The compartments that matter
Total body water is roughly 50–60% of body weight in many adults but varies with age, sex and adiposity. About two-thirds is intracellular and one-third extracellular; only a minority of ECF is intravascular. This is why a litre of crystalloid does not remain as a litre of plasma volume.
Tonicity at the bedside
- Sodium is the principal extracellular cation and the main determinant of extracellular tonicity.
- Water shifts across cell membranes according to effective osmoles; rapid changes in tonicity can injure the brain.
- Hyperglycaemia raises measured osmolality and lowers measured sodium by translocational water shift; interpret sodium in context.
Assess volume, not just BP
- Look for capillary refill, pulse pressure, peripheral temperature, JVP, lung crackles, oedema, mucosa, postural symptoms and mental state.
- Trend urine output, creatinine, sodium/chloride, lactate when relevant, weight and cumulative balance.
- A normal BP does not exclude hypovolaemia; oedema does not prove adequate effective circulating volume.