Body Fluid Compartments & Electrolytes — Study Notes

I. Overview of Body Composition

Component	% of Body Weight (Male)	% of Body Weight (Female)	Notes
Water	~60%	~50%	Lower in females due to ↑adipose tissue and ↓muscle mass
Solids	~40–50%	~50–60%	Proteins, lipids, carbohydrates, and minerals

• Example: Average adult male → ~40 L total body water (TBW)

• Solids = structural and metabolic components of the body.

II. Fluid Compartments

Compartment	% of TBW	Approx. Volume	Description
Intracellular Fluid (ICF)	⅔ of TBW	~25 L	Fluid inside cells (where the nucleus and organelles are)
Extracellular Fluid (ECF)	⅓ of TBW	~15 L	Fluid outside cells (includes plasma + interstitial fluid)

Subdivisions of ECF

Subdivision	% of ECF	Approx. Volume	Description
Plasma	~20%	~3 L	Fluid within the vascular system (blood vessels)
Interstitial Fluid	~80%	~12 L	Fluid between cells

III. Fluid Intake and Output

Fluid Intake (~2500 mL/day)

• Sources: Drinking water, food, and GI absorption

• Water first enters ECF from the GI tract before equilibrating with ICF.

Fluid Output (~2500 mL/day)

Route	Approx. Loss	Notes
Kidneys (urine)	~1500 mL	Major route
Skin (sweat/evaporation)	~600 mL	↑ with heat/exercise
Lungs (respiration)	~300 mL	Insensible loss
GI tract (feces)	~100 mL	↑ in diarrhea/vomiting

IV. Movement of Water Between Compartments

• Mechanism: Passive movement by osmosis (no ATP required).

• Direction: From low solute → high solute concentration.

• Membrane: Semi-permeable, allowing water movement but limiting solute passage.

 Osmosis Example:
More solute in one compartment → water shifts there until osmotic balance achieved.

V. The Lymphatic System and Fluid Balance

• Excess interstitial fluid → collected by lymphatic vessels → returned to venous circulation.

• Prevents edema and maintains normal plasma volume.

VI. Electrolyte Distribution

A. Major Electrolytes and Concentrations (mmol/L)

Electrolyte	ECF (Plasma/Interstitial)	ICF (Inside Cells)	Notes
Na⁺	145	12	Major extracellular cation
K⁺	4	155	Major intracellular cation
Ca²⁺	2.5	<0.5	Involved in muscle contraction & signaling
Cl⁻	115	4	Major extracellular anion
HCO₃⁻	24	10	Key in acid-base balance
PO₄³⁻ (Phosphate)	1–2	100	Major intracellular anion
Proteins (e.g., Albumin)	High in plasma	High in ICF	Create oncotic pressure and osmotic gradients

VII. Sodium–Potassium Pump (Na⁺/K⁺-ATPase)

• Function: Maintains ion gradients across cell membranes.

• Purpose:

  • Preserves resting membrane potential

  • Maintains osmotic equilibrium

  • Enables cell excitability (neurons, muscles)

VIII. Summary Table: Principal Ions

Compartment	Main Cation	Main Anion
ECF	Sodium (Na⁺)	Chloride (Cl⁻)
ICF	Potassium (K⁺)	Phosphate (PO₄³⁻) & Negatively charged proteins

IX. Clinical Importance of Electrolytes

• Hyperkalemia / Hypercalcemia: May cause cardiac arrhythmias → potentially fatal.

• Hypo/Hypernatremia: Affects neurological function and brain cell volume.

• Bicarbonate (HCO₃⁻): Crucial for acid–base homeostasis.

X. Key Concepts Summary

• Total Body Water (TBW): ~60% body weight (men), ~50% (women).

• Compartments: ICF (⅔ TBW), ECF (⅓ TBW).

• Plasma: 20% of ECF; Interstitial: 80% of ECF.

• Main Ions:

  • Na⁺ → main ECF cation

  • K⁺ → main ICF cation

  • Cl⁻ → main ECF anion

  • PO₄³⁻ & proteins → main ICF anions

• Water moves: Passively via osmosis (no ATP).

• Na⁺/K⁺ pump: Maintains ionic gradients using ATP.

• Albumin: Main plasma protein maintaining osmotic pressure.

• Homeostatic control: Monitors ECF, not ICF, to maintain balance.