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

lymphatic drainage systems, use the same mechanisms for interstitial fluid exchange as other areas of the body. The milking action of the moving structures, such as the lungs, continually forces fluid and plasma proteins back into lymphatic channels, helping to keep these cavities empty. Thus any obstruction of lymph flow causes fluid to accumulate in the serous cavities. Although the accu- mulation of third-space fluids produces a gain in body weight, it does not contribute to the body's fluid reserve or function. The prefix hydro- may be used to indicate the pres- ence of excessive fluid in one of the serous cavitis, with hydrothorax referring to fluid in the pleural cavity. An accumulation of fluid in the peritoneal cavity as asci- tes and the transudation of fluid into the serous cavities is referred to as effusion . Effusion can contain blood, plasma proteins, inflammatory cells (i.e., pus), and extracellular fluid. ■■ Body fluids, which contain water and electrolytes, are distributed between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments of the body, with two thirds being contained in the ICF and one third in the ECF. ■■ The cell membrane serves as a selective barrier to the movement of substances between the ICF and ECF. Lipid-soluble substances (e.g., oxygen [O 2 ] and carbon dioxide [CO 2 ]), which dissolve in the lipid layer of the cell membrane, pass directly through the membrane. Electrolytes, such as sodium [Na + ] and potassium [K + ], rely on transport proteins located in the cell membrane for movement across the membrane, accounting for the compartmental difference in their concentrations. ■■ Water moves across a semipermeable membrane by osmosis moving from the side with the greater concentration of water and lesser concentration of solute particles to the side having the lesser water concentration and greater solute concentration. Osmolarity refers to the osmotic activity that nondiffusible particles exert in pulling water from one side of a semipermeable membrane to the other and tonicity to the tension or effect that the osmotic pressure of a solution with nondiffusible solutes exerts on cell size because of water movement. Cells remain the same size when placed in an isotonic solution with the same osmolarity as the ICF; swell when placed in a hypotonic solution that has an osmolality less than the SUMMARY CONCEPTS

Water and Sodium Balance The distribution of body fluids between the ICF and ECF compartments relies on the concentration of ECF water and sodium. Water provides approximately 90% to 93% of the volume of body fluids and sodium salts approximately 90% to 95% of ECF solutes. Normally, equivalent changes in sodium and water are such that the volume and osmolality of ECF are main- tained within a normal range. Positive water balance (intake greater than output) results in a decrease in body fluid osmolality and ECF sodium concentration due to diluting effects of the excess water. 7 Likewise, a negative water balance (intake less than output) results in an increase in body fluid osmolality and ECF sodium concentration. Regulation of Water Balance Total body water (TBW) accounts for a large percent- age of body weight. In young men, TBW accounts for approximates 60% of body weight and decreases to approximately 52% in elderly men. Because women usually have less lean muscle mass than men, TBW accounts for only about 50% of body weight in young women and decreases to approximately 46% in elderly women. 1,3 Obesity decreases TBW, with levels some- times as low as 30% to 40% of body weight in adults (Fig. 8-6). Infants normally have more TBW than older chil- dren or adults. Total body water constitutes approxi- mately 75% to 80% of body weight in full-term infants and an even greater percentage in premature infants. In addition to having a greater percentage of body water than adults, infants have more than half of their TBW in their ECF compartment, as compared ICF; and shrink when placed in a hypertonic solution that has an osmolality greater than the ICF. ■■ Edema represents the accumulation of fluid volume in the interstitial spaces of the ECF resulting from: (1) an increase in capillary filtration pressure, (2) a decrease capillary colloidal osmotic pressure, (3) an increase in capillary permeability, or (4) obstructed lymphatic flow.The effect that edema exerts on body function is determined by its location, with edema of the brain, larynx, or lungs representing an acute life-threatening situation. ■■ Third spacing represents the loss or trapping of ECF in the transcellular space, such as in the pericardial sac, the peritoneal cavity, or the pleural cavity.

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