 in order to activate the kidneys to do something about the osmolarity of the extracellular fluid, you have to have some kind of sensory receptor that's responsible for being able to detect the osmolarity of the blood in this case. So I am going to draw you a picture. First of all, we're going to note that we're talking about the extracellular fluid, basically the concentration of the extracellular fluid. And here's my little diagram I'm going to make for you. There are sensory receptors that are found in blood vessels in the hypothalamus. Now, I'm drawing two blood vessels, so I can draw you two different scenarios, but really this is the same blood vessel because it is lined with sensory receptors. They're called osmo receptors. And I'm just making a little diagrammatic view here that these are my little osmo receptors. And where are we again? What are we talking about? We're in the hypothalamus. So you can imagine I'm going to do two different scenarios. In my first scenario, I'm going to set the osmolarity of the blood plasma at, let's just say, 270 milliosmoles. 280 is isosmotic. That's kind of the goal that you aim for. 310 milliosmoles is definitely more concentrated than your blood should normally be. Now, we've been talking about 300 milliosmoles, like we've been talking about the filtrate being 300 milliosmoles. That's just because 300 milliosmoles is easier to write than 280. And we're not really so concerned with the exact numbers in the kidneys, especially when we're talking about that incredible medullary concentration gradient. But I think it is fair game to know that 280 really is our set point. So these osmoreceptors basically are mechanoreceptors that respond to stretch. And look where they're found. Now, you have to take a deep breath and imagine this. They respond to stretch. They don't actually respond to osmolarity. They don't like say to the blood, I'm going to take a sample of the blood and I'm going to calculate the concentration of the plasma to find out whether or not I should send a message to the hypothalamus or not. They respond to stretch. And so I want you to look at my two different scenarios. First of all, which one is more concentrated? Much more concentrated here. Do you agree with that? We have a higher concentration of the extracellular fluid. We're going to have more particles in the same volume if the osmolarity is 310 milliosmoles, as opposed to 270 milliosmoles. Now, this is the place where you're going to have to take a little bit of a leap. I want you to think about how might you get to 310 milliosmoles? What might you do to increase the concentration of the blood? There's two things. You can increase particles or you can decrease volume. Whoa, where'd that come from? I'm leaving that there. It's phenomenal. It's a beautiful little circle. All right, either one of those things is going to... Increasing the particle, decreasing the volume is going to increase the concentration of this zone. Now, if you do these things, this is the one that your stretch receptors are going to detect. So you tell me, do you think that at a higher concentration, are you likely to be stretching more or stretching less? Please tell me that you agree. If you're going to stretch less, you're going to have no stretch because it's more concentrated. To get more concentrated, a great way to do it is to decrease volume. If you decrease the volume, then you're not going to be stretching as much in that vessel as opposed to having fewer particles or increased volume. That's how we can get a lower concentration that is associated with stretch. So these Osmo receptors are just responding to stretch. They're actually stretch receptors. And here's the cool thing. Let's make our Osmo receptors purple because purple's a great color. They are, I don't know what their structure is, but essentially somehow they're associated with an afferent neuron. But when stretching happens, action potentials are not fired. So it's sitting here and it stretches. Oops, I want this to be very good. We've got stretch. That means there is going to be no action potential. Whatever. If, now this is the same vessel, but this time I'm drawing it down here, here's my neuron associated with my stretch receptors. And now I have no stretch. Got to get around that wild circle I just made. If I stop stretching, if I shrink, the message that the afferent neuron sends is holy, we better send an action potential. We better send an action potential to the hypothalamus and say, dude, I ain't stretching, you better do something about this. I think it's getting too concentrated in this mug. The hypothalamus is then going to say, dun, dun, dun, dun, dun. I'm on this. I will take care of this for you. And there are two things that the hypothalamus will do in response or two things that the hypothalamus can do in response to this stretch. Are you good with that? That's how the hypothalamus can coordinate action. All right. We're going to come back to talk about aldosterone first. I'm feeling a little sad. I'm wishing that we could talk about vasopressin first, but aldosterone's on my list first. So we will go there first.