Greetings. So, today we want to talk about how we move water across the plasma membrane. And remember the last time we said that the plasma membrane was a bilayer of lipids, so that it was a hydrophobic barrier around all of the cells of the body. In order to move a molecule that's, that's hydrophobic hydrophilic across this barrier we had to have some the of protein which was a transporter or a pump. In the case of water, water is the hydrophilic molecule, it's a polar molecule and so, water by itself, is very, very, very slow to move across the plasma membrane. But, but water has its very own specific transporter and this specific transporter is called aquaporin. The aquaporin is present in essentially all cells of the body. So, water is able to move across the plasma membrane very rapidly through this through this channel. Because the channel is open at all times, it is not a gated channel. So, the movement of water, because its so important to the understanding the, how we control the volume of cells, has its very own special name and that's called osmosis. And so, today we're going to talk about osmosis. And secondly we want to consider the terms osmolarity and tonicity, which are terms which will govern the movement of water. And then lastly we want to talk about how these effective solutes, this, these are solutes which are not permeable to the, to the plasma membrane. are going to regulate the flu, the size of the fluid compartments of the body. Alright so, the first thing we have to think about, and this is not always intuitive, and that is the concentration of water. Because water is going to be moving by facilitated diffusion, through that aquaporin channel. The aquaporin channel is open at all times so, it is not a gated channel. The concentration of water has, is highest in pure water. And when we add a solute to water, we will then decrease the concentration of the water. So, for instance, if this particular, if this particular vessel is one liter in volume and the vessel over here is one liter in volume, by adding the sodium to the second vessel, this is to this one. Then what happens, is that the concentration of the water is less in, in vessel two than it is in vessel one. And I know you're all sitting there saying well, that's pretty obvious but sometimes it's confusing. So, let's just think about that and try to remember that water, the highest, the highest concentration of water is pure water. Alright so, let's look at it, at why this can be very important. So, if we have two different compartments, so we have two cells. We have cell one and we have cell two, in cell one, we have two sodium ions and in cell two we have four. Now, these have equal compartments so, they're the same size. So, let's say this is one liter in size and this is one liter in size. If we allow these two cells to come together and we have a membrane between them that allows the movement both of the solute and of the water. Then the solute and the water will reach equilibrium. So, the sodium will diffuse from one cell to the other. So, we'll have then an equal number of sodiums in cell one as we have in cell two. And water will also distribute equally between the two so, that the concentration of the sodium will bec, will be equal in both cases. And that's pretty straightforward. Now, notice that we did not change the volume of the two compartments, so they're each one liter in size. But what happens if we take our same two, two cells and we put them together, but now we put a membrane between those cells, which is not permeable to the solute. It is permeable to water so, the sodiums all stay in compartment two. And the, and, the water now that's in compartment one, is able to leave compartment one, and enter into compartment two. To dilute compartment two so, that the concentration in two is equal to the concentration in one. And that's what's shown here. Now notice, that by doing so, we now have changed the volume of compartment two. So, where this used to be compartment one, used to be one liter, now let's say it's 500 milliliters, it's half. And, compartment two is now increased by another half, so it's now one and a half liters. So, the diffusion of water then is, is ,is a facilitated diffusion. The fusion of water requires the aquaporin channels and it will cause the change in the compartment size, when the membrane is impermeable to the solute. And this is a key, key thing to remember. Because this is, if we were talking about cells in your body, the cells in your body are going to respond in exactly the same way. So, that if we have a concentration change in the ECF, such that you eat, you eat a lot of sodium, so that the ECF now has a lot of sodium in it. The water that is within the cells, will leave the cells and move to the ECF. So, that the concentration of the sodium will be the, will, will, will be equalized. The concentrations between the ECF and the ICF will be equal and we'll talk about this in just a second. So, osmosis then is the movement of water, it can fer, it occurs by diffusion only so, we're going from a high concentration of water to a low concentration of water. We use aquaporin channels [UNKNOWN] facilitated diffusion so that, that moves very quickly when we have these aquaporin channels present. And the channels are not gated, the channels are always open so, we have a patent opening between the cells. The highest concentration of water is pure water. So, we want to talk about two separate separate concepts, one is the osmolarity of the solution and the second is the tenacity of the solution. So, when we calculate osmolarity of the solution, we need to calculate how much, how many particles are within the solution, not just the number of moles that are within the solution. Normally when you think of the solution, you think of the molarity of the solution and that's the number of moles per volume and that's what shown here. But the osmolarity, then we also consider the number of particles, okay, so let's, let's think about this. So, we have a solution when we have a one molar solution of sodium chloride and one molar solution of sodium chloride, the sodium and the chloride dissociate into two particles. Those two particles then, means that we will have a 2 OsM solution of NaCl . There's also term called osmolality and in, and in biological systems, we really don't make a very large distinction between osmolality and osmolarity. The difference is that in osmolarity, we're talking about one liter for our volume and in osmolality we're talking about a kilogram of water for our volume. But we will consider, in this course, we're consider them to be essentially equivalent. The other thing that we're going to consider is that in the body the osmolarity of the cell is about 300 mOsM. So, the body is going to be 300 mOsM and, and if I put this this cell into a solution of ECF and the ECF is 300 mOsM, then that solution is isosmotic to the cell. Because it's the same osmolarity as the cell. If I had the ECF is actually 200 mOsM, then it is more dilute than the cell and so, it would then be called