Extracellular Edema

Extracellular Edema

Extracellular fluid edema occurs when there is excess fluid accumulation in the extracellular spaces. There are two general causes of extracellular edema: (1) abnormal leakage of fluid from the plasma to the inter-stitial spaces across the capillaries, and (2) failure of the lymphatics to return fluid from the interstitium back into the blood. The most common clinical cause of interstitial fluid accumulation is excessive capillary fluid filtration.

Factors That Can Increase Capillary Filtration

To understand the causes of excessive capillary filtra-tion, it is useful to review the determinants of capillary filtration. Mathematically, cap-illary filtration rate can be expressed as

Filtration = K_f x (P_c – P_if – π_c + p_if),

where K_f is the capillary filtration coefficient (the product of the permeability and surface area of the capillaries), P_c is the capillary hydrostatic pressure, π_if is the interstitial fluid hydrostatic pressure, p_c is the capillary plasma colloid osmotic pressure, and p_if is the interstitial fluid colloid osmotic pressure. From this equation, one can see that any one of the followingchanges can increase the capillary filtration rate:

a.Increased capillary filtration coefficient.

b.              Increased capillary hydrostatic pressure.

c. Decreased plasma colloid osmotic pressure.

Lymphatic Blockage Causes Edema

When lymphatic blockage occurs, edema can become especially severe because plasma proteins that leak into the interstitium have no other way to be removed. The rise in protein concentration raises the colloid osmotic pressure of the interstitial fluid, which draws even more fluid out of the capillaries.

        Blockage of lymph flow can be especially severe with infections of the lymph nodes, such as occurs with infection by filaria nematodes. Blockage of the lymph vessels can occur in certain types of cancer or after surgery in which lymph vessels are removed or obstructed. For example, large numbers of lymph vessels are removed during radical mastectomy, impairing removal of fluid from the breast and arm areas and causing edema and swelling of the tissue spaces. A few lymph vessels eventually regrow after this type of surgery, so that the interstitial edema is usually temporary.