we're going to use this graphical representation to describe the changes that may occur as we either add or subtract different types of fluid from our body so on this graphic we see on the x-axis this is going to represent volume and any change on the y-axis represents a change in osmolarity notice we've divided the two compartments into the largest compartment of fluid which is the intracellular fluid and then also the extracellular fluid we are not distinguishing between the plasma and interstitial fluid because the barrier between those two allows the free movement of both water and ions and so they basically function as one unit the barrier between the intracellular fluid and extracellular fluid though is our plasma membrane although it will allow the free movement of water it does not allow the movement of ions now any difference between the osmotic pressure between the intracellular fluid and extracellular fluid will cause water to shift between these two compartments so these two compartments the osmolarity will always equilibrate as we're looking at the potential changes that may occur in these fluid compartments there are several different factors that we need to consider first we need to identify the change that's occurring in the extracellular fluid so we need to ask was water lost or gained and was solute lost or gained next as a result of those changes the next question we ask is will that change result in an increase a decrease or no change in the extracellular fluid osmolarity if a change in osmolarity does occur then we have to describe the direction that water must move in order to re-establish equilibrium between the intracellular and extracellular fluid osmolarity let's first look at this system if we were to say we are losing an isotonic fluid so if we're losing an isotonic fluid notice we've shown this extracellular fluid compartment has decreased in volume so remember the volume is on the x-axis so a lost in isotonic fluid results in a decrease in volume of the extracellular fluid but because this was an isotonic fluid that was lost there's been no change in osmolarity so in this particular example we'll have a decrease in total body water a decrease in the extracellular fluid compartment volume no change in osmolarity and if there's no change in osmolarity then there will be no shift of water into or out of the intracellular fluid compartment so that will also remain the same in this situation we're going to say we are losing a hypotonic fluid so in this case we're losing more water than solute and for learning purposes if you'd like to think if we say we're losing a hypotonic fluid just imagine losing pure water so in that situation we once again are going to see a decrease in volume but notice in this case because we've lost pure water we are seeing an increase in osmolarity and as a result of this increase in osmolarity this is then set up a gradient across the cell membrane anytime there's a gradient in the solute concentration across the cell membrane water is going to have to move so in this particular situation we can imagine this as being our cells being surrounded or bathed by a hypertonic fluid so in order to try and equalize that out we're going to see water moving from the intracellular fluid compartment to the extracellular fluid compartment now realize as we're graphically depicting this we're artificially breaking this down into very discrete steps so here in this example we're showing the extracellular fluid compartment osmolarity increasing in the ICF the intracellular fluid compartment being normal and that again is just for instructional purposes in reality this is going to be more of a fluid dynamic system but for learning I think it's helpful to really break this down into very discreet steps so we've said the fluid from the intracellular fluid compartment is going to then cross over and go to the extracellular fluid and as a result it's going to result in the osmolarity of those two compartments equaling out but if we're going to describe what happens to each of these compartments our total body water has decreased because we've lost fluid our extracellular fluid volume has decreased the osmolarity has increased and as a result the fluid has left the intracellular compartment and so we'll see a decrease in the intracellular fluid compartment as well if we Infuse a hypotonic fluid so in this case imagine we're infusing water that's going to result in an increase in volume of our extracellular fluid and notice that it's caused a drop in plasma in the osmolarity of the extracellular fluid compartment as a result of this we now have basically set up a situation where our cells the intracellular fluid compartment is surrounded by hypotonic fluid water is therefore going to have to move and in this case water is going to move into the cells so overall we've seen an increase in total body water an increase in extracellular fluid a decrease in osmolarity and our intracellular fluid compartment has also increased how about if we Infuse a hypertonic fluid in this case we're increasing both volume and osmolarity so we've set up a osmotic gradient across the cell membrane which means water has to move in this case we can imagine our cells are surrounded by a hypertonic solution and in that case water is going to leave the cell and it's going to result in the osmolarity overall equilibrating but notice that we've equilibrated at a higher value so the total body water has increased the extracellular fluid volume has increased osmolarity is increased and our intracellular fluid volume has decreased now don't spend time trying to critique the exact change in these volumes that we've given here that's not the purpose of these graphs these are not meant to be perfectly accurate they're meant to show general concepts with regards to changes in these various compartments foreign