Body fluid compartments II

Body Fluid Compartments I

Learning Objectives

• To learn and understand:

  • The volume and solute content of the intracellular and extracellular fluid compartments.

  • The principles used in measurement of body fluid compartments and how to calculate the different volumes.

  • The barriers that exist between the different fluid compartments and how the differences in solute composition are maintained.

Overview of Body Composition

• In a non-obese human, approximately 60% of body weight (BW) is composed of water.

• Breakdown of body weight in a standard lean 70 kg man:

  • Proteins: ~18% BW

  • Minerals: ~7% BW

  • Fat: ~15% BW

  • % of water decreases as % of body fat increases.

• Total water content in a 70 kg man:

  • Total Volume of Water (TB W) = ~42 L

  • Assumption: 1 L of water weighs 1 kg.

Fluid Distribution

• Water is distributed in two compartments:

  • Intracellular Fluid (ICF): ~66.67% TB H2O = ~28L

  • Extracellular Fluid (ECF): ~33.33% TB H2O = ~14L

• ECF can be further divided into:

  1. Plasma: 4-5% BW (~25% of ECF = ~3L)

  2. Interstitial Fluid (IF): 13.5% BW (~75% of ECF = ~10.5L)

• Transcellular fluids (~1% BW): such as synovial fluid (joint), cerebrospinal fluid, intra-ocular fluids, etc.

Blood Volume Calculation

• If Hematocrit is 45%, then 55% of blood is plasma.

  • Given Plasma Volume = 3L, then Blood Volume =
    $Blood ext{ } Volume = \frac{3L}{1.0 - 0.45} = 5.5L \approx 8\% BW$

Electrolyte Composition of Body Fluids

• Concentration of major electrolytes (mEq/L):

  • Intracellular Fluid (ICF):

  • Na+: 15

  • K+: 120

  • Cl-: 20

  • Ca²⁺: all bound

  • Extracellular Fluid (ECF):

  • Na+: 145

  • K+: 4.5

  • Cl-: 116

  • Ca²⁺: 1.2 (*2.5 in total, including bound)

  • The cytoplasm composition is to be understood in terms of the above ions.

Representation of Distribution

• Figure 2: Visual representation of the distribution of major electrolytes between cytoplasm and extracellular spaces.

• Figure 3: Main body fluid compartments:

  • Intracellular: High in K⁺ and low in Na⁺; total protein concentration is 30 g/dL.

  • Extracellular: Low in K⁺ and high in Na⁺; total protein concentration is 7 g/dL.

Measurement of Body Fluid Compartments

• Dilution Principle:

  • An exact quantity of a soluble substance (X) is dissolved in an unknown volume of water (v); after equilibration, concentration (C) can be measured.

  • Volume equation:
    $v = \frac{X}{C}$

  • In vivo correction:
    $v = \frac{X - \text{quantity of X excreted}}{C}$

Criteria for Measurement

1. Substance X must be freely distributed throughout the entire compartment.

2. Must be non-toxic.

3. Must not be metabolized.

4. Must be easily measurable.

Example Calculation of Fluid Volume

• To measure an unknown volume:

  • Add 100g of blue dye.

  • Mix until the concentration equilibrates.

  • If the concentration of the dye is 20g per litre:

  • Volume calculation:
    $\text{Volume} = \frac{100g}{20g/L} = 5L$

Specific Measurement Techniques

• Plasma Volume Measurement:

  • Clinically important subcompartment of ECF. Used agents: 131 Iodine-labelled albumin or Evans Blue dye (binds to plasma proteins).

• ECF Volume Measurement:

  • Use inulin (sugar foreign to the body) or radiolabeled sodium/chloride, must not enter cells.

• Total Body Water Measurement:

  • Use tritiated water or heavy water (D₂O), must freely cross cell membranes.

• Intracellular Fluid Volume Calculation:

  • ICFV = TB W - ECFV

Example Calculation of Total Body Water

• To measure total body H₂O:

  • 100 ml of deuterium oxide (D₂O) was injected into a normal, lean man weighing 84 kg.

  • After 2 hours, D₂O concentration in plasma was 0.2%.

  • Loss of D₂O averaged to 4% of total dose.

• Calculation Steps:

  1. Final Concentration Calculation:

  • Amount added = 100 ml

  • Amount lost = 4 ml

  • Amount retained = 96 ml

  • Final D₂O concentration =
    $\frac{0.2\text{ml}}{100\text{ml plasma}}$

  1. Volume of Distribution Calculation:

  • Volume of Distribution =
    $\frac{\text{Amt. Added - Loss}}{\text{Final Concentration}} = \frac{96\text{ml}}{\frac{0.2}{100}} = 48000 \text{ ml} = 48 \text{ liters} \approx 57.1\% BW$

Fluid Exchange and Regulation

• The capillary wall functions as a semipermeable membrane that separates plasma from interstitial fluid, allowing for the exchange of materials through:

  • Passive movement, controlled by chemical concentration gradients and physical pressures (hydrostatic and osmotic pressures) across the capillary wall.

Key Processes in Capillaries

• At the arterial end:

  • Hydrostatic pressure (Pc) is higher than colloid osmotic pressure (πc),

  • Resulting in filtration: water and solute are forced into the interstitium.

• Mid-capillary:

  • As blood flows, Pc decreases, concentration of impermeable plasma proteins increases, leading to πc rising.

• At the venous end:

  • When πc exceeds Pc, fluid movement reverses, resulting in reabsorption of fluid back into the capillary.

• Fluid homeostasis is maintained by ensuring that filtration and reabsorption rates are roughly equal; any excess is drained into lymphatics and returned to circulation.