Chapter 18
Fluid and Electrolyte Balance

BODY FLUIDS

˜Water is most abundant body compound

ØReferences to “average” body water volume based on a healthy, nonobese, 70-kg male

ØWater is 60% of body weight in males; 50% in females
(Table 18-1)

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

˜Variation in total body water is related to:

ØTotal body weight of individual

ØFat content of body—the more fat the less water (adipose
tissue is low in water content)

ØGender—female body has about 10% less water than male
body (Figure 18-2)

ØAge—in a newborn infant, water may account for 80% of total body weight. In older adults, water per pound of weight decreases (muscle tissue—high in water—replaced by fat,
which is lower in water)

BODY FLUID COMPARTMENTS

˜Two major fluid compartments (Table 18-1)

ØExtracellular fluid (ECF)

•Types:

†Plasma

†Interstitial fluid (IF)

†Transcellular fluid—lymph; joint fluids; cerebrospinal fluid; eye humors

•Called internal environment of body

•Surrounds cells and transports substances to and from them

ØIntracellular fluid (ICF)

•Largest fluid compartment

•Located inside cells

•Serves as solvent to facilitate intracellular chemical reactions

 

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

 

 

˜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 conditions

˜Importance of electrolytes in body fluids

ØNonelectrolytes—organic substances that do not break up
or dissociate when placed in water solution (e.g., glucose)

ØElectrolytes—compounds that break up or dissociate in water solution into separate particles called ions (e.g., ordinary table salt or sodium chloride)

ØIons—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-])

˜Importance of electrolytes in body fluids (cont.)

ØElectrolyte composition of blood plasma—
Table 18-3

ØSodium—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

FLUID IMBALANCES

 

˜Dehydration—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 period

˜Overhydration—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)