Greetings. Today we have our last lecture in the urinary system. This lecture deals how the kidney function provides the homeostatic basis of electrolyte and fluid balance. Today we want to talk about how it regulates fluid balance within the body. That is, how we regulate the size of the fluid compartments of the body. Some of the things that we have to think about today will be to recall 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. This is because we're interested in how we generate an interstitial ionic gradient, the 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 consider the conditions that activate it. This is the system that coordinates 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. The first thing that you should think about is that as you go through your specific daily activities, you take in about 2 liters of food and fluid in a given day. 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 leaving the body and secondly the kidney needs to regulate ions. In particular we're interested in water, the amount of water that enters the body, and how the kidney can either conserve water for the body, or excrete water if it is in excess. The thing to remember is that the kidney can conserve water but it can't replenish water. What I mean by that is that as you go through your day, you lose water. You have a thirst mechanism which causes you to drink water. This brings water into the body, to replenish the water loss. 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 that only water that can be excreted by the kidney is the water loss that can be regulated. That means as we sweat, as we speak, if you defecate, you lose water from the body. It's not something that's regulated. Usually this is a very small amount of water versus the amount of water, total water, that's within the body. Only the water that the kidney's excreting in the urine is water that we can regulate. 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. The other thing to remember is that the osmolarity of the normal urine changes with the body's needs. If the body has an excess of volume, excess of fluid, you have dilute urine. If the body has a need for retaining water or has a dehydration state, then it needs to hold water within the body. You have a very concentrated urine. That's what is shown here on this table. At Bowman's capsule, we have 180 liters per day coming into the filtrate. It has an osmolarity of 300 milliosmolar. By the end of the collecting duct, 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. It can have of an osmolarity as dilute as 50 milliosmolar, or as concentrated as 1,200 milliosmolar. How in the world does this happen? In order to understand this, we have to look at the structure and the function of the nephron. 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, that the normal kidney generates by the juxtamedullary nephron. a standing osmotic gradient within the medulla. This gradient is 300 mOsM down to 1,200 mOsM. The way that it does this is by moving ions and water out from the renal tubule, that is, out from the filtrate. It does this in a very interesting manner. Let's follow how the structure works. We start then with our glomerulus. The glomerulus is surrounded by Bowman's Capsule. We have an afferent arteriole and we have the efferent arteriole. This is our normal structure. Then, from Bowman's Capsule We have the proximal convoluted tubule. The proximal convoluted tubule, descends into the medulla, and as it descends into the medulla, it becomes the thin loop of Henle. Ths is the descending thin loop of Henle. The tubule does a hairpin turn and comes back up to the cortex as the thick ascending loop of Henle. Eventually the tubule drains into the distal convoluted tubule. It then enters, drains into the collecting duct. That's what shown here. These are the major regions of the tubule. So first. the filtrate enters into Bowman's Capsule with an osmolarity of 300 milliosmolar. In the proximal convoluted tubule, we all know that reabsorption can occur. In fact this is the dominant site for reabsorption of fluid and ions. THat process occurs in an iso-osmotic manner. Note we have no change between the filtrate, at 300 milliosmolar, and the interstitium in the cortex which is 300 milliosmolar. But once the tubule descends down into the medulla then the epithelial cells are no longer freely permeable to ions. Water is able to move out of the filtrate and into the interstitial space. But ions are no longer able to freely move across the tubule epithelium. So, the sodium then that's within the descending Thin Loop of Henle becomes more concentrated. The concentration of sodium becomes higher and higher as water moves out from the tubule due to this extracellular gradient of 300 to 1200 milliosmoles within the interstitium. Water is moving down its concentration gradients through open aquaporin channels present within the luminal surfaces of these epithelial cells. Water moves across these cells and enters into the interstitium of the medulla. By the time we reach the bottom loop of the thin loop of Henle, the filtrate is very concentrated. It is able to equilibrate with the interstitial fluid space. The