Greetings. So today we have our last lecture in the urinary system where we're dealing directly with the kidney's function for a homeostatic basis of electrolyte and fluid balance. And today what we want to talk about predominantly is how it regulates fluid balance within the body. And that is regulating the size of the fluid compartments of the body. So some of the things that we have to think about today will be to recall of the location of the juxtamedullary nephrons, because these are the nephrons that we'll be mostly concerned with. And secondly, we want to explain the change in the permeability along the renal tubules to ions and water. And this is because we're interested in how we're generating then that interstitial ionic gradient that osmotic gradient that sits within the medulla of the kidney. And third, we want to explain the importance of that osmotic gradient within the kidney medulla for the concentration of urine and how that works. And then, fourth we want to explain what's called RAAS or R-A-A-S. This is the renin-angiotensin-aldosterone system. And the conditions that activate it. And this is the system that's going to coordinate the functions of the kidney with that of the rest of the body in maintaining blood pressure. And then, five, we want to explain the hormonal regulation of ECF volume. And six, define diuresis and explain the different causes of diuresis. So we have quite a few things to consider on our list. So let's get started. So the first thing that you should think about is that as you go through your specific daily activities, you're taking in about 2 liters of food and fluid in a given day, and the intake to the body of this material has to be equal to the output from the body of this material in order for us to maintain mass balance. The kidney's function is to regulate the fluid volumes that is to regulate the amount of fluid that's coming in to the body with that which is leading the body and secondly the kidney needs to regulate ions and. In particular we're interested in the water, that is the amount of water that's entering the body, and how the kidney then can conserve water for the body, or excrete water if it has an excess. The thing to remember is that the kidney can conserve water but it can't replenish water. And so, what I mean by that is that as you go through your day, and you lose water, then you have a thirst mechanism which causes you to drink water. That is to bring back water into the body, to replenish it. The kidney cannot make water. It can move water from presumptive urine space back into the blood space, but that has a limited function. The second thing is, is the only water that can be excreted by the kidney is the water that can be regulated. And that means as we're sweating, as I'm speaking, if you defecate, you lose water from the body. It's not something that's regulated. And usually it's a very small amount of water versus the amount of water, total water that's within the body. But only the water that the kidney's excreting in the urine is the water that we can regulate. And we can regulate it up to a certain point, but you can't make solid bits of uric acid as some of the desert mice are able to do so. So humans always put out a fluid component, which is urine. And the other thing to remember is that the osmolarity of the normal urine is going to change with the body's needs. If the body has an excess of volume, excess of fluid, you have dilute urine. And if the body has a need for retaining water or has a dehydration type of a state that it needs to hold water within the body, you have a very concentrated urine. And that's what's shown here on this table, where at Bowman's capsule, we have 180 liters per day coming into the filtrate. And that it has an osmolarity of 300 milliosmole. And that by the end of the collecting duct, that is our final urine, what's being put out into the ureter and then eventually bladder, urethra and out to the outside world, is usually only a half to 1.5 liters per day and that can have of molarity as dilute as 50 milliosmolar, but as concentrated as 1,200 milliosmolar. So how in the world does this happen and so this is in order to understand this, we have to look at the structure and the function then of the nephron, and in particular we're interested in the juxtamedullary nephron. These are the ones that send their tubules deep into the medulla of the kidney. So the first thing is, is that the normal kidney generates by the juxtamedullary nephron. It generates a standing osmotic gradient within the medulla. And this gradient will be 300 mOsM down to 1,200 mOsM. And the way that it does this is by moving ions and water out from the renal tubule, that is a way out from the filtrate. But it does hit in a very interesting nano. So let's follow how the structure works. So we start then with our glomerulus, and the glomerulus is surrounded by Bowman's Capsule. And we have our afferent arteriole and we have the efferent arteriole. And this is our normal structure. And then, from that, we have from Bowman's Capsule. We have the proximal convoluted tubule. The proximal convoluted tubule, then descends into the medulla, and it descends into the medulla. It becomes the thin loop of Henle, and we have the descending thin loop of Henle. It does a hairpin turn and then as it comes back up to the