Body Fluid Compartments and Osmosis
Core facts on fluid compartment volumes, osmolarity, tonicity and predicting water shifts.
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Questions Covered in This Set
11 cards to master
What is the 60-40-20 rule?
60% of body weight is total body water, 40% is intracellular fluid, 20% is extracellular fluid.
In a 70 kg adult, give TBW, ICF and ECF volumes.
TBW ≈ 42 L; ICF ≈ 28 L (2/3 of TBW); ECF ≈ 14 L (1/3 of TBW).
How does the ECF divide?
Roughly 3:1 into interstitial fluid (~10.5 L) and plasma (~3.5 L); blood volume ≈ 5 L with red cells.
What distinguishes the cell membrane from the capillary wall as a barrier?
Cell membrane: freely water-permeable but solute-selective (Na⁺/K⁺-ATPase keeps Na⁺ out, K⁺ in). Capillary wall: leaky to water and small solutes but retains plasma proteins (albumin).
What governs fluid distribution between plasma and interstitium?
Starling forces — hydrostatic and oncotic (protein) pressures — not NaCl.
Bedside formula for plasma osmolality (US units)
2 × [Na⁺] + glucose/18 + BUN/2.8 ≈ 290 mOsm/kg normally (285–295).
Osmolarity vs osmolality
Osmolarity = particles per litre of solution (mOsm/L); osmolality = per kg of water (mOsm/kg); nearly identical in dilute body fluids.
What is tonicity?
Effective osmolality — only solutes that cannot cross the cell membrane (e.g. Na⁺ salts, mannitol, glucose without insulin) count; urea and ethanol do not.
Why is [Na⁺] a report on body water rather than body sodium?
Na⁺ and its anions dominate plasma osmolality, so hyponatraemia usually means too much water, not too little salt.
Effect of hyperglycaemia on measured sodium
For every 100 mg/dL rise in glucose, [Na⁺] falls ~1.6–2.4 mmol/L — translocational hyponatraemia in a hypertonic patient.
Steps of the Darrow–Yannet method
1) Add disturbance to ECF only; 2) new total osmoles ÷ new total body water = new osmolarity; 3) back-calculate each compartment's volume from its osmole content.
