Lecture 16: Excretion PDF

LECTURE 16: Excretion SLIDE 1: Today we will continue our study of osmoregulation. This time focusing mostly on the process of excretion by the kidneys. SLIDE 2: What challenges do organisms face that results in this process of excretion? We have to understand that organisms need to remove their n...

LECTURE 16: Excretion SLIDE 1: Today we will continue our study of osmoregulation. This time focusing mostly on the process of excretion by the kidneys. SLIDE 2: What challenges do organisms face that results in this process of excretion? We have to understand that organisms need to remove their nitrogenous waste while at the same time, maintaining a suitable volume and concentration of the extracellular fluid that we discussed? SLIDE 3: In addressing process and the responses to these challenges, we will tackle the following learning objectives. We\'ll compare how nitrogenous waste is removed from different types of organisms. We\'ll look at the processes involved, such as filtration, reabsorption, and secretion, talking mostly about the directionality of movement of any particular substance. We\'ll look in detail about the structure of the nephron, the functional unit of the mammalian kidney. We\'ll look at the key steps in urine formation, looking at what forces contribute to creating urine. We\'ll revisit our principles of osmosis and diffusion to explain water and ion movement in the kidney. We\'ll then apply knowledge of normal functions of the various nephron regions to create scenarios that could result in an increase in urine production. Finally, we\'ll identify the steps in generating this salt gradient that is necessary for the nephron to function properly SLIDE 4: Where do we begin? We need to remember that in order to regulate the volume and the concentration of ECF, you can modulate 2 things: The first is water intake, which we discussed as we detailed the mechanism of thirst. Second, we can regulate water output, which is mostly regulation of excretion. That is where our focus will be now. SLIDE 5: First, what is excretion? It is the removal of nitrogenous wastes that result from the metabolism of proteins and nucleic acids, both nitrogen containing. This is separate from elimination, which is the removal of digestive waste. Because nitrogenous wastes break down into toxic ammonia, they must be treated differently. Let's first have a look at the three different forms of nitrogenous wastes that can be produced, and how they are adapted to the animal's that produce them. What we see is that there is a set of compromises that animals make in their choice of nitrogenous waste production. SLIDE 6: All three forms (ammonia, urea, and uric acid) need to be eliminated from the system. However, they vary in toxicity, solubility, and energy requirements to produce it. First, we see highly toxic ammonia, that is very water soluble and requires very little energy to produce. I should mention that due its high toxicity, it must be diluted and removed with a large volume of water and can therefore be very dehydrating. Let's now look at uric acid. Uric acid is not toxic at all, is not very water-soluble and does not require much water to remove at all. However, it is very energetically costly to produce. Let's contrast those nitrogenous wastes with urea, which is intermediate in all of those properties. It is moderately toxic, usually when in high amounts, somewhat water soluble, and requires a modest amount of water in which to be disposed. Its production requires a moderate amount of energy. You can see the choice of nitrogenous waste an animal excretes is a balance of different factors. How then are each of these nitrogenous waste products adapted to the animals that use them? We see ammonia excreted primarily by aquatic organisms the fish. The threat of water loss is not high with them therefore they can excrete the large amounts of water along with the ammonia that is necessary. Urea is primarily used by the mammals including us. Terrestrial animals can produce urea with the necessary input of energy and through mechanisms adapted to minimize water loss, and they excrete urea while maintaining the proper water balance. We see uric acid being the primary nitrogenous waste produced by egg laying animals, the reptiles including birds as well as egg-laying mammals. With the advent of the amniotic egg brought on a challenge of how the developing young can dispose of their nitrogenous waste while within the egg. Because of its low solubility and tendency to precipitate, uric acid solves that challenge. You can see that it is worth the high energy investment to protect the developing young. We know that each of these nitrogenous wastes tend to exist in some form in all different organisms, however the majority of the waste products is in the form that we discussed. SLIDE 7: Before we focus on the vertebrate kidney as the main organ of excretion, let's recognize that all animals are faced with the challenge of excretion. Different structures have evolved in different groups to remove nitrogenous wastes and balance fluid levels. In flatworms, the planaria, we see a network of dead-end tubes, or flame bulbs, that make up protonephridia and function in excretion, and osmoregulation. In earthworms, we see a metanephridia system with tubes in each segment of the work, that functions in excretion and osmoregulation as well. In the insects, we see tubes call Malpighian tubules functioning in excretion and osmoregulation. The Malpighian tubules of mosquitos are often studied as a model organism in renal physiology due to their great effectiveness. They are responsible for