Body Fluid Compartments

Volume and Solute Content of Fluid Compartments:
- Understand the volume and solute content of the intracellular fluid (ICF) and extracellular fluid (ECF) compartments.
Principles of Measurement:
- Comprehend the principles used to measure body fluid compartments and how to calculate their different volumes.
Barriers Between Compartments:
- Understand the barriers that exist between different fluid compartments.
- Learn how differences in solute composition are maintained across these compartments.

Total Body Water (TBW):
- Approximately 60% of body weight (BW) is water in a non-obese human.
- Breakdown of Body Composition:
  - Proteins: ~18% BW
  - Minerals: ~7% BW
  - Fat: ~15% BW (Note: % water decreases as body fat increases).
- Water Weight in a Lean 70 kg Man:
- Total: ~42 L H2O (1L weighs 1Kg).
Fluid Distribution:
- Water is distributed in 2 compartments:
- Intracellular Fluid (ICF): ~66.67% TBH2O = ~28 L
- Extracellular Fluid (ECF): ~33.33% TBH2O = 14 L
  - Components of ECF:
  1. Plasma: 4-5% BW (~25% ECF = 3L)
  2. Interstitial Fluid (IF): 13.5% BW (~75% ECF = 10.5L)
  - Note: Transcellular fluids (e.g., synovial, cerebrospinal, intra-ocular fluids) are ignored here (~1% BW).
- Blood Volume Calculation:
- If hematocrit is 45%, then 55% of the blood is considered plasma.
- If plasma volume is 3L, then blood volume can be calculated as:
  Blood Volume = \frac{3L}{1.0 - 0.45} = 5.5L \approx 8\% BW

Major Electrolytes:
- Intracellular Fluid (ICF):
- Na⁺: 15 mEq/L
- K⁺: 120 mEq/L
- Cl^-: 20 mEq/L
- Ca²⁺: all bound
- Mg²⁺: 1 mEq/L
- Protein: 30 g/dl
- Extracellular Fluid (ECF):
- Na⁺: 145 mEq/L
- K⁺: 4.5 mEq/L
- Cl^-: 116 mEq/L
- Ca²⁺: 1.2 mEq/L
- Mg²⁺: 0.6 mEq/L
- Protein: 1 g/dl
- Osmolarity (Posm):
- Typical range: 285-310 mosm/Kg H2O
Fluid Movement:
- Water moves freely between all fluid compartments due to osmotic gradients.

Dilution Principle:
- Volume ( v ) can be calculated by dissolving a known quantity of substance ( X ) into an unknown volume of water ( v ) after achieving equilibrium:
  v = \frac{X}{C}
- In a living organism, adjustments must be made for any substance removed during equilibration:
  v = \frac{X - quantity \ of \ X \ excreted}{C}
Criteria for Measurement:
1. Substance X must be freely distributed throughout the entire compartment.
2. Substance must be non-toxic.
3. Substance must not be metabolized.
4. Substance must be easily measurable.

Example Procedure to Measure Volume:
1. Add 100g of blue dye to the fluid.
2. Mix until the concentration equilibrates.
3. Measure concentration of dye: 20g/litre.
4. Calculate volume:
- Volume = \frac{100g}{20g/l} = 5 \text{ litres}

Plasma Volume Measurement (Clinically Important):
- Use 131 Iodine-labeled albumin or Evans Blue dye which binds to plasma proteins and remains within plasma.
Total ECF Volume Measurement:
- Use inulin (a foreign sugar). It cannot enter cells but equilibrates throughout plasma and interstitial fluid.
Total Body H2O Measurement:
- Use tritiated H2O or heavy water ( D_2O ), which crosses cell membranes freely.
Calculating Intracellular Fluid Volume (ICFV):
- ICFV = TBH2O - ECFV

Scenario: Inject 100 ml of deuterium oxide (D2O) in a normal, lean man weighing 84 kg. After 2 hours, the concentration in plasma water is 0.2%. Average urinary, respiratory, and cutaneous losses of D2O are estimated at 4% of the total dose.
Calculation Steps:
1. Final D2O Concentration Calculation:
  - Total Amount Added = 100 ml
  - Amount Lost = 4 ml
  - Retained Amount = 100 ml - 4 ml = 96 ml
2. Final D2O Concentration Calculated:
  - Final D2O Concentration = \frac{0.2 ml D2O}{100 ml plasma H2O}
3. Volume of Distribution:
  - Volume = \frac{Amt. Added - Loss}{Final Conc.} = \frac{96 ml}{(0.2/100)} = 48000 ml = 48 Liters
4. Percentage of Body Weight:
  - \frac{48 L}{84 kg} \times 100 \approx 57.1\% BW

Plasma Membrane:
- The plasma membrane acts as a barrier between intracellular and extracellular fluids consisting of a complex lipid bilayer containing transporters, carriers, and pores, permitting both active and passive transport.
Capillary Wall:
- Serves as a barrier between plasma and interstitial fluid;
- Allows exchange via passive movement, determined by concentration gradients and physical pressures (hydrostatic and colloid osmotic pressures).

Hydrostatic Pressure (Pc):
- Higher at the arterial end of the capillary, drives water and solutes out into interstitial space (filtration).
Colloid Osmotic Pressure (πc):
- Increases as fluid leaves, due to concentration of impermeable plasma proteins, causing reabsorption when πc exceeds Pc.
Fluid Dynamics:
- Over a capillary length, filtration and reabsorption are generally equal, preventing accumulation of fluid in the interstitium.
- Excess fluid is drained into lymphatics to return to circulation.

Total body water (TBW) constitutes approximately 60% of body weight (BW) and is divided between intracellular fluid (ICF, ~40% BW) and extracellular fluid (ECF, ~20% BW).
ICF and ECF are separated by the cell membrane, which maintains concentration differences through various transport mechanisms.
The ECF is divided into plasma and interstitial fluid compartments, where fluid movement is influenced by Starling forces (hydrostatic and oncotic pressures) and solute movement occurs via convection and diffusion along concentration gradients.