interstitial fluid is at 1,200 milliosmolar. The filtrate is at 1,200 milliosmolar. Here we have free movement of sodium and urea and water across these cells. The filtrate now is 1,200 milliosmolar. As the filtrate then starts to ascend back towards the cortex, it goes into the thick ascending loop of Henle. In a thick ascending loop of Henle, the epithelial cells no longer express aquaporin channels. There's no aquaporin channels on the luminal surfaces of these cells. Consequently, water cannot move across this epithelium. Water now is trapped within the tubular filtrate. But we have a secondary active transport of sodium, potassium, and chloride in this region. This co-transporter is moving sodium, potassium, and chloride out of the filtrate and into the interstitial space. It does so such that, by the time the filtrate enters back into the cortex in the distal convoluted tubule, it now is at 100 milliosmolar. We now have a hyposmotic filtrate. We had a hyperosmotic condition in the thin loop of Henle deep in the medulla, the deep region of the medulla. In the cortex, we now have a hyposmotic condition where the filtrate is at 100 milliosmolar. If nothing happens to the filtrate from this point on, then the urine that's expelled from the body will be 100 milliosmolar and that urine will be 18 liters per day. We do not change the osmolarity of the filtarte if we don't do anything to the epithelial cells in the collecting ducts, or in the distal convoluted tubules. If there are no aquaporin channels present in this region, then whatever is in the filtrate, will be expelled from the body as urine. That will be 100 milliosmolar solution and it will be 18 liters per day. There is, in fact, a pathological condition where the individual has diabetes insipidus. 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. We'll talk about it in just a few minutes. The other thing that we should think about 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. It should be diluting the amount of sodium and urea 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 descend down into the medulla from the juxtamedullary nephrons. These tubules have the capillary bed, the second capillary bed, descending with them. In the cortex, the peritubular capillary follows the tubule, and then once down into the medulla, the blood descends 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. Consequently, the blood is becoming higher and higher and higher in the concentration of sodium. Such that, by the time the blood reaches the hair pin loop deep within the medulla, the blood plasma is 1,200 milliosmolar. The blood is also able to pick up the urea in this region. Urea prevents the blood cells from shrinking. The blood cells pick up urea. The sodium gradient is only 600 mOsmolar. We have minimal shrinkage of the red blood cells, despite it's in a very hypertonic condition. As the blood returns to the cortex, it picks up water that is exiting the descending thin loop of Henle. So water leaves the descending thin loop of Henle, and is picked up by the blood. The blood of this capillary bed then returns to the cortex, and eventually leaves the kindey through the renal vein. This particular hairpin turn and the descending regions of blood flow within the medulla is so important it has it's own special name. The capllaries are the vasa recta. These are the vasa recta. Within the medulla that second capillary bed picks up all of the fluid and the sodium that's removed from the filtrate. Now once we're back into the cortex, the blood is 300 milliosmolar. The blood is isosmotic with the cortex interstitium. In the case where the body wants to make concentrated urine, then this distal region is regulated. What happens is that it's the body secretes the hormone anti-diuretic hormone or vasopressin. You all know about this hormone. 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 to bring new water back into the body. But you also secrete antidiuretic hormone. Antidiuretic hormone works on the distal convoluted tubule and in the collecting duct. Here, within these cells, it increases the copy number for aquaporin on the luminal surface of these cell. It's moves, it is mobilizing the aquaporin channels to the luminal surfaces of the cells. In the presence of aquaporin channels, water now can move across these cells. But we all know you have to have an osmotic gradient for the water to move. So for water to move from the distal convoluted tubule out into the interstitium, which is 300 milliosmoles, we have to have a gradient. We know that the gradient is present. Inside this tubule, the filtrate is 100 milliosmolar, so there is a gradient for water to move. It is 100 milliosmolar in the filtrate, 300 milliosmolar In the interstitium of the cortex. If you insert aquaporin channels, water moves from the filtrate into the interstitium of the cortex where it's picked up by the peritubular capillaries. In the collecting duct, when we insert the aquaporin channel, there is also an osmotic gradient. This gradient is 300 to 1,200 milliosmolars. Because the collecting duct goes straight through the medulla to terminate in the tip of the medulla at the beginning of the ureter. This delivers the urine directly into the ureter. The filtrate, the urine, is then picked up by the ureter and delivered to the bladder. So the osmotic gradient then is absolutely critical for concentrating urine. 