Chapter 18 Fluid and Electrolyte Balance. Copyright © 2005 Mosby, Inc. All rights reserved. 2 Mosby items and derived items © 2008 by Mosby, Inc., an

Chapter 18Fluid and Electrolyte Balance

Copyright © 2005 Mosby, Inc. All rights reserved. 2Mosby items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

BODY FLUIDSWater is most abundant body compound References to “average” body water volume based on a

healthy, nonobese, 70-kg male Water is 60% of body weight in males; 50% in females (Table

18-1) Volume averages 40 L in a 70-kg male (Figure 18-1)

Variation in total body water is related to: Total body weight of individual Fat content of body—the more fat the less water (adipose

tissue is low in water content) Gender—female body has about 10% less water than male

body (Figure 18-2) Age—in a newborn infant, water may account for 80% of total

body weight. In the elderly, water per pound of weight decreases (muscle tissue—high in water—replaced by fat, which is lower in water)

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BODY FLUID COMPARTMENTSTwo major fluid compartments (Table 18-1) Extracellular fluid (ECF)

Types: Plasma Interstitial fluid (IF) Miscellaneous—lymph; joint fluids; cerebrospinal fluid; eye

humors Called internal environment of body Surrounds cells and transports substances to and from

them Intracellular fluid (ICF)

Largest fluid compartment Located inside cells Serves as solvent to facilitate intracellular chemical

reactions

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MECHANISMS THAT MAINTAIN FLUID BALANCE

Fluid output, mainly urine volume, adjusts to fluid intake; antidiuretic hormone (ADH) from posterior pituitary gland acts to increase kidney tubule reabsorption of sodium and water from tubular urine into blood, thereby tending to increase ECF (and total body fluid) by decreasing urine volume (Figure 18-6)

ECF electrolyte concentration (mainly Na+ concentration) influences ECF volume; an increase in ECF Na+ tends to increase ECF volume by increasing movement of water out of ICF and by increasing ADH secretion, which decreases urine volume, and this, in turn, increases ECF volume

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Capillary blood pressure pushes water out of blood, into IF; blood protein concentration pulls water into blood from IF; hence, these two forces regulate plasma and IF volume under usual conditionsImportance of electrolytes in body fluids Nonelectrolytes—organic substances that do not break up or

dissociate when placed in water solution (e.g., glucose) Electrolytes—compounds that break up or dissociate in water

solution into separate particles called ions (e.g., ordinary table salt or sodium chloride)

Ions—the dissociated particles of an electrolyte that carry an electrical charge (e.g., sodium ion [Na+])

Positively charged ions (e.g., potassium [K+] and sodium [Na+]) Negatively charged particles (ions) (e.g., chloride [Cl] and

bicarbonate [HCO3])

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Importance of electrolytes in body fluids (cont.) Electrolyte composition of blood plasma—Table 18-3 Sodium—most abundant and important positively

charged ion of plasma Normal plasma level—142 mEq/L Average daily intake (diet)—100 mEq Chief method of regulation—kidney Aldosterone increases Na+ reabsorption in kidney tubules

(Figure 18-6) Sodium-containing internal secretions (Figure 18-7)

Capillary blood pressure and blood proteins

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FLUID IMBALANCESDehydration—total volume of body fluids less than normal; IF volume shrinks first, and then if treatment is not given, ICF volume and plasma volume decrease; dehydration occurs when fluid output exceeds intake for an extended periodOverhydration—total volume of body fluids greater than normal; overhydration occurs when fluid intake exceeds output; various factors may cause this (e.g., giving excessive amounts of intravenous fluids or giving them too rapidly may increase intake above output)

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Chapter 18 Fluid and Electrolyte Balance. Copyright © 2005 Mosby, Inc. All rights reserved. 2 Mosby items and derived items © 2008 by Mosby, Inc., an