Osmometry, Osmolality and Copeptin Flashcards
Define osmosis
The process by which solvent moves from an area of high solute concentration, through a semi-permeable membrane, to an area of low solute concentration
Most cell membranes in the body are freely permeable to water. What is the exception to this rule?
The cells lining the collecting ducts of the kidney - their membranes only become permeable to water in the presence of ADH
What is the difference between osmolarity and osmolality? What do osmometers measure? What do osmotic calculations provide?
Osmolarity = osmoles per litre of solution
Osmolality = osmoles per kg of water
Osmometers measure osmolality
Osmotic calculations provide osmolarity
What is a colligative property?
A property of a solution based on the concentration of solute particles of the solution
List the different types of colligative properties
Osmotic pressure, vapour pressure, raising boiling point, decreasing freezing point
What is the reason for the lack of good agreement between calculated and measured osmolality?
The following (incorrect) assumptions are made
1. All important osmotically active species are accounted for
2. All potential dissociations are complete
3. The anions associated with Na+ and K+ are free to contribute to osmolality and are not part of a macromolecule eg protein
4. The activity of each species is the same as concentration ie the ions exhibit ideal behaviour
5. The millimolal concentration of each ion is the same a its millimolar concentration (not true as plasma is only 93% water)
Why is the agreement between measured and calculated osmolality reasonable for serum/plasma but NOT urine?
Calculated osmolality is based on 3 measurements, and the error/imprecision of each one is additive. The combined error of 3 measurements is greater than the error of measured osmolality. The imprecision of urine measurements is greater than that of serum/plasma measurements and therefore the accumulated error when three urine measurements are performed is too great to provide a reasonably accurate estimate of urine osmolality.
Components of an osmometer
- Thermostatically controlled freezing chamber/Peltier device
- Agitator (seeds the freezing of the sample)
- Thermistor probe
- Galvanometer
- Potentiometer
What is a thermistor?
A resistor whose resistance varies rapidly and predictably with temperature. Becomes a better conductor as temp rises.
What is the role of the galvanometer?
To display the freezing curve. A guide when the measuring potentiometer is used.
Process of osmometry?
- Thermistor probe inserted into sample
- Sample with probe lowered into cooling block and supercooled to -7C
- Piston agitates tube to initiate freezing
- Freezing occurs to slush stage
- Expected temperature changes:
a) Initial increase due to release of heat of fusion
b) Initial heat release prevents further freezing - temperature plateaus
c) As cooling in the cooling block continues, the temperature will then decrease as the solution freezes - Temperature at equilibrium is the freezing point of the solution, and osmolality calculated from this temperature
How does the addition of solute lower freezing temperature?
Addition of solute particles disrupts the formation of intermolecular bonds between water molecules
How much is freezing point temperature decreased by for each mole of solute in a kg of water?
1.86 deg C
Normal osm and freezing point of blood?
285 mosm/kg, -0.53deg C
What is a Peltier device?
A solid-state heat pump.
