Porth's Essentials of Pathophysiology, 4e

165

Disorders of Fluid, Electrolyte, and Acid–Base Balance

C h a p t e r 8

Exchange

Lymph Drainage. The lymphatic system represents an accessory sys- tem by which fluid can be returned to the circulatory system. Normally the forces moving fluid out of the capillary into the interstitium are greater than those returning fluid to the capillary. Any excess fluids and osmotically active plasma proteins that may have leaked into the inter- stitium are picked up by vessels of the lymphatic system and returned to the circulation. Without the function of the lymphatic system, excessive amounts of fluid would accumulate in the interstitial spaces. 3

Capillary filtration pressure

Arterial end

Venous end

Interstitial fluids

Fluid and proteins not reabsorbed into capillaries

Lymphatic vessel

( text continued from page 163 )

Increased Capillary Filtration Pressure. Edema due to increased capillary filtration pressure is usually the result of increased vascular volume. It is commonly seen in conditions such as congestive heart failure that produce fluid retention and venous congestion (see Chapter 20). Edema does not usually become evi- dent until the interstitial volume has been increased by 2.5 to 3 L. 4 Dependent edema describes an accu- mulation of fluid in the lower parts of the body. Because of the effects of gravity, edema that results from increased capillary pressure commonly causes dependent edema. For example, edema of the ankles and feet becomes more pronounced during prolonged periods of standing. Lower extremity edema due to an increase in venous pressure is a common complication of venous insufficiency resulting from thrombophle- bitis (development of a blood clot in one of the deep leg veins). Decreased Capillary Colloidal Osmotic Pressure. Plasma proteins exert the osmotic force needed to pull fluid back into the capillary from the tissue spaces. The plasma proteins constitute a mixture of albumin, globu- lins, and fibrinogen. Albumin, the smallest of the plasma proteins, has a molecular weight of 69,000; globulins have molecular weights of approximately 140,000; and fibrinogen has a molecular weight of 400,000. 2 Because of its lower molecular weight, 1 g of albumin has approximately twice as many osmotically active molecules as 1 g of globulin and almost six times as many as 1 g of fibrinogen. Also, the concentration of albumin (approximately 4.5 g/dL) is greater than that of the globulins (2.5 g/dL) and fibrinogen (0.3 mg/dL). 2

Therefore, albumin has the greatest effect on the colloi- dal osmotic pressure. Edema caused by decreased capillary colloidal osmotic pressure usually is the result of inadequate production or excessive loss of plasma proteins, mainly albumin. The plasma proteins are synthesized in the liver. In persons with severe liver failure, the impaired synthesis of albumin results in a decrease in colloidal osmotic pressure. In starvation and mal- nutrition, edema develops because there is a lack of amino acids for plasma protein synthesis. The most common site of plasma protein loss is the kidney. In kidney diseases such as nephrotic syndrome, the glo- merular capillaries become permeable to the plasma proteins, particularly albumin (see Chapter 25). When this occurs, large amounts of albumin are filtered out of the blood and lost in the urine. An excessive loss of plasma proteins also occurs when large areas of skin are injured or destroyed, such as occurs during the early stages of a burn. 5 Because the plasma proteins are evenly distributed throughout the body and are not affected by the force of gravity, edema that is caused by a decrease in capillary colloidal osmotic pressure tends to produce generalized edema involving the face as well the legs and feet. Increased Capillary Permeability. When the capil- lary pores become enlarged or the integrity of the capillary wall is damaged, capillary permeability is increased. When this occurs, plasma proteins and other osmotically active particles leak into the inter- stitial spaces, pulling fluid out of the capillary into the interstitium. Among the conditions that increase