Greetings. Today we want to talk about how water moves across the plasma membrane. Remember in the last lecture we said the plasma membrane is a bilayer of lipids which forms a hydrophobic barrier around each cell in the body. In order to move a molecule that is hydrophilic across this barrier we need a protein which functions as either a transporter or a pump. Consider water. Water is a hydrophilic, polar molecule. Water by itself, is very, very slow to move across the plasma membrane. But, water has its very own specific transporter. This specific transporter is called aquaporin. Aquaporin is present in essentially all cells of the body. Water is able to move across the plasma membrane very rapidly through this channel because the channel is open at all times. It is not a gated channel. The movement of water, is really important to understand because this is how the volume of each cell is controlled. The movement of water has special name. It is called osmosis. Today we're going to talk about osmosis. In addition, we want to consider the terms osmolarity and tonicity. These terms refer to the the factors that govern the movement of water. And lastly we want to talk about how effective solutes, solutes which are not permeable to the plasma membrane. regulate the flow of water and thereby 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, is the concentration of water. Water is going to move by facilitated diffusion through the aquaporin channel. The aquaporin channel is open at all times; it is not a gated channel. The concentration of water is highest in pure water. When we add a solute to water, we decrease the concentration of the water. So, for instance, if this particular vessel is one liter in volume and the vessel over here is one liter in volume. If we add sodium to the second vessel, then what happens to the concentration of water? It is less in vessel number two than it is in vessel number 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 has the highest concentration in pure water. Alright. So let's look at why this can be very important. Consider that we have two different compartments, that are 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. At this point they are equal compartments. They're the same size. Let's say this each 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 be will be equal in both cells. 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. Now when we add sodium it all stays in compartment #two. But the water now that's in compartment #one, is able to leave compartment #one, and enter into compartment #two. This added water dilutes compartment #two. Eventually the concentration of sodium in cell #two is equal to the concentration in cell# one. And that's what's shown here. Notice, that by doing so, we have changed the volume of compartment #two. So, where this compartment #one, used to be one liter, now let's say it's 500 milliliters, i.e., it's half. And, compartment #two is now increased by another half, so it's now one and a half liters. The diffusion of water is a facilitated diffusion. The diffusion of water requires the aquaporin channels. The movement of water will cause the change in the compartment size, when the membrane is impermeable to the solute. This is a key, key thing to remember. Because if we were talking about cells in your body, the cells in your body are going to respond in exactly the same way. Let's consider an example of a concentration change in the ECF. Suppose that you eat a lot of sodium such 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. This will continue until the concentration of the sodium will be equal in the ECF and ICF. The sodium concentration in the ECF and the ICF will be equal. What about the water? We'll talk about this in just a second. So, osmosis then is the movement of water, it occurs by diffusion only. Water moves so, from a high concentration of water to a low concentration of water. We use aquaporin channels for this facilitated diffusion. Water 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 tonicity of the solution. When we calculate osmolarity of the solution, we need to calculate 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 for osmolarity, we also consider the number of particles. Okay, so let's, let's think about this. We have a solution of one molar of sodium chloride. In the one molar solution of sodium chloride, the sodium and the chloride dissociate into two particles. Those two particles. This means that we have a 2 OsM solution of NaCl. There's also a term called osmolality. 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. In this course, we will 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 fluid compartments of the body are 300 mOsM. If I put a 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 change the ECF to 200 mOsM, then it is more dilute than the cell. The ECF would then be called hypo-osmotic to the cell. If the ECF solution that I put the cell into is 400 mOsM, then the ECF is hyper-osmotic solution relative to the cell. Okay so, iso meaning the same or equal, hypo meaning less and hyper means more. Now, when we're calculating osmolarity, we calculate all of the molecules that are within the solution. So, if I have 1Molar sodium chloride solution and I add to that one mole of urea. Note that urea does not dissociate into more than one particle. This solution becomes a 3 OsM solution. Recall that the 1 M Sodium Chloride is 2 OsM plus 1 mole of urea makes a 3 OsM solution. Now, let's consider the difference between osmolarity and tonicity. In 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 to enter the cell. 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. It is 300 mOsM. The solution is 300 mOsM. The solution is isotonic to the red blood cell. It's the same tonicity as the cell. If I then dilute the solution that the cell is sitting in, then the solution could go down to let's say 200 mOsM. When that happens, the red blood cell which is at 300 will take in water because the solution now is hypotonic to the red blood cell. Water is going to move from a higher concentration, which is outside of the cell, across the membrane and into the red blood cell. Our red blood cell swells. Conversely, if I put our red blood cell into a solution where the solution is now 400 mOsM. This solution is hypertonic to the cell. The cell will shrink. Water will move out of the cell and into its environment. So, remember that water moves to balance the water cincentration. It moves to balance the concentration of the solution that's around the cell. Water moves rapidly across that aquaporin channel. So, for tonicity then, we have to consider the non-penetrating molecules only. So, in a solution where we have 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 as they were running their race. 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 life threatening situation where water started moving into the neurons of the brain. This caused the neurons of the brain to swell. Three of these runners actually died from this. This is a really important point. We need to adjust the ECF tonicity, that is the amount of solutes within the ECF. And therefore within the vasculature (IVF), such that it's compatible with life. When we have very high salt concnteration 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 table, so that you can think about these concepts. The first one example is that we give an IV, that is by needle, a solution directly into the vein of an individual. We give them isotonic saline. So, saline is 300 mOsM. So, it's 300 mOsM added to the body. The total body water volume increases. The effect on the ECF osmolarity is no change. But the ECF volume is going to increase, because we are perfusing, directly putting this 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 isotonic to the cells. There is no loss of water or gain of water by the cells. In the next situation we have diarrhea. Now there is a loss of fluid. We are losing isosmotic fluid from the body, from the from the anus. The total body water volume decreases. Effect on the osmolarity, again, is no change. There is no change in the osmolarity because we're losing an isosmotic 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 enters the body. So, the sodium coming into the body, enters the ECF. The osmolarity of the ECF increases, because all of the sodium is going to go into the ECF. The total water 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. Under these conditions, what happens to the ECF volume? I've brought a lot of sodium into the ECF volume. The volume in the ECF increases. The water comes from the cells. As water moves from the cells into the ECF, then the cells shrink. What about the last case? Think about what would happen in the last case, where we have excess sweating. There is a hypotonic fluid loss from the body. You figure out how this affects the ECF volume and the cell volumes. Are they going to be effected? Alright so what's our general concepts? First is we have two fluid compartments of the body. We have the intracellular fluid and we have the extracellular fluid compartments. These are in osmotic balance. Second concept is that 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. The cellular volume is 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, 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. These general concepts will recur as we go through the rest of the course. As we consider the gastrointestinal tract and the renal system you may want to come back and look over this lecture and the lecture on transporters. All right so see you next time.
