Porth's Essentials of Pathophysiology, 4e

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Disorders of Fluid, Electrolyte, and Acid–Base Balance

C h a p t e r 8

A third, usually minor, subdivision of the ECF com- partment is the transcellular compartment. It includes the cerebrospinal fluid and fluid contained in the var- ious body spaces, such as the peritoneal, pleural, and pericardial cavities, and joint spaces. Normally, only about 1% of ECF is in the transcellular space. This amount can increase considerably in conditions such as ascites, in which large amounts of fluid are seques- tered in the peritoneal cavity. When the transcellular fluid compartment becomes considerably enlarged, it is referred to as a third space, because this fluid is not readily available for exchange with the rest of the ECF. Capillary/Interstitial Fluid Exchange The transfer of water between the vascular and inter- stitial compartments occurs at the capillary level. There are four main forces that control the movement of water between the capillary and interstitial spaces: (1) the capillary filtration pressure , which pushes water out of the capillary into the interstitial spaces; (2) the capillary colloidal osmotic pressure , which pulls water back into the capillary; (3) the interstitial or tissue hydrostatic pressure , which opposes the movement of water out of the capillary; and (4) the interstitial col- loidal osmotic pressure , which pulls water out of the capillary into the interstitial spaces. 1–3 Normally, the combination of these four forces is such that only a small excess of fluid remains in the interstitial compart- ment. This excess fluid is removed from the interstitium by the lymphatic system and returned to the systemic circulation. Capillary filtration refers to the movement of water through capillary pores because of hydrostatic pressure, rather than an osmotic force. The capillary filtration pressure (about 30 to 40 mm Hg at the arterial end, 10 to 15 mm Hg at the venous end, and 25 mm Hg in the middle) is the pressure pushing water out of the capillary into the interstitial spaces. 2 It reflects the arte- rial and venous pressures, the precapillary (arterioles) and postcapillary (venules) resistances, and the force of gravity. 2 A rise in arterial or venous pressure increases capillary pressures throughout the body, whereas the force of gravity increases the capillary pressure in the dependent or lower parts of the body. In a person who is standing absolutely still, the weight of blood in the vas- cular column causes an increase of 1 mm Hg pressure for every 13.6 mm of distance from the heart. 2 Since this pressure results from the weight of water, it is called hydrostatic pressure. In the adult who is standing abso- lutely still, the pressure in the veins of the feet can reach 90 mm Hg. The capillary colloidal osmotic or oncotic pressure (about 28 mm Hg) is the osmotic pressure generated by the plasma proteins that are too large to pass through the pores of the capillary wall. 2 The term colloidal osmotic pressure differentiates this type of osmotic pressure from the osmotic pressure that develops at the cell membrane from the presence of electrolytes and nonelectrolytes.

Because plasma proteins do not normally penetrate the capillary pores and because their concentration is greater in the plasma than in the interstitial fluids, it is capillary colloidal osmotic pressure that pulls fluids back into the capillary. The interstitial fluid pressure (about −3 mm Hg) and interstitial colloidal osmotic pressure (about 8 mm Hg) contribute to movement of water into and out of the interstitial spaces. 2 The interstitial hydrostatic fluid pressure , which is normally negative, contributes to the outward movement of water into the intersti- tial spaces. The interstitial colloidal osmotic pressure , which reflects the small amount of plasma proteins that normally escape into the interstitial spaces from the capillary, also pulls water out of the capillary into the tissue spaces. The lymphatic system represents an accessory route whereby fluid from the interstitial spaces can return to the circulation. More importantly, the lymphatic sys- tem provides a means for removing plasma proteins and osmotically active particulate matter from the tis- sue spaces, neither of which can be reabsorbed into the capillaries. Edema Edema can be defined as palpable swelling produced by an increase in interstitial fluid volume. The physiologic mechanisms that contribute to edema formation include factors that: (1) increase the capillary filtration pressure, (2) decrease the capillary colloidal osmotic pressure, (3) increase capillary permeability, or (4) produce obstruction to lymph flow. The causes of edema are summarized in Chart 8-1.

CHART 8-1   Common Causes of Edema Increased Capillary Pressure Increased vascular volume (e.g., heart failure, kidney  disease)

Venous obstruction (e.g., thrombophlebitis) Liver disease with portal vein obstruction Acute pulmonary edema

Decreased Colloidal Osmotic Pressure Increased loss of plasma proteins (e.g., protein-  losing kidney diseases, extensive burns) Decreased production of plasma proteins (liver  disease, malnutrition) Increased Capillary Permeability Inflammation Allergic reactions (e.g., hives, angioneurotic edema) Malignancy (e.g., ascites, pleural effusion) Tissue injury and burns Obstruction of Lymphatic Flow Malignant obstruction of lymphatic structures Surgical removal of lymph nodes

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