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Body fluid compartments

Created: 5/4/2004
Updated: 28/4/2012

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

Dilution principle

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

Plasma (mmol/L) ICF (mmol/L)
Na+ 143 9
K+ 5 135
Ca2+ 1.3 <0.8
Mg2+ 0.9 25
Cl- 103 9
HCO3- 24 9
HPO42- 0.4 74
Sulphate- 0.4 19
Proteinate- 1.14 64

Body fluid composition in neonates

ECF exceeds 30% and ICF is <40%. These differences are greatest in premature babies, when ECF exceeds ICF.

 Body Fluids

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


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