the rapid processing of protein-rich blood meals with each mosquito bite. Much of what we know about ion transport across our kidney tissues comes from these studies. SLIDE 8: We saw that each of these excretory systems are based on a tubular system. This is the case in the vertebrate system as well. The basic function of all excretory structures is seen here. If we boil excretion down to its primary function, we see it is a progressive refining of a filtrate. This is best accomplished via a tube. In this case, it is blood containing the nutrients that enters the tube. Upon initial filtration, we now have the filtrate. The challenge is that before excreting the filtrate that contains the nitrogenous wastes that we want to get rid of, we need to make sure we don't also remove substances and extracellular fluid that we need to retain. Therefore, different segments of the tube perform different functions so that materials that are slated for removal remain within the tube while materials necessary to the organism are reclaimed. We call these functions reabsorption and secretion. It's important to recognize that reabsorption refers to the movement of fluids and materials from the tube to the blood, being reclaimed by the body. And secretion means movement from the blood to the tube, materials then are destined for removal. This is a delicate balance that we are going to study here in the vertebrate kidney. SLIDE 9: We will now focus on the mammalian kidney for the remainder of our discussion. Let's look at its anatomy. First, in all vertebrates, we see that kidneys, or kidney tissue, is located in the dorsal part of the body. We call that retroperitoneal, behind the peritoneum, and it\'s a paired organ. Surgeons often reach the kidney via an incision in the back. It is that far back behind our abdominal cavity. Kidneys have a characteristic bean shape and have a blood supply via a renal artery and renal vein, bringing and removing blood from the organ. For reasons that will become clear shortly, approximately one third of the cardiac output of the heart passes through the kidney. The kidney is drained by a structure called a ureter, which is a tube that carries the newly formed urine from the kidney to the bladder. Where does the tubular arrangement of the kidney come into play? SLIDE 10: At the gross anatomical level, we see a formed bean shaped organ when we look at the kidney. But that kidney is made up of a million tubes -- the nephron, one of which is seen here. Within the kidney, these 1 million or so nephrons are interspersed with blood vessels, arranged in such a way that fosters exchange that will result in the production of urine that does not remove any more water than is necessary. It's important to note there that kidney can be divided into two regions -- the cortex, seen here as the outer part of the kidney, and the medulla -- or the inner, middle part of the kidney. In mammals, the nephrons extend from the cortex into the medulla, and back to the cortex again. This arrangement is incredibly important in the accomplishment of the role of creating hyperosmotic urine by kidney. SLIDE 11: We will now focus in on one nephron because it is important to understand how this works in order to understand how the mammalian kidney is able to accomplish its main goal -- to remove nitrogenous waste within urine that is hyperosmotic to the blood. This means that as the filtrate travels through the tube, it is carefully balanced with solutes and fluid. To gain this understanding, let's first identify the different components of the nephron -- each with different properties and different functions. Blood enters the nephron through a ball of capillaries called the glomerulus, which is surrounded by Bowman's capsule. It is here that the nonselective filtering of blood occurs and filtrate results. After the filtrate leaves Bowman's capsule, it enters the proximal distal tubule which leads into to a unique part of the nephron called the loop of Henle, which has a distinct descending limb and ascending limb. Leaving the ascending limb of the loop of Henle, filtrate enters the distal convoluted tubule which empties into the last part of the nephron, the collecting duct. Alll of the collecting ducts of all of the nephrons of each kidney merge at the renal pelvis, seen here, and empty into the ureter for removal. SLIDE 12: If we stretch that nephron out and oversimplify what it may look like, we get something like this. This type of image reinforces the names of each segment. The loop of Henle is certainly a loop. And the convoluted tubules appear convoluted, rather than straight. The proximal tubule is nearer to the glomerulus starting point than is the more distant, distal tubule. SLIDE 13: Let's start to look at each of the individual steps of urine formation in the simplified diagram first. Here we see the initial filtration of the blood occurring as we said in the capillaries of the glomerulus. This is nonselective filtering, basically anything in the blood that is small enough to pass through the capillaries, does and enters into the filtrate. Large substances like proteins, for example, are filtered out. Another thing to remember is that the movement of blood at this point was powered by the contraction of the heart as it was delivered through the arterial system. The filtrate then enters the proximal tubule. Here is where most, about 75%, of water is reabsorbed, remember being reclaimed by the body. In addition to water, some nutrients and ions are reabsorbed here as well. The loop of Henle is responsible for setting up the osmotic gradient to influence further reabsorption of water. We will