Greetings. So today we start our last of the organ systems and this is the urinary system. And what we want to talk about in today's specific lecture is that the structure of the kidney, which is the dominant organ within this system, is the one that generates urine, and then its homeostatic functions. This urinary system is the one which balances your water and your electrolytes within the body. It removes then the waste products that are fluid waste products. So let's see the learning objectives. The first will describe the structure of the kidney itself. Secondly, we'll talk about its homeostatic functions. This is the general overview of the kidney and its role in homeostasis. And third, we'll describe the structure of the kidney nephron. This is the actual filtering unit. This is the small structure that filters the blood. And then lastly, we'll explain the regional functions of this kidney tubule, which is a portion of the nephron. All right, so let's get started. So the first thing then is that the system has two kidneys and the kidneys are connected by ureters, which are tubules. So here are two kidneys, and they have tubules which are these ureters coming into the bladder and the bladder then is going to store the urine that's generated by the kidneys. And then from the bladder we will excrete urine to the outside world. The kidney's function is to filter the blood, and to remove that excess fluid and excess electrolytes, so this is its fluid balance and its electrolyte balance. The electrolytes that we're balancing is not only sodium and potassium and calcium and so forth, but also protons. That is, one of the kidneys major functions is to balance PH, and it works in conjunction with the respiratory tract. And we'll talk about its PH balancing in the next sets of lectures. Then the kidney is also to remove liquid waste products and that is as the tissues undergo metabolism, they generate waste products such as urea and this needs to be removed from the body. It's also a mechanism for excreting drugs, such as morphine or penicillin, and for getting rid of vitamins and other organic structures. And then lastly, the kidney has some very important endocrine functions. The first of these is that makes a hormone which is called erythropoietin, and it does so in response to hypoxia. So low oxygen tension causes the kidney to secrete erythropoietin. Erythropoietin works in the bone marrow to increase the production of red blood cells. The second of these is called renin, and the renin is actually an enzyme, it's a hormone that is secreted into the blood, but it is an enzyme. And it will start a cascade which will end up with two major vascular constrictors circulating within the blood stream. Renin is secreted in response to low blood pressure. So, the functions of this system are going to be to raise the blood pressure by raising blood volume. And the third is that the kidney activates vitamin D3. So the vitamin D3 is made as an inactive form from the skin in response to light, and then this moiety moves to the kidney where it's final activation. Vitamin D3 works in the GI tract to help you to absorb calcium across the epithelium of the GI tract. And then that calcium then is used for calcification of the bone. Okay, so what's our kidney structure? So in our body the kidneys are located at the small of your back and they are retroperitoneal. And as I say, we usually have two kidneys in a given individual, but there are individuals who have only one. The kidney has, on the outside aspect of the organ, there is a capsule, so it's an encapsulated structure. And on one side of the structure there's an indentation, and this indentation or concave surface is called the hilus. At the hilus the renal artery enters into the organ and the renal vein exits the organ. This is the location for the exiting of the ureter, which is sort of a funnel like structure which gathers all of the urine that is generated within the kidney and then transports it to the bladder for storage. And so this is occurring then here at this at hilar region. Within the kidney structure itself, the organ can be divided into two zones. There is the renal cortex, which is the outermost area, and this is shown here. And then an innermost area which is towards the center of the organ and this is called medulla. So we have a cortex and a medulla structure. Now the kidney houses about one million nefrons which are the little filtering units that will generate the urine. This is the place where blood will be filtered and then the fluid that's generated will be lost from the body by excretion through the ureter to the bladder to the urethra and out of the body. These filtering units are located partially within the cortex and partially within the medulla. And we'll talk about this regional distribution later in the lectures. Now the kidney structure of the nephron is shown here. And this stylized drawing shows the two major components to the little nephrons, or to the filtration units. The first is that we have a vascular component, and this vascular component is that we have a capillary bed, a little tuft of capillaries, which is called the glomerulus. This capillary is fed by an efferent arteriole, and the blood that's entering from the renal artery eventually makes it to the afferent arteriole. The afferent arteriole receives the blood, the blood goes into the glomerulus and then it exits from the glomerulus, or that capillary bed by an efferent arteriole. It then enters into a second capillary bed. And this is a portal system where we have two capillary beds which are connected by an arteriole. This is a unique structure, it's the only place in the body where this structure occurs, and it is important to the functioning of the actual nephron. The second component of this nephron is that we have a renal tubule, and the renal tubule is a blind-ended structure that surrounds the glomerulus. So this tubule comes up and it makes a little capsule, or cap, around the glomerulus. This cap, or capsule, is called Bowman's Capsule, and that's what's shown here. And then the renal tubule extends from that capsule and eventually the renal tubule then will drain into another duct, another tubule, which is called the collecting duct. Several nephrons will