Plasma Osmolality

Plasma Osmolality

Osmolarity:

osmotic pressure of a solution expressed in osmols or milliosmols per kilogram of solution

Osmolality:

osmotic pressure of a solution expressed in osmols or milliosmols per kilogram of water

Osmole:

· quantity of a substance in solution in the form of molecules &/or ions that has the same osmotic pressure as one mole of an ideal nonelectrolyte

eg 1 mole dextrose in 1L —> 1 osmole/L

eg 1 mole NaCl in 1L —> 2 osmol/L

eg 1 mole CaCl₂ —> 3 osmole/L

eg 1 osmole/L solution CaCl₂ —> ¹/₃ mole or ¹¹¹/₃ = 37 gm

Plasma Osmolality: (280 — 285 mOsm/Kg)

· As cell membranes in general are freely permeable to water, the osmolality of the ECF is approximately equal to that of the ICF

· Therefore, plasma osmolality is a guide to intracellular osmolality

· Plasma osmolality may be approximated by:

Plasma osmolality (mOsm/Kg) = 2([Na⁺] + [K⁺]) plasma + [BUN] + [Glucose]

· Plasma hypertonicity 2^o hypernatraemia indicates cellular dehydration

· Plasma hypotonicity 2^o hyponatremia indicates cellular swelling

· Hyperglycemia: ECF osmolality rises and exceeds ICF osmolality (since glucose only penetrates cells slowly) prompting a shift of water from ICF to ECF. Thus plasma Na⁺ concentration falls in proportion to the dilution of the ECF

· Azotemia: urea rapidly penetrates cells; since the ECF [urea] = ECF [urea] there is no net shift of water from the cells despite the increased plasma osmolality