Explain why and how ECF osmolality is maintained.

Video transcript

Hi, everybody. Let's look at our next problem. Explain how A DH helps maintain the osmolality of body fluids. And we have three different answer choices. A B and C with events in different orders. And there's actually different events in each answer. Choice. So each of the series of events connected with arrows leading from one to the next. So choice A is concentrated ECF then A DH released, then increased water reabsorption in the kidneys, then reduced urine output, then diluted ECF then osmolality back to normal levels. Choice B is concentrated ECF then A DH released then which triggers thirst, then prompts the individual to drink fluids, then restores the fluid balance and dilutes the ECF thus lowering its osm to normal C concentrated ECF, then A DH released, then increased absorption of water from the intestines, then bringing the osmolality back to normal or toy. None of the above. So to sort through which of these choices is correct, let's think about what A does. So a DH is antidiuretic hormone and if we think about what a diuretic is, it's something that makes the body eliminate water. So A DH does the opposite. It makes the body retain more water. And how does it do this? It causes more aquaporin to be expressed in the cells of the ductal convoluted tubule of the kidney aquaporin are water transport proteins. So, if there's more of them present, more water is reabsorbed, buy the kidney. Now, if we think of this in terms of osmolality of body fluids, what is osmolality? It's the solute concentration and these fluids. So when the solute concentration is higher, there's less water content in the bodily fluids. And so when osm is high, less water, when osmolality is low, there's more water in those fluids. So if more water is being reabsorbed by the kidney, this would take place or you'd want this to take place. You want this when osmolality is high in those bodily fluids because retaining more water will then dilute the bodily fluids and bring down the osmolality. So with that in mind, let's look at our answer choices. All three of them A B and C start with the same first two steps concentrated evecf, then A DH released. As we said, that makes sense when the osm is high, which is what happens when the ECF is concentrated, then ADH is released. The effect of it. When we look at choice A would be increased water reabsorption in the kidneys. As we saw below more water is reabsorbed by the kidney when A DH is released. So step three also correct in choice. A increased water, re absorption by the kidneys means reduced urine output. As choice A says, less is being eliminated, more is being brought back into the body. When you're reabsorbing more water, then that extracellular fluid is being diluted and that ends up resulting in lowering the osmolality of bodily fluids. And so in our last step, osmolality is back to normal levels. So we see choice A has all the steps in the correct order that you would expect ac after the release of A DH. So choice A is going to be our answer. We can also eliminate choice D none of the above to be thorough. Let's just look at choices B and C again, they both have the release of A DH. When the ECF is concentrated in choice B, the third step is which triggers thirst. But this is a different mechanism for adjusting the osm of bodily fluids. Thirst is triggered when the hypothalamus gets a message from osmo receptors. These receptors are in the plasma membrane and they respond to a change in the pressure on the plasma membrane. So when the blood volume is up, uh the plasma membrane expands and the body knows there's more water present. When the blood volume drops, the plasma membrane is not as stretched. And the osmo receptors send this trigger to the hypothalamus thirst center and a sensation of thirst is triggered. The rest of the steps are correct that prompts the individuals who drink fluids restores the fluid balance and dilutes the ECF osm lowered. But again, that's not an effect of a DH, but of triggering the hypothalamus and the thirst center there. So that's why choice B isn't correct. And then finally, choice C. The third step, there is after A DH is released, there's an increased absorption of water from the intestines, but this is not correct. It increases the reabsorption of water from the kidneys, not the intestines. So that's why choice C is not correct. So again, our correct explanation of how A DH helps maintain the osmolality of bodily fluids is in choice. A which has the correct effect of A DH being increased water reabsorption in the kidneys leading to the dilution of the ECF and the return of osmolality back to normal levels. See you in the next video.