Expansion
Total body water 60 per cent, two-thirds intracellular, one-third extracellular
Expansion
For a 70 kg man, using the 60-40-20 rule:
- Total body water: 60 per cent of body weight, about 42 litres
- Intracellular fluid: 40 per cent of weight, about 28 litres (two-thirds of total)
- Extracellular fluid: 20 per cent of weight, about 14 litres (one-third), subdivided into:
- Interstitial: about 10.5 litres (three-quarters of extracellular)
- Plasma: about 3.5 litres (one-quarter)
Total body water is lower in women (about 50 per cent) and the obese, since fat contains little water, and higher in neonates (about 75 per cent).
Distribution of infused fluid follows these proportions, which is the practical point:
- 5 per cent dextrose distributes through total body water, so only about one-twelfth stays in plasma
- 0.9 per cent saline stays in the extracellular compartment, so about a quarter stays in plasma
- Colloid stays largely in plasma initially
That is why a litre of dextrose is nearly useless for resuscitating a shocked patient while a litre of saline is far more effective.