We’re going to compare Aldosterone and ADH or Antidiuretic Hormone side by side. To do this I think it would be helpful if we just do a little recap on how these two work because it’s going to help inform exactly what they do. So if we have a little nephron here, a little tubule, and these are the cells lining the tubule and it’s going to eventually send the urine on its way out. Next to it I have a little blood vessel, and just to save myself from drawing it twice I’m going to cut and paste this over to this side right here. In this tubule we know that on one side, on the aldosterone side, we have water permeability. This membrane that seperates these two, these layer of cells are water permeable. Over here on the ADH side, we know that they are NOT water permeable. The reason I’m saying it’s different is because we know that, although they look the same these are different parts of the nephron. The way that aldosterone works, the main thing it does, it’s going to pull in sodium and spit out into the urine potassium. That’s the main activity of aldosterone in terms of capturing sodium and what happens is that sodium, we know, is not permeable to membranes. Not able to cross membranes very easily. That’s actually really important because if it can’t cross membranes, then that means that it’s going to contribute to tonicity because we know that the ions that cannot cross membranes are the ones that are the biggest contributors to tonicity. In fact, this is actually very important because potassium by comparison can “slightly” cross membranes. So if you have one ion that cannot cross membranes at all and you give away the ion that can slightly cross membranes, then your tonicity goes up because overall you’re getting more ions that can stay in the blood vessel and because they can stay in the blood vessel, it can contribute to tonicity. Water is going to be driven into the blood vessels. So that’s really how aldosterone is dragging water into the blood vessel through increased tonicity. By comparison, the ADH is just using water channels if water is unable to get across otherwise, if you through in some water channels then you have no problem gathering water. So these are the key differences: one of them uses an osmole to drag water across and that’s why we always say “Water follows sodium.” and the other is just using channels. Let me make a little bit of space here. . . let’s see if we can create some space down here I’m going to create two categories: one category is Volume and the other is Osmolarity. We know osmolarity actually is simply a fraction. It’s just osmoles divided by volume. We have the same thing in the other column and I’m going to do that for this side as well: osmolarity is osmoles divided by volume. We’re going to see how these two work and whether one or both will be effected by the hormones. We know that the way that aldosterone works is by raising the osmoles. It’s going to change this, going to increase the osmoles and as a result it will increase the volume. So actually, both osmoles and volume are effected. Let me start out by just circling this box because we know that the volume is effected. because osmoles and volume are both affected and that they’re proportional to one another we usually don’t think of osmolarity being effected by aldosterone because both the numerator and denominator are going to go up if there’s a lot of aldosterone or down if there’s not any aldosterone around. So osmolarity doesn’t really get effected by aldosterone. Now, ADH is a little different. In ADH we have volume going up, that’s really the primary thing that’s happening So we’d say we have a volume change here but in terms of osmoles, you haven’t really changed the osmoles with ADH, not directly. If you haven’t changed the osmoles and you have changed the volume then osmolarity is changed. If you just change the denominator, but not the numerator then the number will change. So that’s why ADH effects osmolarity although it doesn’t effect osmoles. Kind of a tricky thing, but I think you can see it now that the numerator doesn’t change, but the overall fraction does. So you have on this side increased volume and you have increased volume on this side and you have decreased osmolarity. So these are the major changes from these hormones. Let me make a little bit more space and we’ll continue this line of reasoning. So if these are the changes, now imagine the scenario where you want to increase volume, but maintain the osmolarity. So if you want to increase volume, but maintain osmolarity which hormone would you use? because we can’t see the hormone [drawings above], let’s just use aldosterone and ADH and I’ll just put “up” arrows and “down” arrows These are the two hormones, I want to increase volume, I would definitely use aldosterone because it doesn’t effect osmolartiy, but I would NOT use ADH I’ll put a little circle with a line through it. I would NOT use ADH because again, I want to maintain osmolarity I would not want to use ADH in that scenario. Now let’s say you wanted to increase volume, REGARDLESS of osmolarity meaning you don’t really care if osmolarity changes and this could be, let’s say you have a big car accident and you’re bleeding out and the only thing you really care about right away is increasing your blood volume, that’s the only thing that matters, so you really want to increase volume and you want to do it fast. Well, in this scenario, you definitely want to use everything that’s available to you, aldosterone and ADH. The fact the osmolarity will go down with ADH really doesn’t matter because we said regardless in this scenario so because of that, I’m going to imploy ADH this time. You can kind of get a sense for how this is going to work, right? Now let’s say you wanted to decrease osmlarity regardless of volume, so I don’t really care if the volume changes a little bit here or there, so in this case, regardless of volume, what would I do? Well, if I didn’t really care about the volume and I just wanted to decrease osmolarity that seems like a no brainer, right? That’s exactly what ADH will do, it will decrease osmolarity and really in this case, I don’t need aldosterone. I said in my phrase here that I don’t care about volume changed so, that’s fine I’ll just use ADH and I’ll tolerate the increase in volume. In the fourth scenario, let’s say you want to decrease osmolarity and maintain volume, you don’t want it to go up or down. This is kind of a tricky one right? because to decrease osmolarity, only one hormone will do that so you’ve got to start with some ADH, but if you want to maintain volume you know that ADH will cause your volume to go up a little bit and if you don’t want it go up, you wanted to maintain volume, you may actually have to decrease aldosterone just a little smidge so that would maintain your volume. Now you can see how the two hormones basically have to work together to get the different outcomes. Depending on what your volume status is and what your osmolarity status is. I can flip around all the arrows or I could say well what about decreased volume and maintained osmolarity or decreased volume regardless of osmolarity? and you would basically just do the opposite of all these things. Any tweak in volume and osmolarity can help you predict what the aldosterone and ADH will be doing.