Actual textbooks will frequently lead you far astray. Caon identifies multiple other such errors in popular physiology resources. Again, IUPAC have a definition for this, and again it is perfect for industrial chemistry and disappointing for a physiology teacher:.
Osmolality: Quotient of the negative natural logarithm of the rational activity of water and the molar mass of water. To unpack this would require a terrifying digression into other definitions eg.
The main difference between this and the definition of osmolarity is that the definition uses the mass, rather than the volume, of the solvent. There are several reasons for this, of which the main is the dependence of solvent volume on things like temperature. Mass, in contrast, is dependably constant. You don't usually measure osmolarity, owing to the meaninglessness and inconsistency of the volume metric, but osmolality can definitely be measured by a variety of instruments.
For this, you would dust off the old automated osmometer. These devices usually measure the depression of the freezing point of the sample, which is another colligative property. For the casual browser, the central arguments of their excellent paper can be oversimplified as follows:. Incidentally, when one applies the common equation to human plasma in order to determine whether their patient has ingested a massive faceful of antifreeze or methylated spirits , the equation they use.
Thus, it is the osmolarity you are calculating here, not osmolality. But then, to calculate the gap, you send a sample to the lab for a measured value, which will be returned to you as osmolality, i. How can you subtract one from the other, and call the result an "osmolar" gap with a straight face?
It boggles the mind. The standard definition of tonicity usually incorporates some mention of osmotic pressure or osmolality difference between solutions. Caon further elaborates by calling it "a semi-quantitative descriptor of the concentration of one solution compared to another".
It is also occasionally called "effective osmolality" , which brings us to the next point: unlike osmolarity, tonicity is only influenced by solutes that cannot cross this semipermeable membrane , because these are the only solutes influencing the osmotic pressure gradient. Overview and Key Difference 2. What is Tonicity 3.
What is Osmolarity 4. Tonicity is a measure of the osmotic pressure gradient by means of water potential of two solutions separated by a semipermeable membrane. It means; the term tonicity describes the relative concentration of solutes I solution which determines the direction and extent of diffusion. This measurement is important in determining the response of cells that are immersed in an external solution.
Unlike the osmotic pressure, the tonicity is only influenced by solutes that cannot pass through the membrane. The solutes that can freely go through the membrane has no influence on the tonicity. It is because, the concentration of these solutes will always remain the same in both sides of the membrane. This is the situation with hyperosmolality due to high urea concentrations as urea crosses cell membranes relatively easily. Hyperglycaemia in untreated diabetics results in ECF which is both hyperosmolar and hypertonic as compared to the normal situation as glucose cannot easily enter cells in these circumstances.
Water moves out of the cells until the osmolar gradient is abolished. In some situations, a more operational definition of tonicity is used to explain the term: though not incorrect this explanation is less versatile and rigorous than the one discussed above.
This is based on the experiment of immersing red cells in various test solutions and observing the result. If the red cells swell and rupture, the test solution is said to be hypotonic compared to normal plasma. If the red cells shrink and become crenated, the solution is said to be hypertonic.
If the red cells stay the same size, the test solution is said to be isotonic with plasma. The red cell membrane is the reference membrane. Red cells placed in normal saline ie 0. Haemolysis does not occur until the saline solution is less then 0.
The point about this definition of tonicity is that it is qualitative and not quantitative. It does imply that permeant solutes will be ineffective because it is essentially a test against a real membrane. A major physiology text Ganong 16th ed. This less rigorous definition is wrong as it does not cover the full sense in which the term tonicity is used. This is really a problem with his definition. Glucose is a permeant solute in the non-diabetic and can easily enter cells.
Water does not leave the cells initially and haemolysis does not occur because there is no osmolar gradient across the cell membrane. The solution is however hypotonic and when the glucose enters cells water does also. If insulin is not present, this movement of glucose does not occur. In this latter case, the solution is isosmolar before infusion and can be considered isotonic after infusion as well. The definition really doesn't add much more then could be achieved by the terms hypo- and hyper-osmolality.
However, the way they express their effects is different from each other. Both terms relate to situations where solutions are separated by permeable membranes where osmosis takes place. Osmolarity takes into consideration all the molecules present in the solution for its collective effect whereas, tonicity only considers the concentration of the non-permeable solute molecules within the solution. Therefore, it can be said that tonicity is a measure of the external environment , whereas osmolarity concerns the given solution itself.
This is the main difference between osmolarity and tonicity. This is because, osmolarity involves the total amount of solutes present whereas the molarity involves the concentration of the compound as a whole. Depending on the characteristics of the surrounding environment, the osmolarity of a given solution can take three different forms. Isosmotic, hyperosmotic and hypoosmotic.
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