look at that process in more detail shortly. More selective reabsorption and secretion occurs in the distal tubule. Here we see more fine tuning of materials toward the end of their passage through the nephron. Finally, filtrate enters the collecting duct which receives hormonal input to regulate water reabsorption to balance volume and concentration of the extracellular fluid. Note that the glomerulus, Bowman's capsule, and both tubules are confined to the cortical region of the nephron while the loop of Henle and the collecting duct extend into the medulla. SLIDE 14: Let\'s have a closer look at each of these segments of the nephron. The first step occurs in Bowman\'s capsule. There are cells here that are involved in the filtration step that occurs. Specialized cells called podocytes, are in contact with the blood that arrives in the glomerulus and essentially build a porous membrane at the interface of the blood created here, this connection between blood and the porous membrane on the other side. On that side is the lumen of the nephron into which those materials that pass through Bowman\'s capsule will land. Remember, large materials like proteins or red blood cells will not pass through while water and small nutrients will. SLIDE 15: Let's look at the filtration step in a bit more detail. Blood arrives at the glomerulus from the afferent arteriole branching off of the renal artery. Let's remember the forces that dictate the movement of fluid at a capillary bed. The blood arriving at the nephron is under approximately 60mmHg of hydrostatic pressure, just like we saw that happens at the capillary beds. There\'s also a high concentration of protein in the circulatory compartment that will bring some water out of Bowman\'s capsule back into circulation by osmosis. This is called the glomerular colloid osmotic pressure. This pressure moves fluid from Bowman\'s capsule back into circulation. This opposes the hydrostatic pressure, that moves fluid from the circulatory compartment into Bowman\'s capsule. Because you push liquid into Bowman\'s capsule, Bowman\'s capsule responds to that. There\'s a pressure pushing back towards the capillary. That\'s Bowman\'s capsule pressure. This is here, we see that it\'s about 18 millimeters of mercury. You have at the end, one pressure pushing one direction for filtrate to be formed, and two pressures - Bowman\'s capsule pressure and the colloid osmotic pressure, making liquids or fluids go back into circulation. The difference between all of that might be something like the original 60 hydrostatic pressure minus18, minus 32 is a net filtration pressure that results then of 10 millimeters of mercury, which means that some of that plasma will get filtered, and the net movement will be towards Bowman\'s capsule. The blood is coming in the kidney and due to those different pressures, osmosis, the heart beating and Bowman\'s capsule pressure, only a small residual pressure stays there and allows little by little some of that extracellular fluid solution to get into Bowman\'s capsule, and that\'s how filtration is accomplished. SLIDE 16: Now that the blood has been filtered, let\'s shift to the pH balance and osmoregulatory function of the nephron. This filtrate or urine gets refined as it travels through the nephron. Remember the nephron is responsible for creating and excreting hyperosmotic urine. Being hyperosmotic to the blood to prevent dehydration. We see there that when blood enters the nephron at the glomerulus in the cortex, it\'s at about 300 milliosmols per liter. By the time it is excreted, it is about four times that, about 1,200 milliosmoles per liter. How does this urine get so concentrated? How does it happen? SLIDE 17: When the proximal tubule receives the filtrate, water, some solutes and nutrients are reabsorbed, and some materials are secreted. Remember most of the reabsorption of water that occurs in the nephron occurs here. In addition, we can see the likely mode of transport of each of the nutrients and materials here. We see bicarbonate being actively reabsorbed, as well as some nutrients. And potassium in this case, being passively reabsorbed, while things like protons and ammonia being secreted into the proximal tubule. It\'s the movement of protons and bicarbonate that can affect pH balance. Some are moved actively as we saw while some diffuse. With the reabsorption of bicarbonate and secretion of protons, again, we can see the pH levels are adjusted. Some toxic material can be secreted, meaning that they are moved into the nephron and are now destined for excretion as they are now continuous with the outside of the body. Upon passage through the distal tubule, the filtrate remains isosmotic to the blood. Therefore, water and salt must have been reabsorbed in equal quantities resulting in decreased volume. We call this isotonic reabsorption, which results in stable osmotic potential of the filtrate at this point. It is the movement of materials out of the proximal tubule that drives the water reabsorption that follows it. SLIDE 18: Next the filtrate arrives at the loop of Henle. This is the critical segment in which the osmotic gradient is set up that results in the production of the hyperosmotic urine. It does so by the selective reabsorption of salt. How is this possible? First, we must understand the different properties of the tissue of the loop of Henle. The loop's permeability varies along it. The descending loop is of consistent thickness and is not very permeable to salt. However, water can cross it. The ascending limb, however, has a lower thin portion and an upper thick portion. The ascending loop is unique in that