cortex, we have the thick ascending loop of Henle and eventually it drains into the distal convoluted tubule and then that enters or drains into the collecting duct and that's what shown here. So these are a major regions of the tubule. So the first thing is the filtrate is it enters into Bowman's Capsule has a osmolarity of 300 milliosmolar. And in the proximal convoluted tubule we all know that reabsorption can occur in fact as the dominant site for reabsorption, a fluid and ions. And that that occurs in an iso-osmotic manner so that we have no change between the filtrate, stays at 300 milliosmolar and that the interstitium in the cortex is 300 milliosmolar. But once the tubule then descends down into the medulla then the epithelial cells are no longer freely permeable to ions. And so, water is able to move out of the filtrate and into the interstitial space. But that the ions are no longer able to freely move across this epithelium. And so, the sodium then that's within the descending Thin Loop of Henle. The concentration of the sodium becomes higher and higher as water moves out from the tubule due to this Extracellular gradient of 300 to 1200 milliosmoles is within the interstitium. So water is moving down this concentration gradients up open channels are present within the luminal surfaces of these epithelial cells. And water can move across these cells and enter into the interstitium of the medulla. By the time we reach the bottom loop of the thin loop of Henle, the filtrated cell now is very concentrated. It's able to equilibrate with the interstitial fluid, and so the interstitial fluid is at 1,200 milliosmole, then the filtrate is at 1,200 milliosmole. So we have a movement now of sodium and of urea and of water freely across these cells so that the filtrate now is 1,200 milliosmole. As the filtrate then starts to ascend back towards the cortex it goes into the thick ascending loop of Henle. And in a thick ascending loop of Henle the epithelial cells no longer express aquaporin channels. So there's no aquaporin channels on the luminal surfaces of these cells, consequently, water cannot move across this epithelium. So water now is trapped within the tubular filtrate, but we have a second active transport of sodium, potassium, and chloride. And this transporter is moving sodium, potassium, and chloride out of the filtrate and into the interstitial space. It does so the the point that, by the time the filtrate enters back into the cortex in the distal convoluted tubule, it now is at 100 milliosmole, so we now have a hyposmotic condition. We had a hyperosmotic condition in the thin loop of Henle down in the bottom of the medulla, the deep region of the medulla. And back up in the cortex, we now have a hyposmotic condition where the filtrate is at 100 milliosmoles. If nothing happens to the filtrate from this point on, then the urine that's expelled from the body will have 100 milliosmole and that urine will be 18 liters per day. So if we do not change the osmolarity, we don't do anything to the epithelial cells in the collecting ducts, or in the proximal convoluted tubules. There is no aquaporin channels present in this region and that means that whatever is in the filtrate, then, will be expelled from the body as urine. And that will be 100 milliosmole solution in that it will be 18 liters per day. There is, in fact, a pathological condition where someone has diabetes insipidus. And in diabetes insipidus, then, this individual will be putting out 18 liters per day of very dilute or hypoosmotic urine. And it's because within the distal convoluted tubule and within the collecting duct there's no aquaporin channels present within this tissue. These are regulated insertions, and we'll talk about it in just a few minutes. The other thing that we should think about in the other case is that what happens to all of the movement of the water that's coming out from the descending thin loop of Henle. Because that would wash out the gradient that should be diluting out the amount sodium and potassium within the gradient. Predominantly sodium, that's within the gradient. And for that, we have to consider what's happening to the blood. So, as I told you, within the cortex we have this peritubular capillary which is aligned with renal tubules. Closely aligned to the renal tubules. This alignment occurs as these tubules are descending down into the medulla from the juxtamedullary nephrons. These tubules have the capillary bed, the second capillary bed is descending with them. So in the cortex, the peritubular capillary follows the tubule, and then once down into the medulla, the blood is descending deep into the medulla. It also does a hairpin turn and then returns to the cortex but in this particular case the flow of the blood is in the opposite direction to the flow of the filtrate. So that as the blood is descending deep into the medulla it's picking up the sodium which is being pumped out in a thick ascending loop of Henle. And so consequently, the blood is becoming higher and higher and higher in concentration of sodium. In such that, by the time the blood reaches the hair pin loop deep within the medulla, the blood now is sitting in 1,200 milliosmole. The blood is also able to pick up the urea, so the blood cells are not shrinking, okay. They're picking up the urea,so this is now a