Osmolality and tonicity

The total solute concentration (tonicity) of body fluids is maintained virtually constant. Body fluid osmolality is defined by the ratio of total body solute to total body water. It is regulated at 280-95 mOsm/l. Osmolality is a colligative property, depending on the number of dissolved solute particles, and not on size or structure of the particles. The maintenance of osmolality is achieved primarily through the regulation of water balance.

Figure 2.1 Body fluid compartments.

Body fluid compartments

In a 75 kg man Total body water = 55%-65% body weight = 45 litres Extracellular fluid = 15 litres; intravascular fluid = 5 litres Interstitial fluid = 10 litres Intracellular fluid = 30 litres Transcellular compartment = 1 litre

Osmolality (in mOsm/l) = n x molar concentration (mmol/l of water) Plasma osmolality (in mOsm/kg) = 2(Na+) + 2(K+) + (glucose) + urea (all in mmol/l)

Osmolar gap = measured plasma osmolality — calculated plasma osmolality The osmolar gap is a measure of unmeasured osmotically active molecules, such as mannitol and alcohols.

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