Fluids, Electrolytes, and Body Compartments – Study Notes

1. Membrane Transport Review

• Passive Transport (no ATP required)

  • Simple diffusion → no carrier, down gradient.

  • Osmosis → diffusion of water, no carrier.

  • Facilitated diffusion → needs a carrier protein.

• Active Transport (ATP required)

  • Primary active transport → direct ATP use (ex: Na⁺/K⁺ ATPase, Ca²⁺ ATPase, H⁺ ATPase in kidney, H⁺/K⁺ ATPase in stomach).

  • Secondary active transport → indirect ATP use; relies on gradients created by primary pumps.

    • Uniport, symport, antiport.

    • Example: Na⁺/Ca²⁺ exchanger (antiport).

2. Key Transport Mechanisms

• Na⁺/K⁺ ATPase: pumps Na⁺ out, K⁺ in.

• Na⁺/Ca²⁺ exchanger: secondary active, Ca²⁺ out when Na⁺ enters.

• Digoxin mechanism: inhibits Na⁺/K⁺ ATPase → Na⁺ accumulates inside cell → ↓ Na⁺ gradient → Ca²⁺ cannot leave via Na⁺/Ca²⁺ exchanger → intracellular Ca²⁺ ↑ → stronger cardiac contraction.

• Beta-blockers effect: can inhibit Na⁺/K⁺ ATPase → ↑ extracellular K⁺ → hyperkalemia.

3. Water Properties – Why Body Uses Water

• High specific heat (thermal capacity) → resists boiling/freezing, stabilizes body temp.

• Capillary action → movement in thin tubes; important for tears, lactation, fluid transport.

• Excellent solvent → electrolytes (Na⁺, K⁺, Cl⁻) dissolve and move with water.

• Redox reactions → water helps generate reactive oxygen species for microbial killing.

4. Body Water Distribution

• Total body water (TBW) ≈ 60% body weight in men, 50% in women, 75% in infants.

• Obese patients → ↓ TBW (more fat, less water).

• Compartments:

  • Intracellular fluid (ICF) = ~40% body weight (⅔ of TBW).

  • Extracellular fluid (ECF) = ~20% body weight (⅓ of TBW).

    • Interstitial = ~15% body weight.

    • Plasma = ~5% body weight.

• Example: 60 kg adult → TBW ~36 L.

  • ICF = 24 L.

  • ECF = 12 L (9 L interstitial, 3 L plasma).

5. Special Clinical Points

• Infants → more water (75%) → more prone to dehydration.

• Interstitial fluid as reservoir → replenishes plasma during blood loss.

• Fluid inside RBCs = intracellular.

• Obligated water → follows electrolytes (Na⁺, Cl⁻).

• Free water → independent of electrolytes, generated in loop of Henle, regulated by ADH.

6. Clinical Applications

• Dehydration in infants → dangerous due to higher % body water.

• Digoxin → ↑ cardiac contractility by raising intracellular Ca²⁺.

• Beta-blockers → risk of hyperkalemia.

• Acid–base: In acidosis, H⁺ enters cells, K⁺ leaves → hyperkalemia.

7. Quick Calculation Example

• Plasma volume = 5% of body weight.

• 60 kg adult → 3 L plasma.

• If 1 L NS is given IV → only ~250 mL enters plasma (¼ of infused volume remains intravascular).

Summary:

• TBW ≈ 60% (men), 50% (women), 75% (infants).

• Most water is intracellular.

• ECF is divided into interstitial (majority) and plasma.

• Transport across membranes occurs via passive, facilitated, primary active, or secondary active processes.

• Pharmacology tie-in: Digoxin ↑ contractility via Na⁺/K⁺ ATPase inhibition; beta-blockers risk hyperkalemia.

• Clinical tie-in: dehydration risks, fluid shifts, and plasma replacement concepts.