that you know and understand these hormones not just for this chapter but for the next chapter on fluid balance so the rate at which water is lost depends mainly on adh adh is the major regulator of water aldosterone is the major regulator of sodium and potassium and when adh levels are low the kidneys produce a very dilute urine remember we learned about diabetes insipidus in which adh isn't being produced and so you produce large amounts of very dilute urine and you can become dehydrated very easily the compensation for low water intake or heavy perspiration is to concentrate the urine and that's what adh does it helps to concentrate urine this involves the ascending limbs of the loop of henle as well as the collecting ducts and the renal medulla the counter current mechanism is also involved in this so in order to understand the counter current mechanism let's just take a look at the nephron up here most of it is going to sit in the cortex the loop goes down into the medulla now in the cortex the solute concentration is about 300 milliosmoles that's how we we unit we use to describe the concentration of the solutes milliosmoles and remember just like when we learned about hypertonic and and hypotonic solutions and the movement of water into red blood cells or out of red blood cells water wants it to be the same on both sides it wants to have the same concentration of solutes on both sides so water is going to move across the membrane to try if it can to try to equalize it and have equilibrium up here in the cortex it's about 300 milliosmoles and so water is going to move across in order to make the concentration here inside the tubule also 300 milliamoles so if if this is too concentrated water will move in if it's too dilute water will move out in order for this to be 300 milliosmoles once it goes down into the cortex though in the medulla i mean there is a gradient there's a gradient all the way down here across the medulla and at the top it's about 350 and it goes 550 750 900 it can go down to about 1200 milliosmoles in concentration so the deeper you go down into the medulla the more concentrated the solutes become on the outside as the um filtrate is moving through the descending loop of henle water is going to leave to try to make it the same on both sides to concentrate this urine the problem is and solutes can move across as well but but water is going to be the main thing that's leaving going down the descending loop of henle as we make the turn and go up this area here that's dark brown water can't leave and if water can't leave and only solutes can through active transport sodium and chloride are going to be pumped out as they're pumped out the concentration inside the thick limb of the ascending loop of henle is going to become lower than outside now we've prevented it from being the same so now we're going to be losing solutes and that helps to maintain this gradient but we're going to keep the water and this whole area where it's dark water can't leave it's impermeable so now we're at 350 now at 150 now it's it's diluted to 100 and when it goes into the collecting duct in the collecting duct this is where adh is going to work and this is where adh is going to either insert aquaporins and concentrate the urine or not it's it's going to act on the principal cells to do that if it doesn't we produce dilute urine and it can be very dilute it can be 65 milliosmoles or it could be concentrated it could be very concentrated it could be up to 1200 milliosmoles depending upon the water balance so it's all because of this gradient now the way that the counter current mechanism plays into this is because the vasa recta is going to be going down around that loop of henle and when the vasa recta that's the blood vessel the capillary that's going around there it has to provide nutrients and oxygen to all those cells and it has to do it without affecting the osmolarity so here you can see the vasorecta and the blood is flowing in the opposite direction of the filtrate as the blood is flowing in the vase of that vasorecta it's also maintaining that gradient that we have in the um descending loop of henle and in the medulla it helps to maintain it so the fact that it's going in the other direction of this that's what makes that's why they call it a counter current mechanism it's going to be picking up sodium chloride and potassium from the part that's impermeable to water and it's going to be picking up water from the descending limb and so it's going to be able to maintain that gradient it's not going to mess up the gradient outside it's going to have the same gradient but it's going to be able to deliver oxygen and nutrients to all the cells if it weren't for the counter current mechanism then this would completely disrupt this gradient by having blood flowing through here so the formation of dilute urine happens when water when the adh isn't working here the principal cells aren't going to reabsorb water if adh is low so when adh is low you produce dilute urine so if you have if you were really over hydrated and you didn't have much adh being released the number of aquaporins in the collecting duct decreases water reabsorption decreases and you produce a large volume of dilute urine if you're dehydrated and have maximal adh being released then the the trigger here is high osmolarity of the fluids that's detected by the hypothalamus causes lots of adh to be released lots of aquaporins lots of water reabsorption and because so much water is going to be reabsorbed you can produce extremely concentrated urine very small volume of concentrated urine because the water is going to be reabsorbed so here you can see it's a hundred hundred hundred the osmolarity here it's a hundred now it's 150 300 300 400 600 900 1200 so it it can either be concentrated because of adh or not concentrated because you don't have much adh so in summary when it comes to water reabsorption the proximal convoluted tubule again is the spot where most things are reabsorbed 65 percent in the loop only about 15 percent remember that's the descending part in the distal convoluted tubule about 10 to 15 collecting duck 5 to 10 with the help of adh so diuretics increase urine flow and they work by a number of different mechanisms the most potent ones are the loop diuretics such as ferocimide which is commonly known as lasix and that inhibits the sodium's importers in the thick ascending limb of the loop of henle so it prevents sodium from leaving and if sodium doesn't leave water isn't going water is going to not be reabsorbed so there are other things that can cause diuresis which is increased urine flow caffeine also inhibits sodium reabsorption which is why when you drink caffeinated drinks you have produce more urine alcohol inhibit secretion of adh and there are also other prescription medicines that can act on other areas of the nephron including the proximal convoluted tubule and the distal convoluted tubule the goals for this lecture describe the effects of adh atrial natural peptide in the raa system on fluid balance and urine production describe from where each of the hormones is released and where in the nephron they act describe how dilute and concentrated urine form and what is the counter current mechanism