internal elastic

lamina

Internal elastic

lamina

Endothelial cells

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 magniﬁ cation view on the right, the dense bod-

ies 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.

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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 ﬂ ow

into capillary beds. They play an important role in regulat-

ing blood pressure and controlling the blood ﬂ ow 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 ﬂ ows into the capil-

lary network.

D. Cui /T. Yang

Internal elastic

Small artery Arteriole

lamina

Tunica

media

D. Cui /T. Yang

Endothelial

cell

Smooth

muscle cell

Lumen of

small vein

Arteriole

Nuclei of smooth

muscle cells

Small arteries

Internal

elastic

lamina

Nuclei of smooth

muscle cells

Internal

elastic

lamina

Arteriole

Lumen of

small vein

Small arteries

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 tis-

sues 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 circu-

larly arranged smooth muscle cells. An IEL is often present.

Smooth muscle is innervated by the autonomic nervous sys-

tem and regulates blood pressure by causing vasoconstriction

(sympathetic nervous system) and vasodilation (parasympa-

thetic nervous system).

Lumen of

venule

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 diam-

eter) and one or two layers of smooth muscle cells in the arte-

riolar wall. Arterioles are the last segment of the arterial sys-

tem. They connect small arteries to the capillary network. The

endothelium of these small vessels is able to sense changes in

blood pressure, blood ﬂ ow, and oxygen tension and to respond

to these changes by releasing signals, such as endothelin (vaso-

constrictor) and nitric oxide (vasodilator). These signals regu-

late the tone of adjacent smooth muscle cells. Changes in muscle

tone control blood ﬂ ow 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.

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Figure 9-11. Arteriole. TEM, 5, 800; inset 3,800

The arteriole in the larger image was ﬁ xed in a dilated state, whereas the inset shows a slightly smaller arteriole that was ﬁ xed in a

contracted state. In both vessels, a thin elastic lamina separates the endothelium from the smooth muscle cells of the tunica media. The

state of contraction of the smooth muscle cells is regulated largely by signals from the endothelial cells. In addition to chemical messen-

gers, such as nitric oxide and endothelin, signaling may also involve direct contacts between endothelial and smooth muscle cells. The

cell in the upper left corner of the larger micrograph is a Schwann cell with several unmyelinated axons embedded in its cytoplasm.

Nucleus of smooth

muscle cell

Nucleus of

Schwann cells

Endothelium-smooth muscle

cell junctions

Internal

elastic

lamina

Internal elastic

lamina

Nucleus of smooth

muscle cell

Nuclei of

endothelial

cells

Nucleus of

endothelial

cell

Endothelium–smooth muscle

cell junctions

Internal

elastic

lamina

Internal elastic

lamina

Nucleus of smooth

muscle cell

Nucleus of

Schwann cell

Nuclei of

endothelial

cells

Nucleus of

endothelial

cell

Nucleus of smooth

muscle cell

SYNOPSIS 9-2 Functions of Endothelium in Blood Vessels

Endothelium provides a permeability barrier and controls the composition of interstitial tissue ﬂ uid. ■

Endothelium allows the movement of leukocytes, ﬂ uid, and immunoglobulins into tissues. ■

Endothelium provides for angiogenesis in the formation and differentiation of new blood vessels. ■

Endothelium provides signals that regulate the tone of adjacent vascular smooth muscle cells that control blood pressure ■

by vasoconstriction (endothelin) and vasodilation (nitric oxide, prostacyclin).

Endothelium provides anticoagulant signals (thrombomodulin, nitrous oxide, and prostacyclin) that inhibit platelet attach-

■

ment and aggregation to prevent blood clotting and allow unobstructed ﬂ ow of blood in normal conditions.

When endothelial cells are injured, they reduce the production of platelet inhibitors and increase the release of stimulators

of platelet activation (e.g., von Willebrand factor, tissue thromboplastin). Exposure of the underlying basal lamina and con-

nective tissue further stimulates platelets to adhere and aggregate. Damage to the endothelium will slow down oxidation and

clearance of low-density lipoprotein, leading to many cardiovascular diseases such as atherosclerosis.

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SYNOPSIS 9-3 Pathological and Clinical Terms for the Circulatory System

Vasculitis ■ : Inﬂ ammation and damage to blood vessels often resulting in ischemia of the tissue served by the affected vessels;

may be related to infection, medications, and various autoimmune disorders.

Angina

■ : Chest pain associated with ischemia of the myocardium, most often due to severe atherosclerosis of the coronary

arteries; the pain of angina, often described as pressure, is typically substernal but may radiate to the arms or neck.

Myocardial infarction

■ : Necrosis of cardiac muscle due to interruption of its blood supply, most often due to coronary

artery atherosclerosis.

