next I want to try and bring some clarity to the use of the terms osmolarity and tonicity so first let's look at osmolarity osmolarity will be used when we are trying to compare the relative solute concentration of two solutions now we typically Express osmolarity in terms of milliosmoles per liter so in this particular picture I've just simply included the miliosmols and let's imagine that each side of the container is one liter so in this situation we have two solutions that have the same amount of solute on each side and we could therefore describe these Solutions as being ISO osmotic and that means that they have the same amount of solute concentration in this setting we again have our two containers but note that they now have different osmolarity so if we were to describe these two solutions we would say that solution a is hyperosmotic compared to solution C or on the other hand we could view it from the other perspective and say that c is hypoosmotic to solution a let's look at this a little bit more in more simplified fashion so that we can make a point so in this setup I've drawn 10 solute particles for each side of the container so for simplicity's sake we will say that each side of a container has 10 milliosmoles per liter so that would be the osmolarity of each of these Solutions and because each solution has the same amount of osmolarity or the same amount of solute particles we would call these Solutions as being ISO osmotic but let's think back to a concept we covered previously and that is is that solute particles are sometimes different with regards to the way that they with their ability to cross a membrane so that was brought in with the concept of reflection coefficient so as you recall a reflection coefficient of one would say that that particular solute is non-permeable to the membrane that we are considering and on the other end of the spectrum a reflection coefficient of zero would indicate that that solute could freely penetrate the membrane so in this particular setup we once again have compartments A and B and if you'll note each compartment A and B has 10 solute particles present so we could use the term correctly and say all of these Solutions both A and B in each of these three setups are ISO osmotic so they are equal amount of solute compared to the other side but I hope you can think forward and see that there's going to be a difference with how these Solutions behave so in our Center selection notice that because we have penetrating solute on one side and non-penetrating on the other the penetrating solute is going to be able to move across the membrane and it will equilibrate with itself on either side and yet the non-penetrating solute cannot move it's going to be stuck on side a so this now is going to cause a difference with regards to whether or not it these solute concentrations are going to cause water to move so in our first example we have equal solute concentrations on either side and therefore there will be no net movement of water same goes for the example on the far right we have the same concentration of solute on either side of the membrane and therefore there'll be no net movement of water but in our middle example we now have a difference in solute concentration and where there's a difference in solute concentration osmosis will occur and water will move so the way we have this drawing water would move from side B to side a so although we initially could describe each of these Solutions as ISO osmotic to the one on the other side you will notice that the makeup of the solute whether or not the solute is penetrating or non-penetrating will have an effect with regards to whether or not water will move so simply knowing the osmolarity the relative osmolarity of two solutions doesn't necessarily give you enough information to determine whether or not water will move or not so in medicine we're primarily concerned with solute that is what we say effective in causing osmosis so we're interested in that solute that will cause the movement of water now a term that helps us with this is the term tonicity so tonicity is a term which is going to indicate a solution's effect on cell size in the tonicity of a solution will be determined by the relative effective osmolarity of that solution and when we're talking about human physiology this means that we're concerned about the relative effective osmolarity of the extracellular fluid compared to the intracellular fluid now I have written here in this first example we have a cell that's drawn and we show that the amount of solute in that cell is 300 milliosmoles that is the the number that we're going to use in our class as our Baseline normal osmolarity so in this particular situation we have a cell which is placed into a solution that also has a osmolarity of 300 milliosmoles and if we were to see what effect the solution would have we find that this solution does not cause any net movement of water and therefore there is no change in cell size a solution that does not cause a change in cell size we refer that solution as an isotonic solution let's take our same normal cell and this time we're going to surround it by a fluid that has 250 millios moles in this case we're going to see that water is going to move into the cell because there's a greater concentration of solute so if a solution causes a cell to swell then we refer to that solution as being a hypotonic solution again we'll take our normal cell in this case we're going to place it in a solution with 350 milliosmoles of solute and if we observe what happens here and we see that as a result of this solution surrounding the cell water moves out of the cell and shrinks in size then we would call this solution a hypertonic solution