Body fluids components and it's function
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Body Fluids: Components and Functions
Overview
Water is the medium of the body's internal environment. In an average young adult male, water accounts for 60% of total body weight (50% in females, ~80% in newborns). The percentage varies inversely with body fat content.
The well-known 60-40-20 rule:
- 60% of body weight = Total Body Water (TBW)
- 40% of body weight = Intracellular Fluid (ICF)
- 20% of body weight = Extracellular Fluid (ECF)
Fluid Compartments
TBW is divided into two major compartments, with a small third:
1. Intracellular Fluid (ICF)
- Volume: ~28 L (~40% of body weight) in a 70-kg person
- Located inside the trillions of cells; collectively treated as one large compartment due to remarkably similar composition across cell types
- Principal cations: K⁺ (140 mOsm/L), Mg²⁺ (20 mOsm/L)
- Principal anions: Phosphate/HPO₄²⁻, proteins, sulfate
- Low Na⁺ (14 mOsm/L), very low Ca²⁺ (0.0001 mOsm/L)
Functions of ICF:
- Site of cellular metabolism (energy production, protein synthesis)
- Contains enzymes, organelles, genetic material
- Maintains electrical potential difference across cell membranes (essential for nerve and muscle function)
- Intracellular K⁺ is critical for resting membrane potential
2. Extracellular Fluid (ECF)
- Volume: ~14 L (~20% of body weight)
- Divided into two major sub-compartments:
| Sub-compartment | Volume | % of Body Weight |
|---|---|---|
| Interstitial fluid | ~11 L (3/4 of ECF) | 15% |
| Plasma | ~3 L (1/4 of ECF) | 5% |
- Principal cation: Na⁺ (142 mOsm/L in plasma)
- Principal anions: Cl⁻ (106 mOsm/L), HCO₃⁻ (24 mOsm/L)
- Plasma has slightly higher protein content than interstitial fluid (Gibbs-Donnan equilibrium)
Functions of ECF:
- Delivers O₂ and nutrients to cells
- Removes metabolic waste products (CO₂, urea, etc.)
- Maintains blood pressure and cardiovascular dynamics (via plasma volume)
- Plasma proteins maintain oncotic pressure and proper fluid distribution between blood and tissues
- Serves as the medium for hormone transport and immune defense
3. Transcellular Fluid (Third Space)
- Volume: ~1-2 L (quantitatively small)
- Includes: cerebrospinal fluid (CSF), synovial fluid, pleural fluid, peritoneal fluid, pericardial fluid, digestive secretions, aqueous humor
Functions of specific transcellular fluids:
- CSF: Cushions the brain/spinal cord, removes waste, maintains CNS ion environment
- Synovial fluid: Lubricates joints, nourishes articular cartilage
- Pleural/pericardial fluid: Reduces friction during respiratory and cardiac movement
- Peritoneal fluid: Lubricates abdominal organs
- Digestive secretions: Aid digestion and absorption
Blood Volume (Special Compartment)
- Blood is considered a separate fluid compartment (contained within the circulatory system)
- Volume: ~5 L (~7-8% of body weight)
- Composition: ~60% plasma + ~40% red blood cells (hematocrit ~0.42 in men, ~0.38 in women)
Electrolyte Composition Compared
| Electrolyte | Plasma | Interstitial | Intracellular |
|---|---|---|---|
| Na⁺ | 142 mOsm/L | 139 | 14 |
| K⁺ | 4.2 | 4.0 | 140 |
| Ca²⁺ | 1.3 | 1.2 | 0.0001 |
| Mg²⁺ | 0.8 | 0.7 | 20 |
| Cl⁻ | 106 | 108 | 4 |
| HCO₃⁻ | 24 | 28.3 | 10 |
| Phosphate | 2 | 2 | 11 |
| Proteins | high | low | high |
The concentration gradient between ICF and ECF is maintained by the Na⁺/K⁺-ATPase pump in cell membranes.
Key Functions of Body Fluids (Summary)
| Function | Description |
|---|---|
| Transport | Carries O₂, nutrients, hormones, and waste products |
| Thermoregulation | Water's high specific heat buffers body temperature; sweat dissipates heat |
| Solvent | Dissolves gases, electrolytes, and organic molecules for metabolic reactions |
| Acid-base balance | Bicarbonate buffering system in plasma and ICF |
| Electrolyte balance | Maintains osmolarity (~285-295 mOsm/L) and cell volume |
| Lubrication | Transcellular fluids reduce friction in joints, pleura, pericardium |
| Protection | CSF cushions the CNS; amniotic fluid protects the fetus |
| Volume/Pressure | Plasma volume determines venous return and cardiac output |
Sources:
- Guyton and Hall Textbook of Medical Physiology
- Ganong's Review of Medical Physiology, 26th Edition
- Costanzo Physiology, 7th Edition
- Schwartz's Principles of Surgery, 11th Edition
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