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Maintenance of Permeability Barrier

Section II - Diseases of Organ Systems

Chapter 11 - Blood Vessels

Frederick J. Schoen MD, PhD

Diseases of arteries are responsible for more morbidity and mortality than any other type of human disease. Disorders of veins less commonly cause clinically significant problems.

Vascular abnormalities cause clinical disease by two principal mechanisms:

Narrowing or completely obstructing the lumens, either progressively (e.g., by atherosclerosis) or precipitously (e.g., by thrombosis or embolism).

Weakening of the walls, leading to dilation or rupture.

To understand the diseases that affect blood vessels, we first consider some of the anatomic and functional characteristics of these highly specialized and dynamic tissues.

Normal

The general architecture and cellular composition of blood vessels are the same throughout the cardiovascular system. However, certain features of the vasculature vary with and reflect

distinct functional requirements at different locations (see below). To withstand the pulsatile flow and higher blood pressures in arteries, arterial walls are generally thicker than the walls of

veins. Arterial wall thickness gradually diminishes as the vessels become smaller, but the ratio of wall thickness to lumen diameter becomes greater.

The basic constituents of the walls of blood vessels are endothelial cells and smooth muscle cells, and extracellular matrix (ECM), including elastin, collagen, and glycosoaminoglycans.

The three concentric layers—intima, media, and adventitia—are most clearly defined in the larger vessels, particularly arteries ( Fig. 11-1 ). In normal arteries, the intima consists of a

single layer of endothelial cells with minimal underlying subendothelial connective tissue. It is separated from the media by a dense elastic membrane called the internal elastic lamina.

The smooth muscle cell layers of the media near the vessel lumen receive oxygen and nutrients by direct diffusion from the vessel lumen, facilitated by holes in the internal elastic

membrane. However, diffusion from the lumen is inadequate for the outer portions of the media in large and medium-sized vessels, therefore these areas are nourished by small arterioles

arising from outside the vessel (called vasa vasorum, literally "vessels of the vessels") coursing into the outer one half to two thirds of the media. The outer limit of the media of most

arteries is a well-defined external elastic lamina. External to the media is the adventitia, consisting of connective tissue with nerve fibers and the vasa vasorum.

Based on their size and structural features, arteries are divided into three types: (1) large or elastic arteries, including the aorta, its large branches (particularly the innominate, subclavian,

common carotid, and iliac), and pulmonary arteries; (2) medium-sized or muscular arteries, comprising other

Figure 11-1The vascular wall. A, Graphic representation of the cross section of a small muscular artery (e.g., renal or coronary artery). B, Photomicrograph of histologic section containing



a portion of an artery (A) and adjacent vein (V). Elastic membranes are stained black (internal elastic membrane of artery highlighted by arrow). Because it is exposed to higher pressures,

the artery has a thicker wall that maintains an open, round lumen, even when blood is absent. Moreover, the elastin of the artery is more organized than in the corresponding vein. In

contrast, the vein has a larger, but collapsed, lumen, and the elastin in its wall is diffusely distributed. (B, Courtesy of Mark Flomenbaum, M.D., Ph.D., Office of the Chief Medical

Examiner, New York City.)

TABLE 11-1-- Endothelial Cell Properties and Functions

Maintenance of Permeability Barrier


Date: 2016-04-22; view: 676


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