Osmolarity is the same as osmotic concentration. This is the measurement of solute concentration of a solution. It is defined as the number of osmoles of solute particles in one litre of solution. Molarity is defined as the number of moles of solutes in a unit volume of a solution.
In osmalolity, osmoles means, number of solute particles. For example, in a 1M sodium chloride solution, there is 1 moles of sodium chloride in 1 L. But when consider the osmolarity, there are 2 osmoles. This is because when sodium chloride is dissolved in a solution, sodium and chloride particles are considered as 2 separate solute particles, thus 2 osmoles.
So for ionic compounds, the molarity and osmolarity will be different. Using our coffee analogy, this would be grams of sugar dissolved in kilograms of coffee. This is closer to what we do in the US, unless you know of a Starbucks that asks:. Also, I drink a lot of coffee.
Probably more than you. I said a few paragraphs earlier that solutes are normally measured by weight. Is an osmole a measurement of weight? Not exactly. An osmole is the number of moles that a solute " contributes to the osmotic pressure of a solution. Remember from your early chemistry and biology classes that osmosis is the diffusion of water.
Water travels from a low solute concentration to a high solute concentration in order to establish equilibrium. Osmotic pressure is what drives osmosis. Also remember that a mol is an amount that is specific to each substance. Mols are useful in organizing and predicting chemical reactions, so you will often see them used in chemistry and biochemistry. That 1 mol of NaCl corresponds to 2 osmoles of NaCl. Because Na and Cl ionize completely. And each individual ion contributes to the overall osmotic pressure of the solution.
Similarly, a mol of CaCl2 divided in 1 liter of water will have an osmolarity of 3 osmoles because you have two Cl ions and one Ca ion that dissociate in the water. An easy way to figure this out is to look at the number of ions in a molecule. For most substances, that will give you the correct number of osmoles. This is the point where you will go into hundreds of more complicated examples in your pharmacy calculations class.
But now that you hopefully understand the concept, I'm going to shift gears and talk about how we actually use osmolarity and osmolality in real live patients.
The difference is subtle between osmolarity and osmolality. I mean there is only one letter different in the words. Osmolality is measuring the number of osmoles in a weight kg of solvent. Osmolarity is measure the number of osmoles in a volume L of solvent.
Do we use both in medicine, or do we prefer one over the other? The answer is "both," sort of. It's a little of Column A and a little of Column B. We actually measure the osmolality of a patient using an osmometer. We then use the osmolality to calculate the osmolarity.
We don't measure the osmolarity of our patients directly. So if you see something hinting to that on a test, it's wrong. In medicine, we tend to use osmolarity instead of osmolality.
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.
This produces the distinction between "effective" and "ineffective" osmoles. Ineffective osmoles are those that are able to cross the semipermeable membrane and equilibrate in both solutions which are being compared, which means they cannot contribute to the osmotic pressure gradient.
Usually, when the discussion of tonicity comes up in textbooks, urea and glucose are offered as examples of such "ineffective" osmoles, as they are supposed to equilibrate effortlessly across body fluid compartments. This is partially true, in the sense that given enough time they will achieve an equilibrium.
Time is the key factor here. Rapid changes in either of these molecules will give rise to major osmotic shifts, producing cerebral oedema in HHS , as one example. The bottom line is that you can have solutions separated by a membrane that have equal osmolality on either side of the membrane, and there will be no osmotic pressure across that membrane, which makes these solutions isotonic, i.
However, it is also possible to have iso-osmolar solutions which are not isotonic. Thus, the infused dextrose is iso-osmolar but hypotonic. They ask for this in the syllabus document, but not in the exam, making you wonder how much you really need to know about it.
In short, from looking at the van 't Hoff equation, one immediately realises that it would not work properly as soon as the membrane separating the two solutions becomes even remotely imperfect, i.
On closer inspection, every membrane in biology is imperfect, which means that the van 't Hoff equation will sometimes, massively overestimate the osmotic pressure gradient between fluid compartments.
The reflection coefficient is an empirically derived value which is a ratio of the measured and predicted osmotic pressures, and which is different for every pair of solute and membrane.
The usual equation to describe it is:. In this fashion, the reflection coefficient is a parameter that describes the departure of a membrane from perfect semipermeability. A reflection coefficient of 1. What, might one ask, is the point of knowing this? The reflection coefficient value is useful for a number of practical and experimental purposes in medicine and physiology.
In this fashion, the reflection coefficient factors into the calculation of the glomerular filtration rate or the fluid exchange in the microcirculation as it slots into the Starling equation.
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