Fluid and Electrolyte Balance: Structure, Regulation, and Clinical Significance

Fluid & Electrolyte Balance

Body Fluid Volumes

Water is the most abundant compound in the human body, playing a critical role in physiological processes. The distribution and volume of body fluids vary based on age, gender, and body composition. In a healthy, non-obese 70-kg male, the total body water averages 40 liters, distributed among plasma, interstitial fluid, and intracellular fluid.

• Plasma: 3 L

• Interstitial fluid (IF): 12 L

• Intracellular fluid (ICF): 25 L

• Percentage of body weight: 80% in newborns, 60% in adult males, 50% in adult females

BODY FLUID	INFANT	ADULT MALE	ADULT FEMALE
Plasma	4	4	4
Interstitial fluid, lymph, and transcellular fluids	26	16	11
Intracellular Fluid	45	40	35
TOTAL	75	60	50

Additional info: The table above shows the percentage of body weight represented by each fluid compartment for infants, adult males, and adult females.

Variation in Total Body Water

Total body water content is influenced by body weight, fat content, gender, and age. Adipose tissue contains less water than muscle, so individuals with higher fat content have lower water content per kilogram. Females generally have about 10% less water than males, and water content decreases with age as muscle is replaced by fat.

Body Fluid Compartments

Body fluids are divided into two main compartments: extracellular fluid (ECF) and intracellular fluid (ICF). ECF includes plasma, interstitial fluid, lymph, joint fluids, cerebrospinal fluid, and eye humors. ICF is the largest compartment, located inside cells, and serves as a solvent for intracellular chemical reactions.

Sources of Fluid Intake and Output

Fluid intake comes from ingested liquids, water in food, and metabolic water produced during cellular respiration. Fluid output occurs through water vapor in respiration, sweating, urine, and feces.

INTAKE	OUTPUT
Water in foods: 700 mL	Lungs (expired air): 350 mL
Ingested liquids: 1500 mL	Skin (diffusion): 350 mL
Water formed by catabolism: 200 mL	Skin (sweat): 100 mL
	Kidneys (urine): 1400 mL
	Intestines (feces): 200 mL
TOTAL: 2400 mL	TOTAL: 2400 mL

Fluid Balance

Fluid balance is maintained by adjusting fluid output, primarily through urine volume. The kidneys, lungs, skin, and large intestine are responsible for fluid output. Hormones such as antidiuretic hormone (ADH) and aldosterone increase kidney reabsorption of sodium and water, decreasing urine volume and increasing ECF.

Atrial natriuretic hormone (ANH) increases urine volume by promoting sodium and water loss, acting as a diuretic hormone.

Regulation of Fluid Intake

Sensory receptors detect changes in ECF volume and concentration, sending signals to the hypothalamus. This triggers thirst and fluid intake. If fluids are not adequately replaced, the body compensates by decreasing saliva and other fluid production, but water is still lost through sweating and expiration.

Capillary Blood Pressure and Blood Proteins

Capillary blood pressure transfers fluid from blood to interstitial fluid, causing a fluid shift. Blood plasma protein concentration contributes to osmotic pressure, attracting water and holding it in the plasma.

Fluid Imbalances

Dehydration occurs when fluid output exceeds intake, causing IF volume to shrink first, followed by ICF and plasma. Overhydration results from fluid intake exceeding output, burdening the heart. Water intoxication is a life-threatening condition caused by severe overhydration and electrolyte imbalance.

Importance of Electrolytes in Body Fluids

Electrolytes are compounds that dissociate in water to form ions, which carry electrical charges. Nonelectrolytes do not dissociate in water. Electrolyte composition of body fluids is essential for physiological functions, including nerve conduction and muscle contraction.

• Cations: Positively charged ions (e.g., potassium [K+], sodium [Na+])

• Anions: Negatively charged ions (e.g., chloride [Cl−], bicarbonate [HCO3−])

Edema and Pitting Edema

Edema is swelling caused by high IF volume. Pitting edema occurs when depressions in the skin do not rapidly refill after pressure is applied. Causes include retention of sodium, increased capillary blood pressure, and decreased plasma protein concentration.

