Chapter Summary

I. ELASTIC ARTERY (CONDUCTING ARTERY)

Among these are the aorta, common carotid, and subclavian arteries.

A. Tunica Intima

Lined by short, polygonal endothelial cells. The subendothelial connective tissue is fibroelastic and houses some longitudinally disposed smooth muscle cells.

Internal elastic lamina is not clearly defined.

B. Tunica Media

Characterized by numerous fenestrated membranes (spiral to concentric sheets of fenestrated elastic membranes). Enmeshed among the elastic material are circularly disposed smooth muscle cells and associated collagenous, reticular, and elastic fibers.

C. Tunica Adventitia

Thin, collagenous connective tissue containing some elastic fibers and a few longitudinally oriented smooth muscle cells. Vasa vasorum (vessels of vessels) are also present.

II. MUSCULAR ARTERY (DISTRIBUTING ARTERY)

Among these are the named arteries, with the exception of the elastic arteries.

A. Tunica Intima

These are lined by polygonal-shaped, flattened endothelial cells that bulge into the lumen during vasoconstriction. The subendothelial connective tissue houses fine collagenous fibers and few longitudinally disposed smooth muscle cells. The internal elastic lamina, clearly evident, is frequently split into two membranes.

B. Tunica Media

Characterized by many layers of circularly disposed smooth muscle cells, with some elastic, reticular, and collagenous fibers among the muscle cells. The external elastic lamina is well defined.

C. Tunica Adventitia

Usually a very thick collagenous and elastic tissue, with some longitudinally oriented smooth muscle fibers. Vasa vasorum are also present.

III. ARTERIOLES

These are arterial vessels whose diameter is less than 100 mm.

A. Tunica Intima

Endothelium and a variable amount of subendothelial connective tissue are always present. The internal elastic lamina is present in larger arterioles but absent in smaller arterioles.

B. Tunica Media

The spirally arranged smooth muscle fibers may be up to three layers thick. An external elastic lamina is present in larger arterioles but absent in smaller arterioles.

C. Tunica Adventitia

This is composed of collagenous and elastic connective tissues, whose thickness approaches that of the tunica media.

IV. CAPILLARIES

Most capillaries in cross-section appear as thin, circular profiles 8 to 10 mm in diameter. Occasionally, a fortuitous section will display an endothelial cell nucleus, a red blood cell, or, very infrequently, a white blood cell. Frequently, capillaries will be collapsed and not evident with the light microscope. Pericytes are usually associated with capillaries.

V. VENULES

Venules possess much larger lumina and thinner walls than corresponding arterioles.

A. Tunica Intima

Endothelium lies on a very thin subendothelial connective tissue layer, which increases with the size of the vessel. Pericytes are frequently associated with smaller venules.

B. Tunica Media

Absent in smaller venules, whereas in larger venules one or two layers of smooth muscle cells may be observed.

C. Tunica Adventitia

Consists of collagenous connective tissue with fibroblasts and some elastic fibers.

196 C I R C U L A T O R Y S Y S T E M

VI. MEDIUM-SIZED VEINS

A. Tunica Intima

The endothelium and a scant amount of subendothelial connective tissue are always present. Occasionally, a thin internal elastic lamina is observed. Valves may be evident.

B. Tunica Media

Much thinner than that of the corresponding artery but does possess a few layers of smooth muscle cells. Occasionally, some of the muscle fibers, instead of being circularly disposed, are longitudinally disposed. Bundles of collagen fibers interspersed with a few elastic fibers are also present.

C. Tunica Adventitia

Composed of collagen and some elastic fibers, which constitute the bulk of the vessel wall. Occasionally, longitudinally oriented smooth muscle cells may be present. Vasa vasorum are noted to penetrate even the tunica media.

VII. LARGE VEINS

A. Tunica Intima

Same as that of medium-sized veins but displays thicker subendothelial connective tissue. Some large veins have well-defined valves.

B. Tunica Media

Not very well defined, although it may present some smooth muscle cells interspersed among collagenous and elastic fibers.

C. Tunica Adventitia

Thickest of the three layers and accounts for most of the vessel wall. May contain longitudinally oriented smooth muscle fiber bundles among the thick layers of collagen and elastic fibers. Vasa vasorum are commonly present.

VIII. HEART

An extremely thick, muscular organ composed of three layers: endocardium, myocardium, and epicardium. The presence of cardiac muscle is characteristic of this organ. Additional structural parameters may include Purkinje fibers, thick valves, atrioventricular and sinoatrial nodes, as well as the chordae tendineae and the thick, connective tissue cardiac skeleton.

IX. LYMPHATIC VESSELS

Lymphatic vessels are either collapsed and therefore not discernible, or they are filled with lymph. In the latter case, they present the appearance of a clear, endothe- lial-lined space resembling a blood vessel. However, the lumina contain no red blood cells, though lymphocytes may be present. The endothelium may display valves.

CHAPTER OUTLINE

Graphics

Graphic 9-1 Lymphoid Tissues p. 208

Graphic 9-2 Lymph Node, Thymus, and Spleen

p. 209

Graphic 9-3 B Memory and Plasma Cell Formation

p. 210

Graphic 9-4 Cytotoxic T-Cell Activation and Killing of Virally Transformed Cells p. 211

Graphic 9-5 Macrophage Activation by TH1 Cells

Lymphoid tissue forms the basis of the immune system of the body and is organized into diffuse and nodular lymphatic tissues (see Graphics 9-1 and 9-2).

The immune system relies on the interactions of its primary cell components, lymphocytes, and antigenpresenting cells (APCs), to effect a cell-mediated immune response against microorganisms, foreign cells, and virally altered cells and humoral immune response, release of antibodies against antigens.

•Antibodies (immunoglobulins), glycoproteins produced by plasma cells, form the principal armamentarium of the humoral immune response. These glycoproteins bind to those antigens for which they are specific, forming antibody-antigen complexes. Each antibody

is composed of two heavy chains and two light chains

possesses a constant region and a variable region

constant regions are the same for all antibodies of the same class (isotype)

variable regions that are identical in all antibodies against a specific antigen but differ from all other antibodies that are specific for different antigens.

There are five classes (isotypes) of immunoglobulins: IgA, IgD, IgE, IgG, and IgM (see Table 9-1). The heavy chains of these isotypes differ from one another in their amino acid composition.

COMPONENTS OF THE IMMUNE SYSTEM

There are two components of the immune system, namely, the innate (nonspecific) immune system and the adaptive (specific) immune system.

The pentameric form activates the complement system.

•The innate immune system is nonspecific in that it is not designed to combat a particular (i.e., a specific)

antigen. It is an evolutionarily older system than its adaptive counterpart; it possesses no immunologic memory but acts in a rapid fashion in response to pathogen-associated molecular patterns that are shared by most pathogenic invaders. The components

of the innate immune system are listed in Table 9-2, and toll-like receptors are also presented in Table 9-3.

•The adaptive immune system is distinguished by four primary characteristics: immunological memory, immunological specificity, immunological diversity, and the capability to differentiate between self and nonself.