Body Fluid Compartments

General Concepts

  • Body Fluid Compartments

    • Focus on understanding the distribution and balance of body fluids.

Learning Objectives

  • At the end of the session, participants should be able to:

    • Identify and describe daily water intake and output as well as maintenance of water balance.

    • List and describe body fluid compartments:

      • Intracellular Fluid (ICF)

      • Extracellular Fluid (ECF)

      • Interstitial Fluid

      • Transcellular Fluid

      • Total Body Water (TBW)

    • Describe the composition of each fluid compartment in terms of volume and ion concentration with graphical representation.

    • Discuss physiological factors influencing body fluid distribution such as age, sex, and adipose tissue as well as pathological factors like dehydration and fluid infusion.

Importance of Fluid Volume Maintenance

  • Maintaining a consistent volume and composition of body fluids is essential for achieving homeostasis.

Body Composition Statistics

  • The human body is composed of different elements:

    • Water: Approximately 60% (Range: 50-70%)

    • Protein: 18% of body weight

    • Minerals: 7% of body weight

    • Fat: 15% of body weight

    • Example: A 70 kg man has approximately 42 L of water (70 kg = 70 L).

Body Water Content Across Lifespan

  • Infants: More than 70% water due to low body fat and low bone mass.

  • Healthy adult males: About 60% water; healthy adult females: around 50% water (differences due to higher fat content in females and less skeletal muscle).

  • With aging, total body water decreases (about 45% in older adults).

Daily Water Intake and Output

  • Intake Sources:

    • Fluids ingested: 2100 mL

    • From metabolism: 200 mL

    • Total intake for normal activity: 2300 mL

  • Output Sources:

    • Insensible losses (skin/lungs): 700 mL

    • Sweat: ranges from 100 mL to 5000 mL during high activity levels

    • Feces: 100 mL

    • Urine: 1400 mL

    • Total output for normal activity: 2300 mL; output can increase to 6600 mL under heavy exertion.

Regulation of Fluid Balance

  • Water Deficit: Leads to hypovolemia and dehydration.

    • Physiological response includes:

      1. Activation of hypothalamic thirst center → Increased fluid intake.

      2. Increased ADH secretion → Increased water reabsorption by kidneys.

  • Water Excess: Leads to hypervolemia and edema.

    • Physiological response includes:

      1. Decreased ADH secretion → Decreased water reabsorption → Increased water excretion by kidneys.

      2. Decreased thirst response.

Body Fluid Compartments

  • Water occupies two main compartments:

    • Intracellular Fluid (ICF)

    • Extracellular Fluid (ECF)

      • ECF is composed of:

        • Plasma (intravascular fluid)

        • Interstitial fluid

Body Fluid Composition and Measurements

  • Total Body Water (TBW) for a 70-kg adult:

    • Total Volume: 42 L (~60% body weight)

    • ICF: 28 L

    • ECF: 14 L (3L plasma, 10.5L interstitial fluid, 0.5L transcellular fluid).

Intracellular and Extracellular Fluid Characteristics

  • ICF:

    • Accounts for 2/3 of TBW, high protein content, low sodium and chloride, chief cation is potassium, and main anion is phosphate.

  • ECF:

    • 1/3 of TBW, sodium is the chief cation, chloride is the chief anion, consists of plasma and interstitial fluid.

Fluid Chemistry and Measurements

  • Water as a universal solvent.

  • Solute classifications:

    • Electrolytes: Inorganic salts, acids, bases, some proteins.

    • Nonelectrolytes: Glucose, lipids, creatinine, urea.

    • Concentration measurements include molarity, osmolarity, and osmolality.

Indicators for Body Fluid Volume Measurement

  • Criteria to use a marker:

    • Must mix throughout the compartment, non-toxic, even mixing, and stable during measurement.

  • Examples of markers for measuring specific compartments include:

    • Plasma volume markers: Evans blue dye

    • ECF markers: Isotonic solutions like sucrose and inulin

    • TBW markers: Heavy water and tritiated water.

Volume Calculations for Body Fluid Compartments

  • Intracellular Volume Calculation: ICF = TBW - ECF

  • Interstitial Fluid Volume Calculation: IF Volume = ECF Volume - Plasma Volume

Changes in Body Fluid Compartments and Edema

  • Factors such as dehydration and IV infusion affect fluid volume leading to conditions like edema.

Edema Definition and Causes

  • Edema: Swelling caused by excessive fluid accumulation between tissue spaces, often seen in ECF but can also involve ICF due to various conditions (e.g., hyponatremia and inflammation).

Starling Forces in Fluid Exchange

  • Fluid movement across capillary walls is regulated by hydrostatic and colloid osmotic pressures.

    • Hydrostatic pressure pushes fluid out.

    • Colloid osmotic pressure pulls fluid in.

Summary of Fluid Exchange Mechanisms

  • Fluid filtration and reabsorption at capillary level:

    • Fluid filters from plasma to interstitium; reabsorption occurs back into plasma.

    • Starling's equation provides a model for understanding fluid dynamics across capillary membranes.