Total Body Water
- Adult male — 60% of body weight (42 L in a 70 kg man)
- Adult female — 50% (more subcutaneous fat, which holds little water)
- Newborn — 75–80%
- Obese — lower; elderly — lower
Fluid Compartments
| Compartment | % body weight | Volume (70 kg) |
|---|
| Intracellular fluid (ICF) | 40% | 28 L |
| Extracellular fluid (ECF) | 20% | 14 L |
| 15% | 10.5 L |
| 5% | 3.5 L |
| 1–2% | 1–2 L |
| Blood volume | 8% | 5 L |
- Transcellular fluid — CSF, synovial, pleural, pericardial, peritoneal, intraocular, GI secretions
Ionic Composition
| Ion | ECF (mEq/L) | ICF (mEq/L) |
|---|
| Na+ | 142 (chief ECF cation) | 10 |
| K+ | 4 | 140 (chief ICF cation) |
| Cl− | 103 (chief ECF anion) | 4 |
| HCO3− | 24 | 10 |
| Ca2+ | 5 | < 1 (free) |
| Protein | 16 | 54 |
| Phosphate | 4 | 75 (chief ICF anion) |
- Osmolality of both compartments = 290–300 mOsm/kg — always equal, since water moves freely
- Plasma differs from interstitial fluid only in its higher protein content
Measurement — the Dilution Principle
Volume = Amount of substance injected / Concentration after equilibration(corrected for any amount excreted or metabolised)
Properties of an ideal marker
- Distributes only in the compartment being measured
- Non-toxic, easily measured
- Not metabolised or excreted rapidly
| Compartment | Substances used |
|---|
| Total body water | Deuterium oxide (D2O), tritiated water, antipyrine |
| ECF | Inulin, mannitol, sucrose, thiosulphate, radioactive Na+, Cl−, Br− |
| Plasma | Evans blue (T-1824), radio-iodinated serum albumin (RISA) |
| Blood volume | 51Cr-labelled RBC; or plasma volume ÷ (1 − haematocrit) |
- ICF cannot be measured directly — no marker enters all cells and stays there
- ICF = Total body water − ECF (measured by difference)
- Interstitial fluid = ECF − Plasma (also by difference)
Daily Water Balance
| Intake | ML/day | Output | ML/day |
|---|
| Drinking | 1400 | Urine | 1500 |
| In food | 800 | Insensible — skin | 400 |
| Metabolic water | 300 | Insensible — lungs | 400 |
| | Faeces | 200 |
| Total | 2500 | Total | 2500 |
- Obligatory urine volume = 500 mL/day — the minimum needed to excrete the daily solute load
Regulation of Body Fluid Volume
- Thirst — hypothalamic osmoreceptors
- ADH — water retention by the collecting duct
- Aldosterone — Na+ and hence water retention
- ANP — natriuresis when the atria are stretched
Applied Aspects
- Dehydration — isotonic (haemorrhage, diarrhoea), hypertonic (water loss > salt), hypotonic (salt loss > water)
- 3rd space loss — fluid sequestered in peritoneum or bowel lumen; lost to circulation though still inside the body
- IV fluids — isotonic saline stays in ECF; 5% dextrose distributes through all compartments once the glucose is metabolised