hypo osmotic to the cell. And if the solution that I put the cell into is 400, then 400 mOsM then, that's a hyper osmotic solution relative to the cell. Okay so, iso meaning the same or equal, hypo meaning less and hyper meaning more. Now, when we're calculating osmolarity, we calculate all of the molecules that are within the solution. So, so, if I have one molar sodium chloride and I add to that a, a molar of urea. And urea does not dissociate into, more than one particle, then that solution becomes a 3 OsM solution. Sodium Chloride plus urea [INAUDIBLE] makes a 3 [INAUDIBLE] OsM solution. Now, now the difference between osmolarity and tonicity is that with tonicity, we do not count all of the particles that are in the solution, we only count the particles that are non-penetrating. And the, the non-penetrating particles means that they cannot go across the plasma membrane. Remember, urea could go across the plasma membrane. But a non-penetrating particles would be the sodium and the chloride. If I have my red blood cell and I put it into a solution that's 300 mOsM, then the red blood cell is happy, because it iself is 300 mOsM and that's going to be a solution that's isotonic. It's the same tonicity as the cell. If I then dilute the solution that the cell is sitting in, then the, the solution could go down to let's say 200 mOsM. And when that happens, the red blood cell which is at 300 will take in water because the, the solution now is hypotonic to the red blood cell. And the water is going to move from a higher concentration, which is outside of the cell, across the membrane and into the red blood cell, and our red blood cell swells. Conversely, if I put our red blood cell into a solution where the solution is now 400 mOsM, now this solution is hypertonic to the cell. The cell will shrink, and the water then will move out of the cell and into the, into the, its environment. So, the cell then is going to try to balance the water, is going to try to balance the concentration of the solution that's around the cell. And it does so by by moving across that aquaporin channel. So, the tonicity then, we have to consider the non-penetrating molecules only. So, in a solution where we have a 1 a 1 mOsM solution of NaCl and we add to it a 1 mOsM solution of urea, that solution would still be only 1 mOsM because we don't consider the urea. The urea can go across the plasma membrane. Alright, so why am I torturing you with this? This is a really important point several years ago there was some runners who were in the Boston Marathon who had over hydrated and they had over hydrated as they were running their race. And what happened is that they diluted down their ECF, they diluted down the blood, the actual osmolarity of their blood. And by diluting down the osmolarity of their blood, then they had a situation where water started moving into the neurons of the brain and the neurons of the brain started to swell. Three of these runners actually died from this, so this is a really important point, that we need to adjust the amount of, of tonicity, that is the amount of solutes within the ECF. And therefore within the vasculature, such that it's compatible with life. And that, it, when we have very high salt within the ECF, water will move from the cells into the ECF and if we have a very dilute solution in the ECF, then water will go the opposite direction and the cells will swell. So, one of the things that the body is going to want to do, is to always maintain the ECF at about 300 mOsM. So, let's go through this tables, so that you can sort of think about what I'm talking about. So, the first one is, is that we are going to give an IV, that is by needle through, into directly into the, into a vein of an individual and we're going to give them isotonic saline. So, this is going to be 300 mOsM. So, it's 300 mOsM our solution, so the total body water, of course will increase and the effect on the ECF osmolarity is that there's no change. But the ECF volume is going to increase, because we are profusing, we're actually putting some solution into the body. Does the volume of the ICF change, does the volume of the cells change? And the answer is no, there's no change in the volume of the cells. Because the solution was iso isotonic to the cells, there's no loss of water or gain of water by the cells. On the other hand, if we have a situation where we have diarrhea, where we have an isotonic loss, so that we're losing an isotonic fluid from the body, from, from the from the anus. The total body water is going to decrease. Effect on the osmolarity, again, there's no change in the osmolarity because we're losing an isotonic solution from the body. The ECF volume decreases, but again there's no change in ICF volume. Now, in the third case, we take in excess amounts of sodium. You eat a really big bag of potato chips, salty, salty potato chips, and you don't drink any water. And as you're taking in all that salt, the sodium then, is coming into the body. So, the sodium is coming into the body, so the ECF, the osmolarity of the ECF increases, because all of the sodium is going to go into the ECF. The volume of the body is not changing, I did not bring in any fluids, so the body volume, total body water is staying the same. So, under these conditions, what happens to the ECF volume? I've brought a lot of sodium into the ECF volume, so now, the volume in the ECF is going to increase. And the water is going to come from the cells. And so, the water moves from the cells into the ECF, and the cells will shrink. Alright so, you think this and you think about what would happen in the last case, where we would have excess sweating, so there is a hypotonic loss from the body. And figure out how, how the ECF volume and the cell volumes are going to, are going to be effected. Alright so, what's our general concepts? So, the first is we have two fluid compartments of the body. We have the intracellular fluid and we have the extracellular fluid and these are in osmotic balance. The second one, is that the water moves by facilitated diffusion through aquaporin channels across most cell membranes and this process is called osmosis. Third, we have a non-permeable solutes are called effective solutes and these will affect the cellular volumes. If cellular volume is critic, critically dependent on the steady state of the effective solutes in the water across the cell membrane. If I increase the number of effective molecules in the ECF, water will move from the cells to the ECF to try to balance the concentration across the two compartments. Last, is that the cells will shrink in hypertonic ECF conditions and the cells will swell in a hypotonic ECF condition. Alright, so, the next time we meet then, we're going to talk about one more of these general concepts. Things that we're going to see as we go through the, through the rest of the, of the course. And once we go into the gastrointestinal tract, and into the renal system you may want to come back and look over this, this lecture and the lecture on the transporters. All right so see you next time.
