Edema is the swelling of tissues that occurs when excessive fluid accumulates within the tissue. Fluid comprised of water and electrolytes, with a very small amount of protein and other macromolecules, normally leaves capillaries and small postcapillary venules by a process called filtration. Filtration is primarily driven by the capillary hydrostatic pressure, and the amount filtered per unit time is additionally influenced by the permeability of the vessel wall (endothelium and basement membrane). The fluid that filters into the tissue flows within the intercellular space (the interstitium) and most of it is reabsorbed at the venular end of capillaries where the hydrostatic pressure is lower. Some of the filtered fluid is taken up by lymphatic vessels and returned to the circulation.
Sometimes patients with severe hypertension will also present with systemic edema because of elevated capillary pressures, although it is important to note that capillary pressure is far more sensitive to elevations in venous pressure than to elevations in arterial pressure.
Finally, edema can be a side effect of vasodilator drugs that are used to treat hypertension. Vasodilation of precapillary resistance vessels increases downstream capillary hydrostatic pressure and fluid filtration.
Causes of Edema
Edema may be caused by:- Increased capillary hydrostatic pressure (as occurs when venous pressures become elevated by gravitational forces, volume expanded states, in heart failure or with venous obstruction)
- Decreased plasma oncotic pressure (as occurs with hypoproteinemia)
- Increased capillary permeability caused by proinflammatory mediators (e.g., histamine, bradykinin) or by damage to the structural integrity of capillaries so that they become more "leaky" (as occurs in tissue trauma, burns, and severe inflammation)
- Lymphatic obstruction (as occurs in filariasis)
Sometimes patients with severe hypertension will also present with systemic edema because of elevated capillary pressures, although it is important to note that capillary pressure is far more sensitive to elevations in venous pressure than to elevations in arterial pressure.
Finally, edema can be a side effect of vasodilator drugs that are used to treat hypertension. Vasodilation of precapillary resistance vessels increases downstream capillary hydrostatic pressure and fluid filtration.
No comments:
Post a Comment