Osmometry Flashcards
What is osmometry and osmotic pressure?
A technique for measuring the concentration of dissolved solute particles that contribute to the osmotic pressure of a solution.
When there is a difference of osmotically active molecules across a membrane that cannot cross, those that can cross move to establish osmotic equilibrium – exerting osmotic pressure. This can be quantified using a manometer. At equilibrium the gravitational pressure of the column of solution in the manometer equals the osmotic pressure and prevents further movement of water. The osmotic pressure is defined as the pressure that would have to be exerted on the high solute side to prevent the flow of water.
Define osmolality.
The concentration per mass of solvent
1 osmolal solution = 1 mmol/kg H2O
Osmolality is used in biological systems which contain a mixture of different substances as comparisons can be made between solutions of different strength, irrespective of the molecular weights of the substances dissolved in them.
The “melon solution” will have a much higher osmolality than the “grape solution” as there will be less water present.
What are colligative properties?
Properties of solutions that depend on the number of molecules in a given volume of solvent rather than the properties of the molecules themselves.
Changes to colligative properties with increased osmolality:
- Osmotic pressure – increased
- Boiling point – increased
- Vapour pressure – decreased
- Freezing point - decreased
Describe how freeing point depression measures osmometry.
- The thermistor’s resistance changes with temperature.
- The sample and thermistor are lowered into a cooling bath.
- The sample is cooled to 7˚C below it’s freezing point.
- Crystallisations is initiated by rapid vibration or solenoid knocking.
- Slush forms around the thermistor, releasing latent heat of fusion and raising the temperature.
- Plateau in temperature = freezing point - when latent heat of fusion and abstraction of heat from the cooling bath are in equilibrium.
- Measurement taken at this point.
- Freezing point is inversely proportional to osmolality.
(Supercool sample to below its freezing point, then apply the vibration, you get a heat of fusion. This latent heat of fusion heats the sample back up. As they equilibrate the temperature plateaus and then eventually the sample does freeze and at this point the machine reads the temperature which is inversely proportional to the osmolality of the sample)
This is a manual method.
How does vapour pressure osmometry work?
Is related to the dew point, rather than vapour pressure. Where the dew point = temp at which condensation occurs. The dew point decreases with decreased vapour pressure.
It is measured using a thermocouple sitting inside a small sealed chamber.
- 8µl sample on filter paper is inserted into the chamber
- Transient heating speeds vapour equilibration
- Junction comes to thermal equilibrium with ambient temperature
- The junction is cooled to below the dew point of the atmosphere
- When cooling is stopped, condensation begins to form warming the thermacouple
- Condensation raises temperature until no more condensation forms = dew point
- Instrument measures due point temperature
LOW OSMOTIC CONCENTRATION, HIGH RELATIVE HUMIDITY, LESS NEGATIVE VAPOUR PRESSURE, MORE CONDENSATION, HIGH Td
HIGH OSMOTIC CONCENTRATION, LOW RELATIVE HUMIDITY, MORE NEGATIVE VAPOUR PRESSURE, LESS CONDENSATION, LOW Td
How does colloid osmotic pressure measure osmometry?
Used in research not clinically.
- The measuring cell consists of 2 chambers separated by a semi-permeable membrane.
- Sample is placed on one side of the membrane and colloid free physiological saline solution is placed on the other reference side.
- Solvent will move via osmosis from the saline side.
- The change in the compartment volumes are directly related to the osmolality in the sample.
- Though it is hard to synthesis a membrane that is selective enough, so is only good for looking at larger molecules such as proteins, so not in routine use in Clinical Labs.
What are the advantages and disadvantages of each type of osmometry measurement?
FPO - advantages:
- Performs rapid andinexpensive measurements
- Simple and reliable performance
- Small sample size (nL to µL range)
- Ideal for dilute biological and aqueous solutions
Disadvantages:
- Samples must be of low viscosity
- Not ideally suited for high molality or colloidal solutions
VPO advantages:
- Performs rapid and inexpensive measurements
- Small sample size (nL to µL range)
- Ideal for dilute biological and aqueous solutions
Disadvantages:
- Less accurate than FPO
- Can’t be used for volatile solutes like alcohols or other organic solvents
- Not ideal for high molality or colloidal solutions
COPO advantages:
- Provides potentially unlimited direct measurement of osmotic pressure and solution osmolality
- Good for colloidal solutions
- No limitation on sample concentration
- Can determine MW of macromolecules
Disadvantages:
- Time consuming and difficult to operate
- Requires large sample volume
- Not applicable for small molecules and aggressive solvents due to membrane porosity and compatibility
- Irreproducible results due to clogging of membrane pores
How is osmometry useful clinically?
Osmolality cant diagnose, but can point you in the right direction as it should be constant.
Useful in:
- Investigation of electrolyte disorders (hyponatraemia and hypernatraemia)
- Urine concentrating ability (in a patient with polyuria or oliguria)
- Screening for toxin ingestion (e.g. Salicylate, Ethanol, Methanol, Ethylene Glycol)
Hydration levels of athletes
To assess whether the athlete needs to drink more water or should not exercise
How does the ADH pathway work?
Osmolarity - moles per litre.
Your body osmolaRity is kept constant by proteins, electrolytes, glucose etc in the blood. If that increases you get thirsty as the osmo receptors in the hypothalamus have stimulated your thirst centre causing you to drink something. Stimulation of the osmo receptors signals to the anterior pituitary gland to release ADH, working to upregulate the aquaporins in the distaltubules to increase the permeability and cause more water to be taken up and not excreted in the urine. This reservation causes concentrated urine. Where as if you have drank lots you will have low osmolality and no ADH production, thus dilute urine.
Cranial diabetes insipidus - brain doesn’t produce ADH. So if you give synthetic ADH (AVH) - the axis should work and the kidneys can concentrate. Alternatively nephrotic diabetes insipidus is where you produce ADH but it doesn’t work on the receptor so addition of AH wouldn’t change results.
Usually perform a parked urine and serum osmo, what are the reference ranges for osmolality?
Serum Osmolality : 275-295 mmol/kg
Urine Osmolality: No reference range
(Completely dependant on osmolality in your serum. High serum osmolality, means high urine osmolality, as you are conserving/reabsorbing water leading to concentrated urine. Usually the cut off is around 50 and 750, but the elderly can’t concentrate as much in the kidneys so it is more likely to be 600).
Osmolality can be calculated with the osmolar gap. Several different formulae are used:
1.86(Na + K) + Urea + Glucose (most accurate)
1.86Na + Urea + Glucose + 9
2(Na+K) + Urea + Glucose
2Na + Urea + Glucose
Osmolar Gap =(measured osmolality-calculated osmolality) Abnormal gap (>10) indicates presence of unmeasured osmotically active substances – often alcohols