it is not permeable to water, a rarity in biological tissues. But it is permeable to salt. What we see happen in the loop of Henle is that salt is actively pumped out of the thick ascending limb. This salt lands in the interstitium of the kidney. Water then moves out of the descending limb as the solute is moved out. It's important to recognize that energy is used in the form of ATP to actively pump that salt out of the ascending limb. Knowing that, it makes more sense why water is leaving the descending limb. It is because of the solute concentration difference between the nephron and the interstitium that was set up by the active pumping of salt into the interstitium. This mechanism is known as countercurrent multiplication. While this is similar to the countercurrent mechanisms that we have seen in past -- in oxygen diffusion, thermoregulation, for example -- in that it is used to maintain a gradient, salt in this case, it differs in that energy is first invested to create the gradient in the first place, the multiplier. The result here is that water and salt are reabsorbed and therefore not too much extracellular fluid is lost. We see that the osmolarity of the interstitial fluid is huge at the bottom and not so high at the top. A vertical concentration gradient has been created. SLIDE 19: Let's take a closer look at countercurrent multiplication as it is a novel strategy. What we see here is the osmolarity of the filtrate compared to that in the interstitium all along the nephron. We see both increase greatly as the filtrate descends into the medulla of the kidney, and then decrease as it makes it way out to the cortex, before increasing again on its final descent through the collecting duct. If we just focus on the osmolarity of the filtrate, this looks pointless. However, what we know happens during the travel through the loop is that total volume was decreased, achieving the goal of minimizing water loss during excretion. This is accomplished by the different properties of epithelial membrane of the loop of Henle While filtrate passes through the descending loop of Henle first, we are going to look at what is happening at the end of the ascending loop first, as it is this portion of the loop of Henle that sets up the conditions for the other consequences and allows us to make sense of what is happening. Here, we see that the transport epithelium is impermeable to water, and is actively pumping out salt, sodium chloride. As we mentioned this salt lands in the interstitium surrounding the entire nephron. Because of its impermeability of water, water remains in the ascending loop. However, the walls of the descending loop are permeable to water and water leaves the descending loop, following that salt from ascending loop. We can now see how osmolarity is increased at the hairpin turn of the loop. Salt then leaves the nephron, is reabsorbed, through the now-permeable walls of the thin lower portion of the ascending limb passively as its concentration is still higher compared to the interstitium. However, as more salt is enters the interstitium, its concentration exceeds that remaining in the nephron, and salt is now actively transported out the thick segment against its gradient. And we are back where we started, with the nephron investing energy (and lots of it!) to set up that concentration gradient to reabsorb water. Whle the kidney invests energy in osmoregulation, it is only at the thick portion of the ascending loop of Henle that it is used. Remember back to the fish and we see a trend of energy investment into salt and water balance in a variety of organisms and we can see its importance. And we can see here, specifically, in the mammalian kidney that it is a key component of the central goal of reducing water loss during excretion. SLIDE 20: If we further break down this strategy of countercurrent multiplication, we can see while it is a continuous process, we can dissect out two separate processes. The first we\'ll call the single effect. Here, we can see and understand how the active movement of salt into the interstitium surrounding both the ascending and descending limbs of the loop of Henle drives the movement of water out of the descending limb. What then would result can be considered a horizontal gradient, if there were no other factors at play. That is what we can see here with descending ascending limb and salt being actively pumped out between the two, but water cannot the ascending limb, so the water leaves the descending limb, which is permeable and that dilutes that solution around. We see a dilute solution in the ascending limb and a more concentrated solution in the descending limb, as a limited amount of salt can enter into the descending limb. SLIDE 21: What happens next? How does this horizontal gradient produce concentrated urine? To see that, we have to put it into the context of what we just learned about the loop of Henle and we can understand how countercurrent multiplication will actually transform the single effect and the horizontal gradient into a vertical gradient. Let's set up the situation where we have this filtrate entering the loop roughly isosmotic with blood, so \~ 300 mOsm, seen here as an inactive nephron in stage 1. Next, in stage 2, those ion pumps are now active, moving salt out of the ascending loop. This decreases the salt concentration in the ascending limb, while increasing it in the interstitium and descending limb, as we discussed is the result of a single effect. We now remember that nothing is static in the kidney as the filtrate is continuously filtered by Bowman's capsule and its flow is powered by the heart. So everything