Chapter 20Fluid and

Electrolyte Balance

Body Fluids

• Water is most abundant body compound– References to “average” body water volume in

reference tables based on a healthy, nonobese, 70-kg male

– Volume averages 40 L in a 70-kg male• Plasma (3 L)• Interstitial fluid (12 L)• Intracellular fluid (25 L)

– Water is 80% of body weight in newborn infant; 60% in adult males; 50% in adult females

Body Fluids

• Variation in total body water is related to:– Total body weight of individual– Fat content of body—the more fat in the body

the less the total water content per kilogram of body weight (adipose tissue is low in water content)

– Sex—female body about 10% less than male body

Body Fluids– Age—in a newborn infant, water may account

for 80% of total body weight. In the elderly, water per kilogram of weight decreases (muscle tissue—high in water—replaced by fat which is lower in water)

Body Fluid Compartments

• Two major fluid compartments– 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

Body Fluid Compartments

– Intracellular 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; 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

Mechanisms That Maintain Fluid Balance

• 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

Mechanisms That Maintain Fluid Balance

• 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

Mechanisms That Maintain Fluid Balance

• Importance 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)

Mechanisms That Maintain Fluid Balance

– 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+])

Mechanisms That Maintain Fluid Balance

– Negatively charged particles (ions) (e.g., chloride [Cl– and bicarbonate [HCO3–])

– Electrolyte composition of blood plasma – 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

Mechanisms That Maintain Fluid Balance

• Aldosterone increases Na+ reabsorption in kidney tubules

• Sodium-containing internal secretions – Capillary blood pressure and blood proteins

Fluid Imbalances

• Dehydration—total volume of body fluids smaller 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

Fluid Imbalances

• Overhydration—total volume of body fluids larger than normal; overhydration occurs when fluid intake exceeds output; various factors may cause this (for example, giving excessive amounts of intravenous fluids or giving them too rapidly may increase intake above output)

Electrolyte Imbalances

• Related to “intake” and “output” of electrolytes; also important is absorption, distribution of electrolytes in body fluids and availability for use by body cells

• Examples– Sodium imbalance

Electrolyte Imbalances• Hypernatremia—blood sodium above 145 mEq/L

– Characterized by relative deficit of water to salt in extracellular fluid

– Causes include overuse of salt tablets; dehydration; and prolonged diarrhea

Electrolyte Imbalances• Hyponatremia—blood sodium below 136 mEq/L

– Results when there is relatively too much water in the extracellular fluid for the amount of sodium present

– Causes include excessive secretion of antidiuretic hormone; massive infusion of sodium free IV solution; burns; and prolonged use of certain diuretics

– Symptoms of both hyper- and hyponatremia are related to CNS malfunction and include headache, confusion, seizures, and coma

Electrolyte Imbalances

– Potassium imbalance• Hyperkalemia—blood potassium above 5.1 mEq/L

– Causes include increased intake; shift potassium from ICF to blood caused by tissue trauma and burns; renal failure

– Clinical signs of hyperkalemia are related to muscle malfunction and include skeletal muscle weakness, paralysis, and cardiac arrest

Electrolyte Imbalances• Hypokalemia—blood potassium below 3.8 mEq/L

– Causes include fasting; diets low in potassium; abuse of laxatives and certain diuretics; diarrhea; vomiting; gastric suction

– Clinical signs include skeletal muscle and cardiac problems; smooth muscle weakness causing abdominal distention; and slow rate of passage of GI contents

Electrolyte Imbalances– Calcium imbalance

• Hypercalcemia—blood calcium levels above 10.5 mg/dL

– Caused by excessive input; increased absorption; shifts of calcium from bone to ECF; Paget disease and other bone tumors; hyperparathyroidism

– Clinical signs related to decreased neuromuscular activity—fatigue; muscle weakness; diminished reflexes; cardiac problems

• Hypocalcemia—blood calcium levels below 8.4 mg/dL

Electrolyte Imbalances– Caused by dietary deficiency; decreased

absorption or availability; increased excretion; pancreatitis; hypoparathyroidism; rickets and osteomalacia; and renal insufficiency

– Clinical signs related to increased neuromuscular irritability—cramping muscle twitching; hyperactive reflexes, and abnormal cardiac rhythms