isotonic conditions, w here we have minimal shrinkage of the cells,but it's in a very hypertonic condition. Then is this cell as the blood is now returning to the cortex, it's picking up water and it's picking up water from the descending tubule. So the water is leaving the descending thin loop of Henle, and is being picked up by the blood. And the blood of this capillary bed then is going to return to the cortex, and eventually leave the cells through the renal thing. This particular hairpin turn or the descending regions of the blood within the medulla is so important it has it's own special name and that's call the vasa recta. These are the vasa recta. Within the medulla is that second capillary bed and it's picking up all of the fluid and the sodium that's being excreted from the filtrate. Now once we're back into the cortex, then the blood is now 300 milliosmoles and so it's isosmotic with the cortex interstitium. So in the case with the body wants to make concentrated urine, then this is a regulated function. And what happens is that it's the body will secrete the hormone anti diuretic hormone or vasopressin and you all know about this. This is a hormone that's secreted from the posterior pituitary in response to a rise in osmolarity within the blood. As the blood osmolarity rises, then there's two signals that are generated from the brain. One is thirst, so that we seek water, we drink water, bring new water back into the body. But you also secrete antidiuretic hormone and antidiuretic hormone works on the distal convoluted tubule and in the collecting duct. And here, within these cells, it will increase the copy number for aquaporin that's out on the luminal surface of the cell. So it's moving, it's mobilizing the aquaporin channels and putting that out onto the luminal surfaces of the cells. In the presence of aquaporin channels, water now can move across the cells but we all know you have to have an osmotic gradient for the water to move. And so for water to move from the distal convoluted tubule out into the interstitium, which is 300 milliosmoles, we have to have a gradient. And we know that the gradient is that inside this tubule it's now 100 milliosmole, so there is a gradient for water to move. So it's 100 milliosmole in the filtrate, 300 milliosmole In the interstitium of the cortex. If you insert aquaporin channels, water moves from the filtrate into the interstitium of the cortex and it's picked up by the peritubular capillaries. In the collecting duct, when we insert the aquaporin channel, there is also a gradient and this gradient is 300 to 1,200 milliosmolars. Because the collecting duct goes straight through the medulla, terminates in the tip of the medulla at the beginning of the ureter and delivers the urine directly into the space. The filtrate, the urine, is then picked up by the ureter and delivered then to the bladder. So the osmotic gradient then is absolutely critical for concentrating urine, and if we didn't have the osmotic gradient in the medulla, then the highest concentration that we would have for concentrating urine would only be 300 milliosmolar. But we are able to concentrate urine up to 1,200 milliosmolar because of the gradient, the standing osmotic gradient that's within the medulla. Now there is another hormone that regulates this particular region of the tubules, and in this particular region, we have what are called principal cells and these principal cells are the cells where the aquaporin channels are being mobilized. These are the epithelial cells and these cells are having aquaporin channels are mobilized unto the lumenal surfaces by antidiuretic hormone. But the second hormone called aldosterone, which is secreted by the adrenal glands. This hormone also acts on these principal cells. And this particular hormone has essentially two functions. As you recall from the adrenal gland lectures, these cells are secreted from the zona glomerulosum and that they're secreted in response to high potassium within the blood or in response to angiotensin II. In response to these stimuli, then what happens is the aldosterone works on the principle cells. And it is going to increase the number of sodium transporters that are on the luminal surfaces of the cells and increase the number of potassium transporters that are on the cell and the sodium-potassium APT which is on the basil surfaces of these cells. Consequently, potassium is secreted. And so we have increased secretion of potassium and we have increased reabsorption of sodium. So sodium leaves the filtrate, moves across the cells, enters the interstitium, and is picked up by the peritubular capillary. And the potassium is leaving the blood, crosses the cells, and is secreted into the urine. So that's one way in which aldosterone then can regulate the amount of potassium that's within the cell, but aldosterone also regulates the volume of the body. That is when low blood volume is perceived by the kidney, the aldosterone system is activated. And that's because aldosterone moves sodium into the body, and when you have aldosterone present and antidiuretic hormone present, you are also moving water, which will follow sodium, and therefore you increase the volume of the body. The coordination of the secretion of aldosterone and antidiuretic hormone is done by the kidney itself and that's what's