Fluid & Electrolyte Balance

Introduction to Body Fluid Balance

Fluid and electrolyte balance is essential for maintaining homeostasis in the human body. Water is the most abundant compound in the body, and its distribution and composition are tightly regulated to support cellular function, transport, and physiological processes.

Body Fluid Volumes and Distribution

Relative Volumes of Body Fluids

• Total body water averages 40 L in a healthy, non-obese 70-kg male, accounting for about 60% of body weight.

• Distribution by compartment:

  • Plasma: 3 L

  • Interstitial fluid (IF): 12 L

  • Intracellular fluid (ICF): 25 L

• Water content varies by age and gender: 80% in newborns, 60% in adult males, 50% in adult females.

Table: Volumes of Body Fluid Compartments

Distribution of Total Body Water

• Extracellular fluid (ECF): Includes plasma, interstitial fluid, lymph, and transcellular fluids; makes up about 34% of total body water.

• Intracellular fluid (ICF): Located inside cells; comprises about 66% of total body water.

Body Fluid Compartments

Extracellular Fluid (ECF)

• Acts as the internal environment, surrounding cells and transporting substances.

• Subdivided into plasma, interstitial fluid, lymph, and transcellular fluids (e.g., cerebrospinal fluid, joint fluids, eye humors).

Intracellular Fluid (ICF)

• Largest fluid compartment, located within cells.

• Serves as a solvent for intracellular chemical reactions.

Fluid Intake and Output

Sources of Fluid Intake

• Liquids we drink

• Water in food

• Metabolic water (produced by cellular respiration)

Sources of Fluid Output

• Water vapor (respiration)

• Sweat (skin)

• Urine (kidneys)

• Feces (intestines)

Table: Typical Daily Water Input and Output

Regulation of Fluid Balance

Regulation of Fluid Output

Fluid output is primarily regulated by the kidneys, but the lungs, skin, and large intestine also contribute. The body adjusts urine volume to match fluid intake, maintaining homeostasis.

• Antidiuretic hormone (ADH): Released from the posterior pituitary when ECF volume is low; increases water reabsorption in the kidneys, reducing urine output.

• Aldosterone: Increases sodium and water reabsorption in the kidneys, expanding ECF volume.

• Atrial natriuretic hormone (ANH): Released from the atrial wall when blood volume is high; promotes sodium and water loss in urine, reducing ECF volume.

Regulation of Fluid Intake

Thirst is regulated by sensory receptors that detect changes in ECF volume and concentration, sending signals to the hypothalamus. Thirst prompts fluid intake, restoring balance.

Factors Affecting Fluid Balance

• Total body weight

• Fat content (adipose tissue contains less water)

• Gender (females have less body water than males)

• Age (infants have more, elderly have less water per kg)

Fluid Imbalances

Dehydration

• Total body fluid volume is lower than normal.

• IF volume decreases first, followed by ICF and plasma if untreated.

• Caused by fluid output exceeding intake for an extended period.

Overhydration

• Total body fluid volume is higher than normal.

• Excess intake burdens the heart and may cause water intoxication (neurological impairment due to electrolyte imbalance).

Electrolytes and Their Importance

Definitions and Roles

• Electrolytes: Compounds that dissociate in water to form ions (e.g., NaCl).

• Nonelectrolytes: Organic substances that do not dissociate in water (e.g., glucose).

• Ions: Charged particles; cations are positive (e.g., Na+, K+), anions are negative (e.g., Cl−, HCO3−).

Electrolyte Imbalances

• Imbalances can result from altered intake, output, absorption, or distribution of electrolytes.

• Common clinical manifestations include edema, muscle weakness, cardiac arrhythmias, and neurological symptoms.

Table: Electrolyte Imbalances

Clinical Signs of Electrolyte Imbalances

• Hypernatremia: CNS symptoms (headache, confusion, seizures, coma).

• Hyponatremia: CNS symptoms (as above); often due to excess water relative to sodium.

• Hyperkalemia: Muscle weakness, paralysis, cardiac arrest.

• Hypokalemia: Cardiac arrhythmias, muscle weakness, GI motility issues.

• Hypercalcemia: Fatigue, muscle weakness, diminished reflexes, cardiac problems.

• Hypocalcemia: Muscle cramping, twitching, hyperactive reflexes, cardiac dysrhythmia.

Summary Table: Key Concepts

Quick Review Questions

• 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?