What is the difference between osmolarity and osmolality?

First of all, it is important to define osmosis. If you have two solutions separated only by a semi-permeable membrane, and one solution is more concentrated than the other, then water will move from the less concentrated solution to the more concentrated one in order to try and balance the concentration in both compartments.

The osmolarity of a given solution simply refers to the concentration of the solution (measured in Osm/L). A high osmolarity indicates a highly concentrated solution and the converse applies.

So, if two solutions were separated only by a semi-permeable membrane, one had an osmolarity of 300 Osm/L and the other 270 Osm/L, in which direction would water flow?

Water would flow from the solution with the lower osmolarity (270 Osm/L) into the solution with the higher osmolarity (300 Osm/L) because, if you remember, the solution with the lower osmolarity is less concentrated and so has more water particles. It might be easier to remember that water always flows from the solution with low osmolarity to the one with higher osmolarity.

Now that we understand osmolarity, it makes it easier to explain osmolality and how it relates to osmolarity. Osmolality is more of a clinical concept used in Medicine. For example when injected fluids into a patient's veins (intravenous/IV fluids) it is important to know the osmolarity of the solution which you are injecting. If the solution has an osmolarity much lower than the blood cells, what do you think would happen?

From the explanation above, we understand that water always moves from low to high osmolarity. This means water would move from the solution into the blood cells. This can cause the blood cells to swell and burst under the pressure of the influx of water. We call this type of solution a 'hypotonic' solution. 'Hypo' means low, so in other words this solution has a lower osmolarity than the blood cells.

In that case, what would happen if we had injected a hypertonic solution instead? Hint: 'Hyper' means high.

A hypertonic solution would have an osmolarity higher than the blood cell's so when you apply the same rule, water will move from the blood cells into the solution and this will cause the cell to shrink.

We have discussed hypertonic and hypotonic solution and there is one other term you should know: isotonic. 'Iso' means equal so, as you can probably guess, isotonic solutions would have the same osmolarity as the blood cells meaning there would be no net flow of water in either direction (which is ideal).

All in all, osmolality refers to the tonicity of the solution with regards to a cell (we used blood cells as the example but it can be any cell). If you know the osmolarity of a cell then you can work out whether a given solution is hypo-, hyper, or isotonic. For example, we know that red blood cells have an osmolarity of 300 Osm/L and we infuse an IV solution of osmolarity 210 Osm/L. How would you describe this solution and what effect would it have on the red blood cells?

Well the solution we are adding has a much lower osmolarity than the red blood cells so it is a hypotonic solution. Hypotonic solutions move water into the cell so this can cause the cell to swell and lyse(burst).