shown in this next diagram. Okay, this is our Renin-Angiotensin Aldosterone System or RAAS. So in this system, [COUGH] excuse me, we have the plasma volume is decreasing. So plasma volume decreases and the kidney senses low plasma volume. When the kidney senses low plasma volume or low filtrate flow within the tubule, are activated and they cause the cells, the smooth muscle cells of the afferent arterial, to secrete renin. Renin is an enzyme that enters into the plasma and it cleaves a protein to form angiotensin I. Angiotensin I is converted to angiotensin II by an enzyme which is called ACE, angiotensin converting enzyme. And this enzyme is present in the lungs, but it's also present in the vasculature and many tissues. Angiotensin II is the signal which will cause the hypothalamus to secrete antidiuretic hormone from the posterior pituitary. And antidiuretic Cronin, as we said, works on the tubules to move through those aquaporin channels to insert them into the luminal surfaces of the cells, and we now have water that can move across the cells. Angiotensin II also causes the adrenal glands to secrete aldosterone, and aldosterone works on the collecting ducts, on the principle cells, and on the distill convoluted tubule principle cells. And it will cause sodium to move into the body. So we're moving sodium and water into the body with aldosterone and so we will increase blood volume. So this RAAS system then is increasing blood volume by mobilizing two hormones, antidiuretic hormone which gives us water and aldosterone which gives us sodium. In addition, it's giving us two major vasoconstrictors. And the vasoconstrictors are Angiotensin II itself and antidiuretic hormone. And these two major vassal constrictors then will cause constriction, increasing TP. Constriction of blood vessels and you'll get increase in the total perfuasis within the system. This will help on our preloads so that we are increasing venous return to the body to correct for low cardiac output or perceived low pressures. Okay, so that's what happens when we have low volume or we have low pressure, but what happens if you have an excess of volume? An excess of volume is an interesting kind of a problem because in an excess of volume means that you're taking in fluid faster than the body can get rid of it. And so this doesn't happen very often but it can become very critical. So for instance I had in my laboratory, I had a post doctoral student who had a kidney stone. And his wife was a nurse and she said, well, let's just drink a lot of water. And if you drink a lot of water, you can flush the kidney stone. And so she gave him a big jug of water and he started drinking the water quickly. And then she gave him a second jug of water and he drank that very quickly. And then he became disoriented, he was having trouble talking and even walking. And she became worried. And so she then brought him into the emergency room. What had happened to him? Well, what happened was he brought in so much volume, so much fluid, that he decreased the electrical concentration of the ions that were within the ECF. And by decreasing those ions, he decreased blood osmolarity to less than 280 milliosmolar. When he did so, then he changed the ion gradient across cells, such as neurons and muscle. And not only that he do that, but we now have an osmotic gradient where is much more dilute in the ECF than it is with the cells. So, the cells start to swell. And so he has a condition now where he can get swelling of the neurons, and they're trapped within the cranium, which is a bony cranium. And so, you can have loss of neurons. Let's look at what happened to the reflex loop. So with the reflex loop, we decreased the osmolarity of the blood. And by doing so, we will shut off ADH. So, ADH now is turned off. And when you turn off ADH, we have the same situation that we had with diabetes insipidus and that is that there's no aquaporin channels within the collecting ducts and within the distal convoluted tubule. And so therefore, you will pee out 18 liters per day, a very dilute urine. Not only that, the extra volume that was within the vasculature stretched the atria of the heart. And by stretching the atria of the heart, the cardiac muscles not smooth muscles, but cardiac muscles of the heart in atrial, these are endocrine cells. And so when they're stretched, they secrete a hormone and the hormone is called Atrial Natriuretic Factor or ANF. It works on the kidney and it works on the afferent arterial to cause them to dilate. So, it dilates the afferent arterials. And when that occurs, what happens to GFR? We increase GFR. And so, you increase then filtration across this region. So not only do we have no ADH, no aqua pouring channels in the collecting ducts, but we now the situation we were increasing the filtration across the glomerulus. So we're making more filtrate. We're making more urine. And by doing so then, it increases the water loss from the body. We lose water and we also lose some sodium. But predominantly, what we're worried about is decreasing the fluid volume from the body and this was the way that the body can compensate. The body can compensate, if it has enough time. If it doesn't have enough time, then this situation can become lethal. And in the Boston Marathon several years ago, they had three runners who had run the