How does water move across membranes?
When two compartments are separated by a semi-permeable membrane, the water in those compartments will respond to the amount of dissolved solute present in each compartment. The greater the amount of solute, the greater the osmotic pressure. Water will move across such a membrane from a compartment of low osmotic pressure to the one with high osmotic pressure until the two compartments have equal pressure. An example of this in the body is the balance of water movement between the capillaries and the tissues.
What is the relationship between molarity, molality, osmolarity and osmolality?
Molarity is the moles of solute per liter of solution. Molality is moles of solute per kilogram of solution. Both of these are based on the molecular weight/mass of the solute. Osmolarity and osmolality are based on the total number of dissolved molecules and ions in the solution. Osmolarity is the number of osmoles (molarity X number of dissolved species present) per liter of solution. Osmolality is the number of osmoles per kilogram of solution. Since it is easier to obtain weight rather than volume in a person, osmolality is the measurement more commonly used clinically.
What are the colligative properties?
When solute is added to pure water, the colligative properties of the solution change: freezing point, boiling point, vapor pressure, and osmotic pressure.
- Freezing point decreases: Think of how salt is added on roadways when it snows—it is to make the temperature at which the snow slush freezes become lower. As long as the temperature stays above that, the slush should remain slush and not solid ice. The more solute in a solution, the lower the temperature at which that solution freezes. It is very easy to measure this freezing point in a clinical sample like serum or urine.
- Boiling point increases: Think of the effect of adding salt to a pot of water being heated—the temperature at which that water boils will be higher than 100°C of the water alone. It is very difficult to measure this boiling point in a clinical sample.
- Vapor pressure decreases: This is the equilibrium pressure of gas molecules from that liquid (from evaporation) above the heated liquid itself. It is difficult (though not impossible) to measure this vapor pressure in a clinical sample.
- Osmotic pressure increases: The greater the amount of solute dissolved in a solution, the greater the pressure that solution will exert against a semi-permeable membrane. We do not have instrumentation to measure this pressure in a clinical sample.
How is the freezing point of a patient sample determined?
- The sample is placed in a cooling chamber which keeps the sample very still while the temperature in the chamber lowers. This is called supercooling since the sample remains unfrozen even when its temperature gets below its freezing point. As long as there is no particulate matter in the sample or nothing suddenly disturbs the sample, a solution can stay liquid even below its actual freezing point.
- At a temperature around -1.86°C, the sample container is suddenly moved, and this instantly begins crystal formation in the sample.
- As the crystals form in the sample, there is heat generated called the heat of fusion. This warms the sample to its actual freezing point.
- When that freezing temperature becomes steady, it is used by the instrument’s circuitry in the calculation:
Sample FP in °C/-0.00186°C = sample mOsm/kg, so a sample whose freezing point is determined to be -0.521°C would have an osmolality of 280 mOsm/kg.
Is it possible to measure osmolality without using a freezing point osmometer?
It is possible to do an estimated osmolality by taking the routinely measured blood values for glucose, sodium, and blood urea nitrogen (BUN), using the following calculation:
2[Na] + glucose/18 + urea nitrogen/ 2.8
So if a patient has a sodium of 143 mmol/L, a glucose of 110 mg/dL and a BUN of 12 mg/dL, his estimated osmolality would be 2[143] + (110/18) + (12/2.8) = 296 mOsmol/kg.
Note that the “18” and “2.8” are the factors needed to convert glucose and BUN into mmol/L.
An osmolal gap would be the difference between the measured osmolality and the estimated osmolality. The primary cause of an increased osmolal gap is toxic alcohol ingestion, such as ethylene glycol (antifreeze), methanol, and ethanol. It can also be seen in some cases of ketoacidosis, renal failure, and shock.
How is osmolality used clinically?
A healthy person will have an osmolality between 275 and 295 mOsm/kg for serum and between 500 and 800 mOsm/kg for urine. Osmolality is used to assess electrolyte and acid-base disorders, status of water regulation by the kidneys, toxicity due to alcohols, and hydration in burn patients or comatose patients. Causes for increased serum osmolality include dehydration from sepsis, fever, sweating, burns, hyperglycemia, diabetes insipidus, uremia, hypernatremia, and/or ethanol/methanol/ethylene glycol ingestion. Causes for low serum osmolality are excess hydration, hyponatremia, and the syndrome of inappropriate antidiuretic hormone (ADH) secretion.
Causes for increased urine osmolality include dehydration, syndrome of inappropriate antidiuretic hormone secretion, hypernatremia, and a high protein diet. Causes for low urine osmolality include diabetes insidious, excessive fluid intake, and acute renal insufficiency.
Water makes up over 50% of the average human body. All chemical reactions in the body occur in this watery environment. Measuring the amount of dissolved solute in a body fluid like serum or urine can give an idea of how well the body is controlling water balance. One method used frequently in the clinical setting is osmometry, and the most common analytical principle is freezing point depression. Your challenge includes discovering the connection between freezing point and osmolality, how freezing point is measured, the components of an osmometer, and how osmolality values correlate with pathologies.)