drain into a single collecting duct, and the collecting duct will gather the urine from these separate nephrons, and then that collecting duct will deposit the urine in the deep portion of the medulla at the beginning of the ureter. And then the urine is then drained from the organ. As we go along the renal tubules, the renal tubules changes in morphology and also it changes in function. So it's has a specific regions and we're gonna deal with each of these regions as we talk about the function of the kidney and how it generates urine. The first of these regions we've already described, and that is the Bowman's Capsule, which is the starting portion of the tubule, the blind-ended structure that completely surrounds the glomerulus. And distal to that we then have an area which is called the proximal convoluted tubule. The proximal convoluted tubule extends then into a very thin tubule which is called the Loop of Henle and this has both a descending portion. Then a central region which makes a hairpin loop and then an ascending portion which is called the thick ascending loop of Henle. So the loop of Henle actually has three components to it. A descending thin loop of Henle, the loop of Henle's hairpin turn, and then the ascending, or thick ascending Loop of Henle. And then we have in the distal to this we have what's called the distal convoluted tubule, and then this structure will drain into the collecting duct, and that's shown here. Now there are actually two different types of nephrons present within the human kidney. 90% of the nephrons that are in the human kidney are almost completely located within the cortex. They have very small amounts of the tubules, that is the loop of Henle, dipping down into the medulla. So these are predominantly located in the cortex, and are called the Cortical Nephrons, and that's 90% of the Nephrons in the human kidney. The second type of Nephron is called the Juxtamedullary Nephron. This Nephron is predominantly located within both regions so that it has in the beginning, we have our glomerulus and bomus capsule, and proximal convoluted tubule within the cortex, but the tubules are predominately descending deep into the medulla and then return back to the cortex for the distal convoluted tubule, and then of course draining into the collecting duct. So this particular nephron is called juxtamedullary, juxta meaning near or next to, the medulla. So the juxtamedullary nephron then is the nephron it has tubules that distend deep within the medulla. Only about 10% of the Nephrons of the human kidney are the Juxtamedullary Nephrons. But these nephrons are extremely important for establishing an osmotic gradient within the medulla. And this osmotic gradient extends from 300 million osmolar to 1,200 milliosmolar, deep within the medulla. This is within the interstitial tissue of the medulla itself. So we have a standing gradient, and the gradient is made up of sodium and urea. This particular osmotic gradient is absolutely critical for the kidney to be able to generate concentrated urine. And we'll discuss that in one of the later lectures. Now the kidney receives about 1 liter per minute of cardiac output. So the input then to the kidney of renal blood flow coming through the renal artery is 1 liter per minute. It's actually gonna be filtering the fluid phase of that blood, and then the fluid phase of the blood, as you all know, is called plasma. And plasma is 60% of the total blood, where the other portion is made up of the red blood cells. And other cellular components. So 600 milliliters, Of plasma per minute, then, are being delivered to the kidney for filtration. And within Bowman's capsule then, in any given day, we have 180 liters per minute of this plasma that will be filtered. And then by the time that you get to the urine. That is the final output of urine from the kidney, will only be putting out about a 0.5 -1.5 liters per day. And that means then that with each pass, we don't filter 100% of the plasma of the blood that's being given to that particular kidney. But in fact, we only take a fraction of it. And that fraction then is going to be used to generate urine, but the majority of that fluid, the majority of the water that's within that filtrate, will be moved back into the body and that way we don't have collapse of the cardiovascular system. And one of the things we’ll talk about in the next few lectures, then, is exactly how the kidney does this. How it’s able to first filter this material, and then move the majority of the fluid back into the body for use by the body. The other thing to notice, is that as we go through this renal tubule, we will change the osmolarity of the filtrate. So the filtrate coming across at the beginning of the tubule that is into Bowman's capsule, will have the same osmolarity as the plasma of the blood, and that is at 300 milli-osmole. By the time we get down to the urine, the final output from the body, the osmolarity of the urine can be variable and it can be as dilute as 50 millismoles but as concentrated as 1200 milliosmoles. And we're gonna discuss how it exactly this occurs. But the change in the osmolarity of the output, that is of the urine. Is matched to the needs of the body so when the body has excess volume or excess fluid we put out very dilute urine. But, when the body has insufficient amounts of fluid, or it has more of a dehydration state, then it would put out a very concentrated urine, and this is regulated by hormones within the body, and we'll talk about that in one of the later lectures. Okay, so let's look at this. I have drawn now your renal tubule so that we can go through and talk about the different regions, and sort of a general overview of how they're working. So the renal tubule then and its vasculature is diagrammed here. So coming into the glomerulus, we have the efferent arteriole. It comes into the capillary tuff which is the glomerulus and that's our first capillary. And then draining from that we have a second arteriole and that's the efferent arteriole. So the blood then is moving in this direction and from the efferent arterial it enters into a second capillary bed and that capillary bed the cortex is called the paratubular capillary, And that's the second capillary. So we have a portal system