The body fluid

Vivian Imbriotis | Jan. 6, 2026

The body contains around 0.6L/kg of fluid (~40L for an adult).

It is a lower proportion of body weight in woman (~0.5L/kg) and obesity.

2/3 is intracellular fluid, which is high in potassium, magnesium, and anionic protein.

1/3 is extracellular fluid, which is divided into

  1. 10% transcellular fluid, which is fluid in epithelial lined cavities (urine, CSF, aqueous humor)
  2. 20% plasma volume (note \(40 \cdot \frac{1}{3} \cdot \frac{1}{5} = 2.\overline{6} \approx 0.5 \cdot 5\) i.e \((1-\text{HCT}) \cdot \text{circulating volume}\)
  3. The remaining 70% is interstitial fluid, the fluid between cells.

Interstitial fluid and plasma are both high in sodium and chloride.