BODY FLUID COMPARTMENT

INTRODUCTION

• A large proportion of the human body is contributed by water.

• The % total amount of water (Total Body Water) is approximately 50-70% of body weight.

  • Total Body Water = % total body water × body weight

  • For example, in a 70 kg man: Total Body Water = 0.7 × 70kg = 49 Litres.

• Total body water is influenced by:

  • Gender

  • Body fat

  • Age

GENDER & BODY FAT

• Total body water percentage correlates inversely with body fat.

  • Higher % of body fat means lower total body water percentage.

• Women tend to have a higher % of body fat than men, leading to lower total body water percentages in women.

• Among body tissues, adipose tissue is the least hydrated.

AGE

• Infants have a higher total body water percentage compared to adults due to their lower body fat and bone mass.

• Total body water percentage declines throughout life after infancy.

• In old age, the total body water percentage decreases further.

FLUID COMPARTMENTS

• Total body water is distributed between two major compartments:

  • Intracellular Fluid (ICF): 2/3 of total body water.

  • Extracellular Fluid (ECF): 1/3 of total body water.

  • If total body water % = 60%, then:

    • ICF = 2/3 × 60% = 40%

    • ECF = 1/3 × 60% = 20%

MAJOR EXTRACELLULAR FLUID COMPARTMENTS

• The ECF is divided into two major sub-compartments:

  • Plasma: Fluid portion of blood that circulates in blood vessels; comprises ¼ of ECF.

  • Interstitial Fluid: Fluid in the microscopic spaces between cells; comprises ¾ of ECF.

  • If ECF = 20%:

    • Plasma = 1/4 × 20% = 5%

    • Interstitial fluid = 3/4 × 20% = 15%.

MINOR EXTRACELLULAR COMPARTMENTS

• The ECF also includes several minor fluid compartments that are relatively small:

  • Lymph.

  • Transcellular fluid (includes fluid such as cerebrospinal fluid, intraocular fluid, synovial fluid, pericardial fluid, intra-pleural fluid, peritoneal fluid, and digestive fluids).

COMPOSITION OF BODY FLUID

• The compositions of ECF & ICF are markedly different:

  • ECF:

    • Major cation: Sodium (Na+).

    • Major anions: Chloride (Cl-) and bicarbonate (HCO3-).

  • ICF:

    • Major cations: Potassium (K+), Magnesium (Mg2+).

    • Major anion: Organic phosphates (HPO4-).

• Ca2+ content is relatively higher in ECF than in ICF.

• ICF contains more protein than ECF.

• Substances high in concentration in ECF are low in ICF and vice versa.

IMPORTANT DIFFERENCES BETWEEN ECF & ICF

UNITS OF MEASURING SOLUTE CONCENTRATIONS

• Measurement of solutes:

  • Moles.

  • Equivalents.

  • Osmoles.

• Concentration of solutes can be expressed as:

  • Moles per liter (mol/L).

  • Equivalents per liter (Eq/L).

  • Osmoles per liter (Osm/L).

• In biological solutions, low concentrations are commonly expressed as:

  • Millimoles per liter (mmol/L).

  • Milliequivalents per liter (mEq/L).

  • Milliosmoles per liter (mOsm/L).

MOLARITY

• Defined as the number of moles of solute per liter of solution.

• Can be expressed in either mol/L or mmol/L.

• 1 mole = mass of element (g) / relative atomic mass.

• 1 mole = 6 × 10^23 molecules.

• 1 mmol = 10^-3 moles = 6 × 10^20 molecules.

• Molarity = number of moles (mol or mmol) / volume (L).

EXAMPLE CALCULATION FOR MOLARITY

• Known mass of NaOH(aq) = 572g, volume = 1.00 L.

• Molar mass of NaOH calculation:

  • Sodium (Na) = 22.99 g/mol.

  • Oxygen (O) = 16.00 g/mol.

  • Hydrogen (H) = 1.01 g/mol.

  • Total molar mass of NaOH = 22.99 + 16.00 + 1.01 = 40.00 g/mol.

• Calculate the number of moles of NaOH:

  • Number of moles = 572g / 40.00g/mol = 14.3 mol.

• Calculate molarity:

  • Molarity = 14.3 mol / 1.00 L = 14.3 M NaOH (aq) solution.

NORMALITY

• Defined as the number of gram or mole equivalents of a solute present in 1 liter of a solution.

• An equivalent is the amount of charged (ionized) solute.

• 1 Equivalent = 1/1000 milliequivalent.

• For example, 1 mole of KCl dissociates into 1 equivalent of potassium (K+) and 1 equivalent of chloride (Cl-).

• Therefore, 1 mmol/L of Cl- corresponds to 1 mEq/L.

OSMOLARITY

• Defined as the concentration of osmotically active particles in a solution.

• Expressed as osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L).

• For example, a solution containing 1mmol/L of NaCl is 2 mOsm/L because NaCl dissociates into 2 particles.

• Total solute concentration (osmolarity) is the same in ICF & ECF, with a normal value of osmolarity in body fluids being 290 mOsm/L.

OSMOSIS

• Defined as the flow of water across a semipermeable membrane due to a difference in solute concentration.

• The concentration difference of impermeant solutes establishes an osmotic pressure difference.

• This osmotic pressure difference drives water flow by osmosis.

OSMOLARITY

• Osmolarity is the concentration of particles (mOsm/L).

• Solutions with the same osmolarity are termed isosmotic.

• A solution with a higher osmolarity is hyperosmotic, while a solution with a lower osmolarity is hyposmotic.

