The health of cells and tissues depends on the circulation of blood, which delivers oxygen and nutrients and removes wastes generated by cellular metabolism. Under normal conditions, as blood passes through capillary beds, proteins in the plasma are retained within the vasculature and there is little net movement of water and electrolytes into the tissues. This balance is often disturbed by pathologic conditions that alter endothelial function, increase vascular hydrostatic pressure, or decrease plasma protein content, all of which promote edema—the accumulation of fluid in tissues resulting from a net movement of water into extravascular spaces. Depending on its severity and location, edema may have minimal or profound effects. In the lower extremities, it may only make one’s shoes feel snugger after a long sedentary day; in the lungs, however, edema fluid can fill alveoli, causing life-threatening hypoxia.
The structural integrity of blood vessels is frequently compromised by trauma. Hemostasis is the process of blood clotting that prevents excessive bleeding after blood-vessel damage. Inadequate hemostasis may result in hemorrhage, which can compromise regional tissue perfusion and, if massive and rapid, may lead to hypotension, shock, and death. Conversely, inappropriate clotting (thrombosis) or migration of clots (embolism) can obstruct blood vessels, potentially causing ischemic cell death (infarction). Indeed, thromboembolism lies at the heart of three major causes of morbidity and death in developed countries: myocardial infarction, pulmonary embolism (PE), and cerebrovascular accident (stroke).
With this as a preface, we begin our discussion of hemodynamic disorders with conditions that increase tissue blood volumes.
Hyperemia and congestion both refer to an increase in blood volume within a tissue, but have different underlying mechanisms. Hyperemia is an active process resulting from arteriolar dilation and increased blood inflow, as occurs at sites of inflammation or in exercising skeletal muscle. Hyperemic tissues are redder than normal because of engorgement with oxygenated blood. Congestion is a passive process resulting from impaired outflow of venous blood from a tissue. It can occur systemically, as in cardiac failure, or locally as a consequence of an isolated venous obstruction. Congested tissues have an abnormal blue-red color (cyanosis) that stems from the accumulation of deoxygenated hemoglobin in the affected area. In long-standing chronic congestion, inadequate tissue perfusion and persistent hypoxia may lead to parenchymal cell death and secondary tissue fibrosis, and the elevated intravascular pressures may cause edema or sometimes rupture capillaries, producing focal hemorrhages.
MORPHOLOGYCutsurfacesofhyperemicorcongestedtissuesfeelwetandtypicallyoozeblood.Onmicroscopicexamination,acute pulmonary congestion ismarkedbyblood-engorgedalveolarcapillariesandvariabledegreesofalveolarseptaledemaandintraalveolarhemorrhage.Inchronic pulmonary congestion, theseptabecomethickenedandfibrotic,andthealveolarspacescontainnumerousmacrophagesladenwithhemosiderin(“heartfailurecells”)derivedfromphagocytosedredcells.Inacute http://ebooksmedicine.net
Fig. 4.1 Liverwithchronicpassivecongestionandhemorrhagicnecrosis.