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. In this particular region, we have what are called principal cells. These principal cells are the cells where the aquaporin channels are being mobilized. These are the epithelial cells and these cells have aquaporin channels are mobilized unto the lumenal surfaces by antidiuretic hormone. There is a second hormone called aldosterone, which is secreted by the adrenal glands. This hormone also acts on these principal cells. This particular hormone has essentially two functions. As you recall from the adrenal gland lectures, aldosterone is secreted by cells in the zona glomerulosum. Aldosterone is secreted in response either to high potassium within the blood or in response to angiotensin II. In response to these stimuli, then what happens is that aldosterone acts on the principle cells. It increases the number of sodium transporters that are on the luminal surfaces of these cells and increases the number of potassium transporters that are on these cell. In addition it increases the sodium-potassium ATPase located on their basal surfaces. Consequently, potassium is secreted. And so we have increased secretion of potassium and we have increased reabsorption of sodium. Sodium leaves the filtrate, moves across the cells, enters the interstitium, and is picked up by the peritubular capillary. Potassium leaves the blood, crosses the principal 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 body, but aldosterone also regulates the volume of the body. When low blood volume is perceived by the kidney, the aldosterone system is activated. Note that aldosterone moves sodium into the body, and when you have aldosterone present and antidiuretic hormone is 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. That's what is shown in this next diagram. This is our Renin-Angiotensin Aldosterone System or RAAS. In this system, [COUGH] excuse me, we have the plasma volume decreased. So plasma volume decreases. The kidney senses low plasma volume. When the kidney senses low plasma volume or low filtrate flow within the tubule, the macula densa are activated. They in turn cause the JG cells, the smooth muscle cells of the afferent arterial, to secrete renin. Renin is an enzyme that enters into the plasma. It cleaves a protein to form Angiotensin I. Angiotensin I is converted to Angiotensin II by another enzyme which is called ACE, Angiotensin Converting Enzyme. This enzyme is present in the lungs, but it's also present in the vasculature and many other tissues. Angiotensin II is the signal which will cause the hypothalamus to secrete antidiuretic hormone from the posterior pituitary. And antidiuretic hormone, as we said, works on the tubules to move those aquaporin channels, to insert them into the luminal surfaces of the CD cells, and we now have water that can move across the CD cells. Angiotensin II also causes the adrenal glands to secrete aldosterone. Aldosterone works on the collecting ducts, on the principal cells, and on the distal convoluted tubule principal cells. It will cause sodium to move into the body. We are moving sodium and water into the body with aldosterone and antidiuretic hormone. Blood volume increases. The 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 gives us two major vasoconstrictors. The vasoconstrictors are Angiotensin II itself and antidiuretic hormone. These two major vasoconstrictors cause constriction of smooth muscle, increasing total peripheral resistance (TPR). Constriction of blood vessels results in an increase the total pressure within the blood system. This will help to increase preloads. We are increasing venous return to the heart to correct for low cardiac output or perceived low arterial pressure. Okay, so that's what happens when we have low volume or low blood pressure. What happens if you have an excess of blood volume? An excess of blood volume is an interesting kind of a problem because an excess of volume means that you're taking in fluid faster than the body can get rid of it. This doesn't happen very often but it can become very critical. For instance I had in my laboratory, a post doctoral student who had a kidney stone. His wife was a nurse. She recommended that he just drink a lot of water. If you drink a lot of water, you can flush the kidney stone. 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. Then he became disoriented. He was having trouble talking and even walking. She became worried. And so she then brought him into the emergency room. What had happened to him? Well, what happened was he engested so much volume, so much fluid, water, that he decreased the electrolytes, the concentration of the ions, that were within the ECF. 