Generality
Osmolarity expresses the concentration of a solution, emphasizing the number of particles dissolved in it independently of its electric charge and size.
A liter of solution containing a mole of glucose will therefore have the same osmolarity as a liter of solution containing a mole of sodium (because a mole, by definition, contains a fixed number of particles - atoms, ions or molecules -, equal to 6, 02x1023). The osmolarity of the two will however be different from a liter of a third solution, containing a mole of kitchen salt; the latter (whose molecular formula is NaCl) in fact dissociates in Na + and Cl-, giving rise to a solution containing the double of particles.
| COMPARISON OF OSMOLARITY | ||
| A) One mole of glucose dissolved in one liter of solution | B) two moles of Sodium dissolved in one liter of solution | C) One mole of NaCl dissolved in one liter of solution |
| A is hyposmotic with respect to B | B is isosmotic with respect to C | C is isosmotic with respect to B |
| A is hyposmotic with respect to C | B is hyperosmotic with respect to A | C is hyperosmotic with respect to A |
Under normal conditions, the osmolarity is identical for all the fluids present in the various compartments of the organism and its value is around 300 mOsM (eventual gradients are canceled by movements of water). These compartments can be divided into intra- and extra-cellular, which contain, respectively, a quantity of water equal to 40% and 20% of body weight; the extracellular compartment is further subdivided into two compartments: the plasma one (1/3) and the interstitial one (2/3).
It is very important that the osmolarity of the various compartments is the same; in fact, if the concentration of solutes increases in the extracellular liquid, the water leaves the cell by osmosis (and shrivels), while in the opposite situation the cell draws water up to burst.
Note : although the number of osmoles per kg ( osmolality ) and not that per liter ( osmolarity ) determines the extent of osmosis, for very diluted solutions - such as body solutions - the quantitative differences between osmolarity and osmolality are below of 1% (because only a small part of their weight comes from the solute). This is why the two terms are often used interchangeably.
The main regulator of plasma osmolarity is the kidney, which produces more or less dilute urine according to the organism's homeostatic needs.
| Plasma osmolarity ≈ 290 mOsm / L * | |
| ELECTROLYTES | NOT ELECTROLYTES |
| Sodium 140 mmol / L | Azotemia 5 mmol / L |
| Potassium 4 mmol / L | Blood glucose 5 mmol / L |
| Chlorine 104 mmol / L | |
| Abstract Fork. 24 mmol / L | |
| Magnesium 1 mmol / L | |
| Calcium 2.5 mmol / L | |
In the extracellular water sector the most important osmole is sodium, while in the intracellular area potassium prevails.
* It must be said, however, that the effective plasma osmolarity (or tonicity) does not correspond to the total one. In fact, only the molecules that cannot freely pass through the semi-permeable membranes between them determine water movements from the most concentrated solution to the least concentrated one. On the contrary, there are others, such as urea, which, although contributing to the determination of osmolarity, are freely permeable (they cross the membranes) and as such fail to create gradients of water.
Urea therefore passes the cellular barrier without problems and is therefore unable to condition water movements on the two sides of the membrane.
To this end, hypothalamic osmoceptors - stimulated by hypersodemia - trigger thirst stimulation and the consequent introduction of water brings the plasma osmolarity back into balance. At the same time, the antidiuretic hormone (or ADH or vasopressin) is released, which acts on the kidney level increasing the reabsorption of water and decreasing, consequently, its elimination in the urine. These, on their part, increase their osmolarity (because they are more concentrated). The kidney has the ability to raise this parameter up to 1200 mOsM / L, or to decrease it up to 50 mOsM / L, depending on the different organic needs.
What's this
- Osmolarity is the measure of the number of particles dissolved in a fluid (volume expressed in liters).
- The osmolarity test reflects the concentration of substances such as sodium, potassium, chlorine, glucose and urea in a blood sample, urine or sometimes feces.
- Plasma osmolarity is used to assess the balance between water and dissolved particles in the blood, and to determine the presence of substances that can cause an imbalance in this state.
Why do you measure
Plasma osmolarity is used to evaluate the organism's water-saline balance and to identify the origin of a significantly increased or decreased urine production. The test is also used to determine the states of hyponatremia (low sodium concentrations), due to depletion through the urine or increase in blood fluids.
Plasma osmolarity is useful as a support in determining the cause of chronic diarrhea and allows monitoring of treatment with osmotically active drugs (as in the case of mannitol, a diuretic used for the therapeutic management of cerebral edema).
Furthermore, the investigation can be used as a toxicological examination, if the ingestion of methanol, glycolethylene, isopropyl alcohol, acetone and drugs, such as acetylsalicylic acid (aspirin), is likely in large quantities.
Normal values
The normal osmolarity values are between 275 and 295 mOsm / L.
Note : the reference interval of the exam can change according to age, sex and instrumentation used in the analysis laboratory. For this reason, it is preferable to consult the ranges listed directly on the report. It should also be remembered that the results of the analyzes must be assessed as a whole by the general practitioner who knows the patient's medical history.
High Osmolarity - Causes
Values of osmolarity higher than the norm could depend on the following conditions or pathologies.
- Hyperglycemia;
- Uremia;
- hypernatremia;
- Diabetes insipid;
- Hyperlactacidemia (lactic acidosis).
Increased values can also be found in the event of:
- Diabetes mellitus;
- Mannitol therapy
- Diabetic ketoacidosis;
- Alcoholic ketoacidosis;
- Kidney failure;
- Dehydration;
- Liver disease;
- Trauma;
- Shock;
- Intoxication from ethanol, glycolethylene, isopropyl alcohol and methanol.
Low Osmolarity - Causes
A decrease in osmolarity can result from:
- Hyponatremia;
- Inappropriate secretion of ADH
How to measure it
Plasma osmolarity is measured following a blood sample taken from a vein in the arm. This parameter can also be determined on a random urine sample or, in some cases, on fresh liquid stools (chilled or frozen within 30 minutes of collection).
Preparation
Sometimes, the examination of plasma osmolarity does not require any preparation; in other cases, it is necessary to observe a fast (no food or drinks except water) for at least 6 hours before taking the test. The doctor will be able to provide the most appropriate instructions for the case.
Interpretation of Results
Plasmatic osmolarity is a dynamic parameter, which fluctuates depending on how the organism responds to the temporary salt-water imbalance and how it corrects it. The test result must be evaluated together with the patient's clinical picture and the results of other tests, such as sodium, glucose and azotemia.
Osmolarity is not diagnostic: it suggests that the patient has an imbalance, but does not highlight the cause. In general, when the value is high, it means that the water has decreased in the blood and / or the solutes have increased. If the osmolarity is reduced, on the other hand, the increase in fluids is likely.
Among the various diseases that may be responsible for an increase in plasma osmolarity, uremia, hyperglycemia, diabetes insipidus, hyperlactacidemia and hypernatremia are more commonly found.
A decrease in osmolarity can derive, instead, above all from the presence in the patient of a state of hyponatremia.