marathon. By the end of the marathon, they had gained weight. They had gained water weight. They drank so much water. That it was in excess of the amount of water that they lost by sweating. And so, what happened was they diluted down their osmolarity of their blood. And by doing so, they then put themselves in a position where the neuron started to swell and three of them died. And so this can become a very serious situation, it can be lethal. And so that brings us to our last topic and this is called diuresis. So diuresis is the increased loss of body water to the urine and it's where we have greater than one milliliter per minute of urine being put out from the body, and we can have three major ways in which this occurs. The first is water diuresis and that's what we were just talking about, and that is when you take in too much fluid. You're taking in an excessive water, then it decreases the osmolarity of plasma and, or it increases the blood volume. That decreases anti-diuretic hormone levels and your urine output increases. The second is where we have osmotic diuresis and this occurs in the condition of diabetes mellitus were the individual has such high circulating levels of glucose in the plasma. That when the filtrate then in the kidney, the amount of glucose that's in that filtrate maxes out all the transporters for glucose. We have saturated those transpires so the excess glucose stays within the renal tubule, it stays within the renal filtrate. It is osmotically active and holds water. And so as the filtrate goes through the tubule, then the water stays in the tubule and we had increase during the output. And the last are drugs, which are called diuretics and these drugs increase the lost of the body water. And primarily, they're acting by changing the reabsorption of sodium. These diuretics target specific parts of the renal tubule. So for instance, you can have a diuretic that inhibits that sodium, potassium, chloride transporter that's in the thick ascending loop of Henle and you can have diuretics that work only on collecting duct tubules where the sodium transporters are altered or inhibited. Under these conditions then, sodium stays in the tubule filtrate. And therefore, holds water. And under those conditions then, we have an increase loss of water from the body. There's also diuretics that you're all familiar with and that is every time you drink something that is caffeinated. So if you're drinking coffee or tea, the caffeine of the coffee affects a very mild effect, but it's on a transporter that's present within the distal convoluted tubule and that transporter is the sodium chloride transporter that's inhibited. And so then, you have a bit of an increase in urine output. So the urine output then, these diuretics. Diuretics are very useful for particular pathological conditions where we have someone, whose volume overloaded, then they give diuretics or we have someone with congestive heart failure. We have too much volume within the system and we want to reduce then after load. We give the individual diuretic and then they pee out the extra volume. So, what are our key concepts? So the first is, is that two-thirds of the body water is in the ICF, the intracellular fluid space and one-third is in the extracellular fluid space and that these two fluid spaces are within osmotic balance. That is water can move freely back and forth in most areas of the body. Secondly, the kidneys primary function is to maintain the body fluid volumes by regulating salt balance and it maintains the osmolarity of the body by regulating water balance. Third, reabsorption and secretion of water, and solutes is governed by gradients, and by secondary active transport. And four, hormones regulate osmolarity and that's the antidiuretic hormone when we have a condition where we have a very high osmolarity, that is an increase in sodium concentration in the blood. An antidiuretic hormone is secreted and we move water back from the filtrate across the kidney tubules, and into the blood. And secondly, the blood fluid volume is maintained by these hormones as well by antidiuretic hormone and aldosterone where sodium and water now are moving back from the filtrate to the body to increase blood volume. And by ANF, which is the atrial natriuretic factor, which causes us to lose water when we have an excess of water or a situation where there is too large of a volume within the system. And five, we have increased urine excretion above one milliliter per minute is called diuresis and that there are several causes of diuresis. One is an excess of water on an intake. The second is that we can have an osmotic condition, such as when we have too much glucose within the renal tubules due to diabetes mellitus. Or we have taken a drug such as a diuretic, which inhibits the movement of sodium across the renal tubules. And by inhibiting the movement of sodium, we then change the distribution of water across the renal tubules. The water then stays within the filtrate and will be excreted from the body. The diuretics of primarily inhibiting sodium absorption across the tubials. So, this ends our consideration of the kidney proper for both water and ion balance. And in the last two lectures when we're dealing with the kidney, we're going to talk about its role in acid-base balance. So, see you then. Bye, bye.
