The body contains around 0.6L/kg of water (~40L for an adult).
It is a lower proportion of body weight in woman (~0.5L/kg) obesity, and the elderly.
The relative sizes of the compartments are determined prinicpally
2/3 (27L) is intracellular fluid, which is high in potassium, magnesium, and anionic protein.
- K 150mM, PO4 100mM, Na only 10mM, no calcium, ample protein
- Osmolality 290
- Size regulated by the movement of free water
1/3 (13L) is extracellular fluid
- Na 140, Cl 100
- HCO3 25, K 3-5, Ca 1.1, Mg 0.5
- Osmolality 290 determined by sodium, chloride, glucose, urea
- Protein variable
- Size regulated by total body sodium
Extracellular fluid is divided into:
- 10% (1L) transcellular fluid, which is fluid in epithelial lined cavities (urine, CSF, aqueous humor). It's composition is organ-specific and regulated by active transport
- 20% plasma volume (note \(40 \cdot \frac{1}{3} \cdot \frac{1}{5} = 2.\overline{6} \approx 0.5 \cdot 5\) i.e \((1-\text{HCT}) \cdot \text{circulating volume}\)
- The remaining 70% is interstitial fluid, the fluid between cells. Composition similar to plasma, but low protein content. Returns to circulation via lymphatics.