Fluid, Electrolyte, and Acid-Base Balance: Body Fluid Compartments and Homeostasis

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 regulation are critical for physiological processes. Electrolytes, which dissociate into ions in solution, play a key role in fluid distribution, nerve conduction, and muscle function.

Body Fluid Volumes and Compartments

Distribution of Body Fluids

The human body contains several fluid compartments, each with distinct volumes and functions. The average total body water in a healthy, non-obese 70-kg male is about 40 liters, distributed as follows:

• Plasma: 3 L

• Interstitial Fluid (IF): 12 L

• Intracellular Fluid (ICF): 25 L

Variation in Total Body Water

Total body water varies with age, gender, and body composition:

• Newborn infants: ~80% of body weight is water

• Adult males: ~60% of body weight is water

• Adult females: ~50% of body weight is water (due to higher fat content)

Volumes of Body Fluid Compartments

The following table summarizes the percentage of body weight represented by each fluid compartment in infants, adult males, and 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

Body Fluid Compartments: ECF and ICF

Body fluids are divided into two main compartments:

• Extracellular Fluid (ECF): Includes plasma, interstitial fluid, lymph, and transcellular fluids. It serves as the internal environment, transporting substances to and from cells.

• Intracellular Fluid (ICF): The largest compartment, located inside cells, facilitating intracellular chemical reactions.

Fluid Intake and Output

Sources of Fluid Intake and Output

Fluid intake and output must be balanced to maintain homeostasis. Sources of intake include ingested liquids, water in food, and metabolic water. Output occurs via the lungs, skin, kidneys, and intestines.

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

Regulation of Fluid Output

Fluid output is primarily regulated by the kidneys, which adjust urine volume in response to fluid intake. Hormones such as antidiuretic hormone (ADH) and aldosterone increase reabsorption of sodium and water, reducing urine output and increasing ECF volume. Atrial natriuretic hormone (ANH) increases urine volume by promoting sodium and water loss.

Regulation of Fluid Intake

Thirst is regulated by sensory receptors that detect changes in ECF volume and concentration, sending signals to the hypothalamus. This triggers the sensation of thirst and increases fluid intake to restore balance.

Fluid Imbalances

Dehydration

Dehydration occurs when fluid output exceeds intake, leading to a decrease in total body fluid volume. Interstitial fluid volume decreases first, followed by ICF and plasma if not corrected. Symptoms include dry skin, decreased skin turgor, and, in severe cases, confusion and coma.

Overhydration

Overhydration results from excessive fluid intake or impaired excretion, causing an increase in total body fluid volume. This can burden the heart and lead to water intoxication, a potentially life-threatening condition due to electrolyte imbalance.

Electrolyte Balance

Electrolytes and Nonelectrolytes

Electrolytes are compounds that dissociate into ions in water, conducting electricity (e.g., sodium chloride). Nonelectrolytes do not dissociate in water (e.g., glucose). Electrolyte balance is crucial for nerve conduction, muscle contraction, and fluid distribution.

Major Ions in Body Fluids

• Cations: Positively charged ions (e.g., sodium Na+, potassium K+, calcium Ca2+)

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

Electrolyte Imbalances

Imbalances in electrolyte concentrations can disrupt cellular function and cause clinical symptoms:

Electrolyte	Imbalance	Blood Concentration	Possible Outcomes
Sodium (Na+)	Hypernatremia	>145 mEq/L	Headache, confusion, seizures
Sodium (Na+)	Hyponatremia	<136 mEq/L	Confusion, seizures, coma
Potassium (K+)	Hyperkalemia	>5.1 mEq/L	Muscle weakness, paralysis, cardiac arrest
Potassium (K+)	Hypokalemia	<3.5 mEq/L	Cardiac arrhythmias, muscle weakness
Calcium (Ca2+)	Hypercalcemia	>10.5 mg/dL	Fatigue, muscle weakness, cardiac problems
Calcium (Ca2+)	Hypocalcemia	<8.4 mg/dL	Muscle cramps, twitching, cardiac arrhythmias

Clinical Application: Edema

Edema is the accumulation of excess interstitial fluid, often due to electrolyte imbalances, increased capillary blood pressure, or decreased plasma protein concentration. Pitting edema is identified when depressions in the skin persist after pressure is applied.

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?