where we have two capillaries, the glomerulus, which is the first capillary in series with a second capillary and it's connected by the efferent arteriole. The second part of the structure is that we have our renal tubule and the blind end of the renal tubule is diagrammed here. And this It forms a cup-like structure called Bowman's capsule. From Bowman's Capsule, then, the tubule extends and the tubule extends all the way along. And the peritubular capillary parallels the extension of the tubule. And it's actually very closely opposed to the capillary all the way along the tubule. The first thing that's going to occur is you'll have a filtration event. And the filtration means that we will take some of the, plasma, the fluid from a plasma. It will cross the epithelial cells that are lining the capillary bed. And then the basal lamina, that is, of the capillary, epithelium, and the epithelial cells which are lining Bowman's Capsule. And then cross the epithelium of Bowman's Capsule, so that's our filtration barrier. And once it goes across this, then some materials are able to go across this filtration barrier. Cells are not able to go across it, large proteins cannot go across it. But small solutes such as glucose, amino acids, all of the salts, they're able to cross through this barrier, as well as water. And once they enter into Bowman's capsule, then, this is called the filtrate. And the filtrate, as it moves along the renal tubule, will be modified in some instances. And one of the modifications is that we can reclaim water. And we can reclaim salts, we can reclaim glucose, amino acids, from the filtrate and move it back to the blood space. And then we're moving back, leaving lumen of the tubule. Across the epithelium of the kidney tubule, and into the interstitial space, and then across that, and into the blood. So it's moving across all these barriers, but the peritubular capillaries closely oppose. So that once it gets across the cells that are lining the renal tubule, it's very quick diffusion to get it into the blood space. This is called reabsorption, all of the glucose, the amino acids will be reabsorbed in a normal kidney. The majority of the bicarbonate will be reabsorbed in a normal kidney. And much of water will be reabsorbed in a normal kidney, within the first section of the tubule, which is called the proximal convoluted tubule. Now there is also the ability of the kidney to move materials directly from the blood to the tubule, bypassing the filtration site. And then this is how a lot of the drugs which are organic compounds, such as morphine, epinephrine, norepinephrine. Things such as vitamins, can move directly from blood space and into the renal tubule. And this is called secretion, so secretion, then, is moving in the opposite direction to reabsorption. So secretion is moving from the blood space back into the tubial, and that's shown here. And then the final product, which is what's excreted from the tubule, is called the urine. So excretion, then, will be the amount that's filtered, minus the amount that's reabsorbed, plus the amount that is secreted. Now, a couple of terms, one is if the filtration rate of a substance is equal to it's excretion rate, then we have no net reabsorption. Because we can't actually measure reabsorption and secretion for any given, substance, what we talk about, then, is the net handling of the renal tubule. So if the filtration rate is equal to the excretion rate for a given substance, then there’s no net reabsorption, and there is no net secretion. If the filtration rate exceeds the excretion rate for a substance, then we have a net reabsorption, and if we have the filtration rate, which is less than the excretion rate, then we have net secretion. All right, so the important terms for you to to remember is first, filtration is the movement of the solute in the water from the blood into the lumen of the renal tubule. And this is how we generate our filtrate. Secondly, reabsorption is the movement of the solute and the water from the lumen of the renal tubule across the epithelium and across the interstitium, back into the blood. And third, secretion is the movement of solutes from the blood, directly into the renal filtrate, bypassing the filtration point. And fourth, excretion, then, is the removal of the solutes and the water from the body as urine. Okay, so what are our general concepts then? So first then the kidneys primary functions are to maintain the fluid volumes of the body by regulating the salt balance to maintain the osmolarity of the body. And by regulating the water balance, so we want to regulate the ionic balance of the body, it's electrolytes. And we want to regulate the amount of water that's in the body, that would regulate not only the osmolarity, but also the size of the fluid space. Secondly, we want to remove waste products, that is products that are generated by metabolism and to detoxify drugs. And that's done, again, by the renal tubule. Thirdly, the kidney will secrete hormones, this includes erythropoietin, which is in response to hypoxia. So this is absolutely required in order to generate red blood cells from the bone marrow. Also we have renin, which is secreted in response to low blood pressure that is sensed by the kidney. And we can have the activation of vitamin D3, which is critical for calcium homeostasis within the body. Fourth, the kidney contains million filtration units which are called nephrons, and that each nephron is fed by this portal system. So that we have an afferent arteriole coming into a glomerulus, which is our first capillary. Drained by an efferent arterial, which then drains into a second capillary bed. And we'll talk about this portal system, in detail, in some of the later lectures. And five, filtration occurs across that first capillary bed, the glomerulus, and that reabsorption and secretion occur along the renal tubule. And so then we're moving materials, then, from the renal tubule, either to the blood, or from the blood to the renal tubule. And this will alter the composition of the filtrate. And the final product which leaves the kidney is the urine. All right, so see you next time. And we'll talk some more in more detail about some of the functions of the kidney itself. See you then.