Atherosclerosis

■ : The most clinically signiﬁ cant form of arteriosclerosis (“hardening of the arteries”), characterized by the

accumulation of lipid within the intima of arteries producing plaques that cause narrowing, or “stenosis,” of the artery.

Arteriosclerosis

■ : A general term referring to any hardening (loss of elasticity) of arteries, especially small arteries. High

blood pressure is the most common cause. Atherosclerosis is one form of arteriosclerosis.

Thrombus

■ : A blood clot formed within an artery or vein as a result of endothelial injury, abnormal blood ﬂ ow, or an

increased tendency to form clots (hypercoagulability); rupture of an atherosclerotic plaque in a coronary artery promotes

the formation of a thrombus.

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 ﬂ ow 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 (car-

diac chest pain), even MI (heart muscle death) (see Fig. 9-4C),

can develop when blood ﬂ ow 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.

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 involve-

ment 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

nonspeciﬁ c, making it difﬁ cult 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 inﬂ ammation. Internal and external

elastic laminae are disrupted, which may lead to aneurysms

and bleeding. Polymorphonuclear leukocytes and mononu-

clear leukocytes are visible in the cellular inﬁ ltrate. Steroids

are the major drugs used to control disease progression.

Fibrous

thickening of the

tunica intima

Central lipid core

(with cholesterol

clefts)

Tunica media

Tunica adventitia

Narrow lumen

Internal elastic

lamina

Inflammatory

leukocytes

Diminished

lumen

of the

small artery

Inflammatory

leukocytes

Fibrous

thickening of the

tunica intima

Disrupted

internal elastic

lamina

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The Capillary System

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. Capil-

laries 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 classiﬁ ed 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.

Arteriole

Muscle fibers

Capillaries

Endothelial cells

Figure 9-13B. Continuous capillary, loose connective tissue. EM 5,100

The caliber of this continuous capillary can be inferred from the erythrocyte that occupies the lumen. Nuclei of two endothelial cells

are prominent, and portions of pericytes can be seen external to the endothelial cells. A continuous layer of endothelial cell cyto-

plasm forms a barrier between the capillary lumen and the surrounding tissue compartment. Junctions between adjacent endothe-

lial cells can be of both the occludens and the adherens types. Marginal folds, such as the one beneath the erythrocyte, are typical

features at the junctions between capillary endothelial cells. Each endothelial cell has numerous caveolae beneath its plasmalemma,

although these structures are difﬁ cult to discern at this magniﬁ cation. The caveolae are a manifestation of active transcytosis of

materials between the plasma and the tissue space.

T. Yang

Basal lamina

Endothelial cell

Caveolae

Erythrocyte

Marginal fold

Pericyte

Intercellular

junction

Intercellular

junction

Basal lamina of

pericyte

Erythrocyte

Marginal fold

Pericyte

Intercellular

junction

Caveolae

Intercellular

junction

Basal lamina of

pericyte

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Figure 9-14A. Fenestrated capillaries, kidney. H&E,

680; inset 1,405

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 fenes-

trated capillaries are the capillary beds in the intestine

(where nutrients are absorbed), the glomerulus of the

kidney (where blood is ﬁ ltered and urine is formed), and

the capillaries in the endocrine organs, such as in the pitu-

itary gland and endocrine pancreas (where hormones are

secreted and released into the capillaries). The glomerular

capillaries in the kidney are shown here.

Lumen of

capillary

Podocyte

Endothelial cell

Endothelial cells

Glomerular

capillaries

Glomerular

capillaries

Podocyte

Endothelial cell

Endothelial cells

Lumen of

capillary

Figure 9-14B. Fenestrated capillary, pituitary. EM, 14,000

The wall of this fenestrated capillary consists of a thin layer of cytoplasm interrupted at frequent intervals by circular pores, termed

fenestrations, of fairly consistent diameter (about 70 nm). This permits freer movement than occurs in continuous capillaries for

some molecules such as peptide hormones. In order to pass between the vascular compartment and the interstitial tissue compart-

ment, dissolved molecules still must pass through the diaphragms that bridge the pores, and the endothelial cells still produce a

complete basal lamina at the interface with the surrounding tissue. Fenestrated capillaries are found in the gastrointestinal tract, the

kidney, and in endocrine glands. The vessel shown here is located in the anterior pituitary.

T. Yang

Diaphragm

Basal lamina

Lumen of a

fenestrated capillary

Secretory granules

containing hormones

Collagen

fibers

Diaphragm of

fenestration

Endothelial cell

cytoplasm

Collagen

fibers

Diaphragm of

fenestration

Lumen of a

fenestrated capillary

Endothelial cell

cytoplasm

Secretory granules

containing hormones

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Figure 9-15A. Discontinuous capillaries, spleen. H&E,

274

Discontinuous capillaries, also called sinusoidal capillaries

(or sinuses), are characterized by large pores, which per-

forate the cytoplasm of endothelial cells, and incomplete

or missing basal laminae. In addition, the sinusoids have

large intercellular clefts (gaps between endothelial cells).

