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PATHOPHYSIOLOGY OF THE VASCULAR SYSTEM
Reduced blood flow through peripheral blood vessels character-izes all peripheral vascular diseases. The physiologic effects of al-tered blood flow depend on the extent to which tissue demands exceed the supply of oxygen and nutrients available. If tissue needs are high, even modestly reduced blood flow may be inadequate to maintain tissue integrity. Tissues then fall prey to ischemia (defi-cient blood supply), become malnourished, and ultimately die if adequate blood flow is not restored.
Inadequate peripheral blood flow occurs when the heart‚Äôs pump-ing action becomes inefficient. Left ventricular failure causes an accumulation of blood in the lungs and a reduction in forward flow or cardiac output, which results in inadequate arterial blood flow to the tissues. Right ventricular failure causes systemic ve-nous congestion and a reduction in forward flow .
Intact, patent, and responsive blood vessels are necessary to de-liver adequate amounts of oxygen to tissues and to remove meta-bolic wastes. Arteries can become obstructed by atherosclerotic plaque, a thrombus, or an embolus. Arteries can become dam-aged or obstructed as a result of chemical or mechanical trauma, infections or inflammatory processes, vasospastic disorders, and congenital malformations. A sudden arterial occlusion causes profound and often irreversible tissue ischemia and tissue death. When arterial occlusions develop gradually, there is less risk for sudden tissue death because collateral circulation has an oppor-tunity to develop and the body adapts to the decreased blood flow.
Venous blood flow can be reduced by a thrombus obstructing the vein, by incompetent venous valves, or by a reduction in the effectiveness of the pumping action of surrounding muscles. De-creased venous blood flow results in increased venous pressure, a subsequent rise in capillary hydrostatic pressure, net filtration of fluid out of the capillaries into the interstitial space, and sub-sequent edema. Edematous tissues cannot receive adequate nu-trition from the blood and consequently are more susceptible to breakdown, injury, and infection. Obstruction of lymphatic ves-sels also results in edema. Lymphatic vessels can become ob-structed by tumor or by damage resulting from mechanical trauma or inflammatory processes.
Aging produces changes in the walls of the blood vessels that affect the transport of oxygen and nutrients to the tissues. The intima thickens as a result of cellular proliferation and fibrosis. Elastin fibers of the media become calcified, thin, and fragmented, and collagen accumulates in the intima and the media. These changes cause the vessels to stiffen, which results in increased peripheral resistance, impaired blood flow, and increased left ventricular workload.
Although many types of peripheral vascular diseases exist, most re-sult in ischemia and produce some of the same symptoms: pain, skin changes, diminished pulse, and possible edema. The type and severity of symptoms depend in part on the type, stage, and extent of the disease process and on the speed with which the disorder develops. Table 31-2 highlights the distinguishing features of ar-terial and venous insufficiency.
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