Sodium-Containing Internal Secretions

Internal secretions such as saliva, gastric, pancreatic, bile, and intestinal fluids contain sodium and contribute to electrolyte balance.

Electrolyte Imbalances

Electrolyte imbalances disrupt homeostasis and are related to intake, output, absorption, and distribution of electrolytes. Common imbalances include hypernatremia, hyponatremia, hyperkalemia, hypokalemia, hypercalcemia, and hypocalcemia.

Electrolyte	Imbalance	Blood Concentration	Possible Outcomes
Sodium (Na+)	Hypernatremia	>145 mEq/L	Headache, confusion, seizures; coma and death in severe cases
Sodium (Na+)	Hyponatremia	<136 mEq/L	Headache, confusion, seizures; coma and death in severe cases
Potassium (K+)	Hyperkalemia	>5.1 mEq/L	Weakening and paralysis of skeletal muscle
Potassium (K+)	Hypokalemia	<3.5 mEq/L	Cardiac dysrhythmia or arrest, gastrointestinal (GI) motility problems
Calcium (Ca++)	Hypercalcemia	>5.26 mEq/L	Fatigue, muscle weakness, diminished reflexes, impaired cardiac conduction
Calcium (Ca++)	Hypocalcemia	<4.2 mEq/L	Muscle cramping and twitching, hyperactive reflexes, cardiac dysrhythmia

Sodium Imbalance

Hypernatremia is characterized by blood sodium >145 mEq/L, often due to dehydration or overuse of salt tablets. Hyponatremia is blood sodium <136 mEq/L, caused by excessive ADH secretion, sodium-free IV infusion, burns, or diuretics. Both conditions can cause CNS symptoms such as headache, confusion, seizures, and coma.

Potassium Imbalance

Hyperkalemia (blood potassium >5.1 mEq/L) results from increased intake, tissue trauma, burns, or renal failure, leading to muscle weakness, paralysis, and cardiac arrest. Hypokalemia (blood potassium <3.5 mEq/L) is caused by fasting, low-potassium diets, laxative abuse, diarrhea, vomiting, or gastric suction, resulting in muscle and cardiac problems, abdominal distention, and slow GI passage.

Calcium Imbalance

Hypercalcemia (blood calcium >10.5 mg/dL) is caused by excessive input, increased absorption, bone disease, or hyperparathyroidism, leading to decreased neuromuscular activity, fatigue, muscle weakness, diminished reflexes, and cardiac problems. Hypocalcemia (blood calcium <8.4 mg/dL) results from dietary deficiency, decreased absorption, increased excretion, pancreatitis, hypoparathyroidism, rickets, osteomalacia, or renal insufficiency, causing increased neuromuscular irritability, cramping, twitching, hyperactive reflexes, and abnormal cardiac rhythms.

Review Questions and Quick Checks

Throughout the chapter, review questions and quick checks reinforce key concepts:

• What are electrolytes and what is electrolyte balance?

• What are the two main fluid compartments of the body?

• What is meant by the term fluid balance?

• What is the largest volume of body fluid?

• Which does the body primarily adjust, fluid intake or fluid output?

• What are the chief ways that fluid leaves the body?

• How does the body maintain fluid balance?

• Name the hormones that regulate urine volume.

• Describe the mechanism that regulates fluid intake.

• What is the difference between an electrolyte and a nonelectrolyte?

• What are some of the major roles of ions in the body?

• Identify the functions of electrolytes in the body.

• What are the causes of hypernatremia? Hyponatremia?

• Hypokalemia may cause what conditions?

• Why is calcium a significant mineral in our body?

Additional info: These questions are designed to test understanding of fluid and electrolyte balance, mechanisms of regulation, and clinical implications of imbalances.