These structures permit free movement of proteins, plasma

constituents, and other materials. In some cases, even cells

can pass through the capillary wall. This type of capillary

has a large diameter (30–60 μm) and may have an irregular

shape due to an incomplete basal lamina, spindle-shaped

endothelial cells, and fenestrated structures. Discontinu-

ous capillaries can be found in the liver, spleen, bone mar-

row, and lymph nodes.

Endothelial

cells

Erythrocytes

Capillaries

(sinusoids)

Capillaries

(sinusoids)

Endothelial

cells

Erythrocytes

Capillaries

(sinusoids)

Capillaries

(sinusoids)

Figure 9-15B. Discontinuous (sinusoidal) capillary. EM, 5,200; inset 3,400

Sinusoidal capillaries vary greatly in appearance, but they all have as common features, lumens of large diameter and gaps or

potential gaps between and sometimes even within endothelial cells. Objects as large as cells can pass through these discontinuities.

The sinusoidal capillary shown here is located in the red pulp of the spleen, and it is termed a red pulp sinusoid. The endothelial

cells are elongated with the long axis parallel to the long axis of the vessel. In cross section, as seen here, the endothelial cells are

nearly circular in shape. There are gaps between cells, such as the one seen at the top edge of this example, but many neighboring

endothelial cells are in close contact. Nevertheless, as seen in the inset, motile blood cells and macrophages can pass readily between

adjacent endothelial cells because of sparseness of intercellular junctions and the incomplete basement membrane.

T. Yang

Migrating cell

Gap

Red pulp

cord

Erythrocyte

Monocyte

Basement

membrane

Basement

membrane

Lumen

Basement

membrane

Endothelial

cells

Endothelial

cells

Erythrocyte

Migrating cell

Monocyte

Basement

membrane

Basement

membrane

Lumen

Basement

membrane

Endothelial

cells

Endothelial

cells

Gap

Red pulp

cord

Endothelial cells

Gap

Incomplete basal lamina

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173

The Venous System

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

inﬂ ammatory response; they allow leukocytes (white blood cells)

to migrate into the affected tissues in cases of inﬂ ammation 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.

Venule

Blood cells

Arteriole

Endothelial cell

Smooth

muscle

cells

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 dis-

tinguishable 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 inﬂ ammation. During

infection or tissue injury, immune cells (such as mast cells and

basophils) release histamine and heparin. In response, the endothe-

lium of microvessels allows ﬂ uid leakage (edema) and relaxation of

smooth muscle cells, leading to vessel dilation (redness and heat).

Endothelial

cell

Lumen of a

small vein

Lumen of a

small vein

Arteriole

Smooth

muscle

nuclei

Smooth muscle nucleus

Exudation is the escape of blood ﬂ uid and protein from the

lumen of the vessel into the interstitial tissue when the cell-to-

cell junctions of the endothelium loosen. Transudation is move-

ment, due to hydrostatic imbalance, of the blood plasma ﬂ uid

from the lumen to the interstitial tissue, with large protein and

cells ﬁ ltered out by the normal endothelial wall.

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 capillar-

ies in the tissues back to the heart. Small veins and venules are

very similar in structure and are sometimes difﬁ cult 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 scat-

tered 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.

D. Cui /T. Yang

Smooth muscle cell

Venule Small vein

Endothelial

cell

Endothelium

Connective tissue

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Organ Systems

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 subendothe-

lial layer. The tunica media contains a few layers of circu-

larly 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.

D. Cui /T. Yang

Tunica

adventitia

Tunica

media

Tunica

intima

Connective tissue

Longitudinal smooth

muscle bundle

Circular smooth

muscle layer

Endothelium

Smooth

muscle bundles

Smooth

muscle bundles

Longitudinal smooth

muscle bundles

Circular smooth

muscle layer

Tunica

intima

Tunica

media

Tunica

adventitia

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 longitudi-

nal 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 con-

nective tissue in the tunica adventitia. In the extremities,

such as the legs and arms, there are many valves at vary-

ing intervals in the innermost layer of veins. They prevent

blood backﬂ ow 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.

Internal

elastic

lamina

Lumen of the

medium artery

Lumen of the

medium vein

Arteriole

Venule

Nerve

fibers

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 compan-

ion artery. This medium vein does not show the promi-

nent tunica adventitia or the longitudinal smooth muscle

bundles usually seen in medium veins.

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