Total body water (TBW) constitutes 55-60% of the body weight in men and 45-50% of the body weight in young women.
Relationship between the volumes of major fluid compartments
Measurements of body fluid compartments
A known amount of tracer is introduced into the space to be measured, and its concentration measured after mixing.
Con1 X Vol1 = Con2 X (Vol1 + Vol2)
Con1 = initial concentration of indicator, Con2 = final concentration of indicator
Vol1 = volume of indicator, Vol2 = volume to be measured.
Agents used for measurement of fluid compartments
[i] TBW is measured using deuterium oxide (heavy water).
[ii] ECF volume is measured using inulin, which is proportionally distributed between plasma volume and interstitial volume.
[iii] Plasma volume can be measured either by radioactive albumin or by Evans blue. These substances neither leave the vascular system nor penetrate the erythrocytes.
[iv] The interstitial fluid volume is determined as the difference between ECF volume and plasma volume.
[v] The ICF volume is obtained by subtracting the ECF volume from the TBW.
Composition of body fluid compartments
Body fluid composition in neonates
ECF exceeds 30% and ICF is <40%. These differences are greatest in premature babies, when ECF exceeds ICF.
Related examination questions
Concerning body fluid compartments:
a) Water constitutes 70% of the total body weight
b) Plasma constitutes a quarter of the ECF volume
c) Sucrose can be used to measure the ECF volume
d) Interstitial fluid volume for a 70 kg man is approximately 9 litres
e) The ECF/ICF volume ratio is smaller in infants and children than it is in adults