Pnc

Figure

If excess water is added to the extracellular fluid compartment, cells gain water by osmosis.

tial fluid. This effect attracts still more fluid into the interstitial spaces.

If the outflow of blood from the liver into the inferior vena cava is blocked, the venous pressure within the liver and portal blood vessels increases greatly. This, in turn, raises pressure in liver sinusoids and intestinal capillaries. As a result, fluid with a high concentration of protein is exuded from the surfaces of the liver and intestine into the peritoneal cavity. This causes a rise in the osmotic pressure of the abdominal fluid, which, in turn, attracts more water into the peritoneal cavity by osmosis. This condition, called ascites, distends the abdomen. It is quite painful.

Edema may also result from increased capillary permeability accompanying inflammation. Recall that inflammation is a response to tissue damage and usually releases chemicals such as histamine from damaged cells. Histamine causes vasodilation and increased capillary permeability, so excess fluid leaks out of the capillary and enters the interstitial spaces. Table 21A summarizes the factors that result in edema. ■

Factor

Cause

Effect

Low plasma protein concentration

Liver disease and failure to synthesize proteins; kidney disease and loss of proteins in urine; lack of proteins in diet due to starvation

Plasma osmotic pressure decreases; less fluid enters venular ends of capillaries by osmosis

Obstruction of lymph vessels

Surgical removal of portions of lymphatic pathways; certain parasitic infections

Back pressure in lymph vessels interferes with movement of fluid from interstitial spaces into lymph capillaries

Increased venous pressure

Venous obstructions or faulty venous valves

Back pressure in veins increases capillary filtration and interferes with return of fluid from interstitial spaces into venular ends of capillaries

Inflammation

Tissue damage

Capillaries become abnormally permeable; fluid leaks from plasma into interstitial spaces

Shier-Butler-Lewis: I V. Absorption and I 21. Water, Electrolyte, and I I © The McGraw-Hill

Human Anatomy and Excretion Acid-Base Balance Companies, 2001

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