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. Not only that he do that, but we now have an osmotic gradient where it is more dilute in the ECF than it is with the cells. The cells start to swell. He now has a condition where there is swelling of the neurons which are trapped within the cranium, a bony cranium. You can have loss of neurons. Let's look at what happened to the reflex loop. With this reflex loop, we decreased the osmolarity of the blood. By doing so, we will shut off antidiuretic hormone (ADH). So, ADH now is turned off. When you turn off ADH, we have the same situation that we had with diabetes insipidus. There's no aquaporin channels within the collecting ducts and within the distal convoluted tubule. Therefore, the body pees out 18 liters per day of a very dilute urine. Not only that, the extra volume that was within the vasculature stretched the atria of the heart. By stretching the atria of the heart, the cardiac muscles, not smooth muscles, but cardiac muscles of the heart in atria, these are endocrine cells. When they're stretched, they secrete a hormone. This hormone is called Atrial Natriuretic Factor or ANF. It works on the kidney nephrons.It works on the afferent arterioles causing them to dilate. So, ANF dilates the afferent arterioles. When that occurs, what happens to GFR? We increase GFR. You increase GFR and thereby filtration across this region. So not only do we have no ADH, no aquaporin channels in the collecting ducts, but we also have increased the filtration across the glomerulus. We're making more filtrate. We're making more urine. And by doing so then, water loss increases 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. This is how the body can compensate for excess water. The body can compensate, if it has enough time. If it doesn't have enough time, then this situation can become lethal. 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. It was in excess of the amount of water that they lost by sweating. What happened was they diluted down their osmolarity of their blood. By doing so, they then put themselves in a position where the neurons started to swell. Three runners died. So this can become a very serious situation; it can be lethal. That brings us to our last topic. This is called diuresis. Diuresis is the increased loss of body water to the urine. It is when we have greater than one milliliter per minute of urine being put out from the body. There are three major ways in which this occurs. The first is water diuresis. That's what we were just talking about. 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. This decreases anti-diuretic hormone levels and your urine output increases. The second is where we have osmotic diuresis. This occurs in the condition of diabetes mellitus were the individual has such high circulating levels of glucose in the plasma. When the filtrate enters the kidney, the amount of glucose that's in that filtrate maxes out all the transporters for glucose. We have saturated those transporters. The excess glucose stays within the renal tubule. It stays within the renal filtrate. It is osmotically active and holds water. As the filtrate goes through the tubule, then the water stays in the tubule lumen. We will have an increase in urine output. And the last are drugs, which are called diuretics. These drugs increase the lost of the body water. Primarily, they act by changing the reabsorption of sodium. These diuretics target specific parts of the renal tubule. For instance, you can have a diuretic that inhibits that sodium, potassium, chloride transporter in the thick ascending loop of Henle. Other diuretics 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 thereby, holds water. And under those conditions then, we have an increased loss of water from the body. There' are also diuretics that you're all familiar with. That is every time you drink something that is caffeinated. If you drink coffee or tea, the caffeine of the coffee or tea affects, a very mild effect, on a transporter that's present within the distal convoluted tubule. That transporter is the sodium chloride transporter. It is inhibited. Consequently you have a bit of an increase in urine output. So to change urine output then, these Diuretics are very useful. In particular pathological conditions where we have someone, who is volume overloaded, then they give diuretics. Similarly if someone has congestive heart failure, there is too much volume within the body. We want to reduce the after load. We can give the individual a diuretic. 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. The antidiuretic hormone is secreted when we have a condition of very high osmolarity, that is, an increase in sodium concentration in the blood. 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. Here antidiuretic hormone and aldosterone move sodium and water back from the filtrate to the body to increase blood volume. And by ANF, which is the atrial natriuretic factor, 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 circulatory system. And five, increased urine excretion above one milliliter per minute is called diuresis. There are several causes of diuresis. One is an excess of water 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 act to primarily inhibit sodium absorption across the tubules. This ends our consideration of the kidney proper for both water and ion balance. In the last two lectures on the the kidney, we're going to talk about its role in acid-base balance. So, see you then. Bye, bye.