SAMPLE PROBLEM: OSMOLARITY COMPARISON

• Solution A: 2 mmol/L urea; Solution B: 1 mmol/L NaCl.

• Solution A (urea does not dissociate): Osmolarity = 2 mOsm/L.

• Solution B (NaCl dissociates): Assuming gNaCl = number of particles per mole = 1.85:

  • Osmolarity = 1.85 Osm/mol × 1 mmol/L = 1.85 mOsm/L.

• Conclusion: Solutions are not isosmotic.

OSMOLALITY

• Defined as the concentration of osmotically active particles expressed as Osm/kg or mOsm/kg.

• 1 kg = 1 L; hence, osmolarity & osmolality have the same numerical values under expected conditions.

OSMOTIC PRESSURE

• Created by the difference in solute concentration across the membrane, generating an osmotic pressure difference.

• This difference is the driving force for osmotic water flow.

• Osmotic pressure is the minimum pressure that stops osmosis, dependent on:

  • Concentration of osmotically active particles.

  • Whether the solute can cross the membrane or not.

OSMOTIC PRESSURE CALCULATION

• Calculated with the Van't Hoff equation: n = gCoRT

  • T = osmotic pressure (atm or mm Hg)

  • g = number of particles per mole in solution (Osm/mol)

  • C = concentration (mmol/L)

  • o = reflection coefficient (0 to 1)

  • R = gas constant (0.082 L-atm/mol-K)

  • T = absolute temperature (K).

REFLECTION COEFFICIENT

• A dimensionless number between 0-1 describing a solute's permeability across a membrane:

  • σ = 1.0:

    • Membrane impermeable to solute.

    • Solute remains in the original solution; maximal effective osmotic pressure.

    • Example: serum albumin & intracellular proteins.

  • σ = 0:

    • Membrane fully permeable.

    • Solute diffuses down the concentration gradient until equilibrium; no effective osmotic pressure.

    • Example: urea.

  • σ between 0 & 1:

    • Solutes exhibit partial permeability.

    • Calculated effective osmotic pressure is less than maximal possible but greater than 0.

SOLUTION TYPES

• Isotonic: Two solutions across a semi-permeable membrane with the same effective pressure.

• Hypotonic: Solution with lower effective pressure.

• Hypertonic: Solution with higher effective pressure.

SHIFTS OF WATER BETWEEN BODY FLUID COMPARTMENTS

• Disturbances altering solute & water balance classified into six categories:

  1. Isosmotic Volume Contraction.

  2. Hyperosmotic Volume Contraction.

  3. Hypoosmotic Volume Contraction.

  4. Isosmotic Volume Expansion.

  5. Hyperosmotic Volume Expansion.

  6. Hypoosmotic Volume Expansion.

EXAMPLES OF VOLUME CONSTRAINTS

1. Isosmotic Volume Contraction

• Cause: Loss of isosmotic fluid, such as in diarrhea.

• Changes in ECF Volume and Concentration:

  • ECF Volume: Decreases.

  • Osmolarity: Remains unchanged.

• Fluid Shift: No shift between compartments.

• Changes in ICF Volume and Concentration:

  • ICF Volume: Unchanged.

  • Osmolarity: Unchanged.

2. Hyperosmotic Volume Contraction

• Cause: Loss of hypoosmotic fluid from ECF, like in sweating.

• Changes in ECF Volume and Concentration:

  • ECF Volume: Decreases.

  • Osmolarity: Increases.

• Fluid Shift: Fluid shifts from ICF to ECF.

• Changes in ICF Volume and Concentration:

  • ICF Volume: Decreases.

  • Osmolarity: Increases.

3. Hyposmotic Volume Contraction

• Cause: Adrenal insufficiency (e.g., aldosterone deficiency), leading to excess sodium loss.

• Changes in ECF Volume and Concentration:

  • ECF Volume: Decreases.

  • Osmolarity: Decreases.

• Fluid Shift: Fluid shifts from ECF to ICF.

• Changes in ICF Volume and Concentration:

  • ICF Volume: Increases.

  • Osmolarity: Decreases.

4. Isosmotic Volume Expansion

• Cause: Infusion of isotonic NaCl.

• Changes in ECF Volume and Concentration:

  • ECF Volume: Increases.

  • Osmolarity: Remains unchanged.

• Fluid Shift: No shift between compartments.

• Changes in ICF Volume and Concentration:

  • ICF Volume: Unchanged.

  • Osmolarity: Unchanged.

5. Hyperosmotic Volume Expansion

• Cause: High NaCl intake.

• Changes in ECF Volume and Concentration:

  • ECF Volume: Increases.

  • Osmolarity: Increases.

• Fluid Shift: Fluid shifts from ICF to ECF.

• Changes in ICF Volume and Concentration:

  • ICF Volume: Decreases.

  • Osmolarity: Increases.

6. Hyposmotic Volume Expansion

• Cause: Syndrome of inappropriate antidiuretic hormone (SIADH), causing excessive water reabsorption.

• Changes in ECF Volume and Concentration:

  • ECF Volume: Increases.

  • Osmolarity: Decreases.

• Fluid Shift: Fluid shifts from ECF to ICF.

• Changes in ICF Volume and Concentration:

  • ICF Volume: Increases.

  • Osmolarity: Decreases.

NORMAL STATE VOLUMES

• Summary of conditions:

  • ICF & ECF Volume Contraction: Sample conditions include diarrhea, water deprivation, adrenal insufficiency.

  • Volume Expansion: Conditions include isotonic NaCl infusion, high NaCl intake, SIADH.