Definition
- Total body fluid splits into two compartments:
- Intracellular fluid (ICF) — about two-thirds of body fluid.
- Extracellular fluid (ECF) — plasma, interstitial fluid and transcellular fluid.
Plasma and Blood Volume
Distribution of ECF in a 70 kg adult male
Compartment
Plasma
- % of ECF
- 25
- % of body weight
- 4 to 5
- Volume
- 3.5 L
Interstitial and transcellular fluid
- % of ECF
- 10.5
- % of body weight
- 15
- Volume
- 10.5 L
- Plasma volume = Blood volume × (100 − Hematocrit)/100.
- Blood volume ≈ 80 mL/kg body weight (about 8% of body weight).
- Red cell volume is found two ways:
- By subtracting plasma volume from blood volume.
- Directly, by injecting red cells tagged with a radioactive isotope (51Cr, 59Fe or 32P) and measuring the tagged fraction after mixing.