will be pushed down the tube as new filtrate is generated. This is what we see in stage 4, as new filtrate arrives at \~ 300mOsm and again is pushed to the bottom of the loop of Henle. So the 400 here goes to the bottom and pushes to the top of the ascending limb, this 200 mOsm less concentrated filtrate. Due to the constant filtration, the single effect doesn\'t stop as more filtrate enters. You could imagine that in one instant, there is still a single effect of pumping ions from the right back in the medulla or the descending limb. But now it pumps ions on a slightly different situation than at the beginning. At the beginning it was 300 everywhere, but now it\'s 200 at the top, 400 at the bottom. So it can pump kind of more at the bottom and less at the top, still pumping more ions at all times, and those ions will be loaded again into the interstitium. As the sequence continues due to the constant flow, what you do is you start to concentrate urine to the bottom of the loop. Because we said here, it\'s permeable to salt and water, what is generated is actually an entire vertical gradient of salt in the medulla. The single effect alone generates a horizontal gradient, but this coupled to the flow of urine during the constant filtration, that urine pushes the more concentrated filtrate generated into the descending limb towards the bottom, therefore generating a vertical gradient that then spreads into the medulla. What we saw then is the combination of the single effect coupled with the constant flow allows the takes advantage of the active pumping of salt out the ascending limb to remove as much water as possible from the filtrate as it is concentrated down its passage into the medulla of the kidney. SLIDE 22: As filtrate exits the loop of Henle, almost all (about 95%!) of reabsorption has occurred and the filtrate is once again isosmotic with the blood. What then occurs in the distal convoluted tubule? Here we see some fine tuning of solutes. The distal tubule aids in selective reabsorption of potassium, calcium, and a bit of salt. It will also, like the proximal tubule, contribute to pH regulation. In addition, we see the effect of hormonal regulation here in the distal tubule on sodium reabsorption This hormone is a mineralocorticoid, which is called aldosterone. This hormone is partially responsible for urine concentration and water/salt retention. We will talk about its regulation and exact effect at a future time. SLIDE 23: The filtrate then reaches the last bit of the nephron, which is the collecting duct. The collecting duct is where most of osmoregulation will happen due to the hyperosmotic interstitial fluid because the collecting duct goes down the exact same gradient as the descending loop of Henle goes through. You see that urine goes through an increasing concentration gradient. Of course, as we saw in the descending loop of Henle then, there is a potential for reabsorbing water. Here in the collecting duct, this reabsorption is under regulation by a by a hormone. In essence, the collecting duct decides whether to use that increasing salt gradient to reabsorb water or not and that is controlled by antidiuretic hormone, also sometimes called vasopressin. The name antidiuretic hormone, however, is more fitting as it implies its function -- diuresis is the production of urine, so antidiuretic opposes that production of urine. How is that possible, where does the urine go? Water is reabsorbed. We can see this is a useful tool to both monitor and address dehydration levels. If you\'re dehydrated, we see ADH is secreted and it will boost reabsorption of water by the collecting duct, resulting in a decrease of urine. SLIDE 24: How is that accomplished? Remember those osmoreceptors in the hypothalamus that sense a change in hydration levels? They will respond to an increase in blood osmolarity. When it rises above a set level, the osmoreceptors will stimulate the release of ADH hormone from a gland associated with the brain, the posterior pituitary gland. ADH then will act on the cells of the collecting duct and stimulate the movement of aquaporins into the transport epithelium that will increase the amount of water to go pass out of the collecting duct and be reabsorbed by the blood, returning the blood osmolarity to its normal point SLIDE 25: Let's see how ADH acts at the cellular level. We see the lumen of the collecting duct here. On the left, we have the blood and capillaries of the medulla. What you have is that if ADH is present, released by the posterior pituitary gland, those aquaporins that are normally sequestered within the cells are brought to the surface of the cell and allow water to flow. First, into the collecting wall, and then out into the interstitium. The effect of this then is to decrease the amount of water within the nephron so that less is excreted with the urine. SLIDE 26: We cannot end our discussion of urine formation in mammals without discussing the fate of urea, the primary nitrogenous waste product. Urea is produced in the liver and transported to the kidney via the circulatory system. Urea is passed into the nephron, destined for excretion. However, in the process, it contributes to the solute gradient, along with salt that we discussed in detail. Here we see that urea is present in the collecting duct from which most is excreted. However, excess urea diffuses into the interstitial fluid of the medulla, increasing the solute gradient for more water to follow and be reabsorbed. Urea then cycles back into the nephron at the ascending limb of the loop of Henle and proceeds through the nephron in a recycling process before ultimately being excreted.