5.8 Osmolarity Flashcards
What is osmosis?
The process of net movement of water molecules due to diffusion between
areas of different concentration separated by a semipermeable membrane.
What is osmotic pressure?
The pressure exerted within a sealed system
of solution in response to the presence of
osmotically active particles
on one side of a semipermeable membrane (kPa).
Define osmolarity
Osmolarity is the
number of osmoles of solute
per litre of solution
- Expressed as osm/L and is influenced by temperature
Volume of solution changes with the
amount of solute added and the
changes in temperature and pressure,
making it difficult to determine.
Define osmolality
- Osmolality is the number of
osmoles per kilogram of solution
and is not dependent on temperature
- Expressed in osm/kg
Amount of the solvent would not change
with temperature and pressure changes,
and for this property, osmolality is easier to evaluate.
What is an osmole?
An osmole is a
unit of measurement that describes the
number of moles of a compound that
contribute to the osmotic pressure of a chemical solution
[number of particles equal to Avogadro’s number (6 × 10²³)]
and that would depress the freezing point of the solvent by 1.86 Kelvin.
What is used clinically: osmolarity or osmolality and why?
The two terms refer to similar concepts;
however, osmolality is the preferred term.
When temperature changes,
volumes will change but mass remains the same.
Under most physiological conditions,
temperature is fairly constant and the
two are functionally very similar
but technically different.
osmolality is used in clinical practice to remove a source of error.
What is the formula for calculation of estimated osmolarity?
Formula for calculated osmolarity = 2 Na + 2 K + Urea + Glucose
Sodium and potassium: `
Sodium and potassium along with chloride are the
most abundant ions in the body and are hence used in the equation.
As chloride normally tags along sodium and potassium,
they are multiplied by a factor of two to account for the chloride in the body fluids.
Urea: Urea is added for a historical reason as it does decrease the freezing
point, although it is not osmotically active.
Glucose: This makes a big difference to osmolarity especially in diabetics.
Proteins are NoT used in the calculation as they are not ionic although
osmotically active.
How is it measured in the lab?
Osmometers utilise the colligative properties
of the osmotically active substances.
The most commonly used osmometers work on
the freezing point depression technique
as they are quick, easy,
and measure the volatile
alcohol along with the other solutes.
What are colligative properties?
The properties of a solution that vary according
to the osmolarity of the solution.
- Depression of the freezing point:
Depressed by 1.86 K per osmole of
solute per kilogram of solvent
- Depression of the freezing point:
- Reduction of vapour pressure (Raoult’s Law) by 0.3 mmHg
The depression of freezing point or reduction of the vapour pressure of a
solvent is proportional to the molar concentration of the solute.
- Elevation of the boiling point by 0.52°C
- Increase in osmotic pressure by 17 000 mmHg
Why is there a difference between the estimated and the calculated osmolarity?
Osmolar gap is the difference between measured and calculated osmolarity,
which is a measure of an osmotically active particle
that is not normally found in the plasma (e.g. ethanol).
What are some conditions that affect osmolarity?
- SIADH:
defined by the nonosmotic release of ADH
with consequent water
retention and hypotonicity
(ADH: osmoreceptors in the supraoptic nuclei of the hypothalamus has
a threshold of 289+/–2.3 mosmol/kg. Above this plasma level, ADH is released)
- Diabetes insipidus:
Neurogenic
(deficiency of ADH synthesis or impaired release),
nephrogenic
(renal resistance of action of ADH);
massive diuresis and hypovolaemia
- TUR syndrome:
excessive absorption of irrigating fluid - Water intoxication:
Self-inflicted, or excess glucose solution - Hyperosmolar states:
Hyperglycaemic nonketotic hyperosmolar coma
What is oncotic pressure
(colloid osmotic pressure)?
The osmotic pressure exerted by plasma proteins.
Measured with an oncometer,
comprising a semipermeable membrane,
which separates the plasma sample from a saline reference solution.
Thus, change of oncotic pressure can be transduced and measured.
What is meant by tonicity?
Osmolality measures the total number of solute particles within a solution,
but tonicity is only influenced by those solute particles that are not able to
cross the membrane separating two solutions.
Urea and glucose freely permeate and therefore are not included in the
calculation of tonicity.
