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University of Toronto Scarborough
Biological Sciences
Jason Brown

SLIDE 2 Mammalian Kidney : Gross Anatomy  There are two distinct sections. The outer layer is the cortex. The change in the tissue you see is the medulla in the inner side.  The medulla leads to another structure called the renal pelvis, where the urine produced by the kidney initially collects.  Renal pelvis continues on to form ureter which takes the urine towards the bladder.  The kidney has a blood supply that comes through the renal artery, and the renal vein carries the blood away from the kidney.  The renal artery is one of the major branches coming off the aorta. The blood coming to the kidneys needs high pressure to allow filtration. SLIDE 3 Mammalian Kidney: Comparative Nephrology  Not all vertebrates have kidneys that look just like mammals.  In the pig kidney you can see the similarity in structure between itself and the human kidney in the previous slide.  If you look at other vertebrates like the snake and the fish, it’s different. They don’t have the compact kidney bean shape found in mammals; their kidneys tend to be longer diffused organs.  In the fish you can see long diffuse kidneys, and in the snake it starts to be a little more compact, and as it gets closer to mammalian kidneys it becomes more compact. This occurs in other types of organs as well because they become more compact as you move up from fish to reptiles and to mammals.  In the fish or snake kidney, there’s no distinction between the cortex and medulla. There’s no medulla in the kidneys of non-mammalian vertebrates so it’s unique to mammals. You might see a little bit of it in birds, but generally reptiles, amphibians and fish have only a cortex. SLIDE 4 Mammalian Kidney: Nephron: The functional unit of the kidney  A kidney is comprised of millions of nephrons; the nephrons together make a kidney.  There are two big parts to a nephron. The red is the blood supply, and the blue is the tubular components of the nephron.  Blood supply component (circulatory component) shown in red o So you have a renal artery bringing blood to kidney, it branches off into what’s called afferent arteriole which take blood away from the renal artery and bring it to the ball of capillaries called glomerulus. This is the site within the nephron where blood is filtered. o The vertebrate kidneys operate by filtration-reabsorption mechanism, meaning the first thing in the formation of urine is that there’s filtration which occurs at the glomerulus where fluid and solutes present in the blood plasma are forced under high pressure, to come out of the capillaries and enter the little space around them o The capillaries in the glomerulus come together to form the efferent arteriole, which supplies the kidney with blood. o The remaining blood supply is divided into 2 regions, the peritubular capillaries, and the vasa recta. o Peritubular capillaries are in the cortex and offer blood to the tubular components of the kidney in the cortex. o The vasa recta is the blood supply that comes down into deep parts of the kidneys and makes a U shaped turn, and the blood flows back towards the cortex and comes back to the renal vein right near the cortex-medullary junction. o In short, blood comes through the artery, through the afferent arteriole into glomerulus where filtration takes place, into efferent arteriole, through peritubular capillaries and vasa recta, and ultimately out through the renal vein.  Tubular component shown in blue o The area that surrounds the glomerulus is called the Bowman’s capsule. The little space you see here is where the first urine is formed by the filtration of glomerulus which accumulates in this capsule and then passes through other tubular component. o It first passes proximal convoluted tubule (close to glomerulus) and continues through and comes to the Loop of Henle, the long structure that descends way down the medulla of the kidney and makes a turn and heads towards the cortex and eventually forms the distal convoluted tubule, and then the urine is dumped into the collecting duct. o The proximal and distal convoluted tubule make up the cortex, and the Loops of Henle makes up the medulla. When we talk about non mammalian vertebrates having no medulla, it’s because they lack the loop of Henle. o On the bottom left you see the nephron of a reptile, there’s no Loop of Henle. This applies to reptiles, amphibians, fish, and to some extent birds as well. SLIDE 5 Mammalian Kidney: Nephron: The functional unit of the kidney  This is what the glomerulus looks like under a light microscope.  Surrounding the glomerulus is the Bowman’s space; this is the space where urine is accumulated in this space.  Surrounding that is all the cross sections of the tubules.  You can distinguish between the proximal and distal convoluted tubule. The proximal tubules appear to have a lumen (opening area in the inside) that’s dirty looking. Distal tubules have a clear clean opening.  The stuff you see inside the proximal convoluted tubules is the brush border. These are microvilli on the surface of these cells extending to the lumen.  What’s happening at these tubules is a tremendous amount of reabsorption. All the water and solutes that get filtered out of by the glomerulus are reabsorbed by the proximal convoluted tubules.  You need a large surface area to do this on the cells so they have microvilli on their surface forming the brush border which makes the lumen look dirty. The distal convoluted tubules lack this. SLIDE 6 Mammalian Kidney: Mechanism of Urine Formation – Glomerular Filtration (or Ultrafitration)  What’s happening at the glomerulus?  First, the formation of urine by vertebrate kidneys by filtration.  You can see the capillaries which make up the glomerulus and you can see the Bowman’s capsule surrounding the glomerulus. The fluid and solutes dissolved in the blood plasma are forced to cross the capillary wall and into the Bowman’s space.  This space is continuous with the tubules, where the fluid is carried off through the rest of the tubular compartments.  There are just 2 layers of cells separating the fluid inside the capillary (blood plasma) from the bowman’s space. o One is the endothelium, which is the single layer of cells surrounding the capillary. o The other is the cells that make the Bowman’s capsule itself. These cells are called podocytes. These podocytes are right on top of the endothelium layer.  The urine flows through the endothelium, through the podocytes, and makes its way into the Bowman’s space and then continues to proximal convoluted tubule.  What causes the fluid present from the capillaries into the Bowman’s space? o One is hydrostatic pressure which results from the fact that the fluid contained in the capillaries is under a lot of pressure due to the heart. o Hydrostatic pressure is high in capillaries, but low in Bowman’s space. Since there’s a hydrostatic pressure gradient, this favors the filtration of fluid from inside the capillaries and into the Bowman’s space.  Hydrostatic pressure in the glomerular capillaries is much higher than that in Bowman’s capsule  Favours filtration o There is another pressure that opposes filtration; this is called the Collod osmotic (oncontic) pressure. This is much higher inside the capillaries than in the Bowman’s space. o Present in the capillaries that can’t get to the Bowman’s space are the plasma proteins. All the proteins dissolved in the blood can’t get filtered out because they’re too large. So there’s a lot of proteins in the capillaries, and very little in Bowman’s space, the osmotic pressure of the capillaries is higher and it prevents water to be filtered from the capillaries.  Colloid osmotic (oncontic) pressure in the glomerular capillaries is much higher than in Bowman’s capsule  Opposes filtration o But, the hydrostatic pressure is much larger than the oncontic pressure, so there is net filtration and a net movement of water from the capillaries into the Bowman’s space.  So how might low blood pressure effect kidney function? o It might lower the gradient of hydrostatic pressure differences, and allow oncontic pressure to become more important and minimize filtration of fluid  Similarly if the oncontic pressure were to drop for example from someone who loses a lot of blood, the blood is lost with the proteins as well. Anything that causes these pressures to change can cause a drastic effect at the rate of filtration of water.  Generally, large amount of water is filtered. The amount of water that is filtered is much greater than the amount of water that is excreted. This vast majority of filtered water is reabsorbed by the tubular components which reabsorbed about 99% of what gets filtered at the glomerulus. SLIDE 7 Mammalian Kidney: Mechanism of Urine Formation – Glomerular Filtration  There are two types of filters present in the nephron which regulate which components found in the blood plasma are going to be able to appear in the urine.  There are two parts in the size-selective filter which is set up by the physical space between the cells in the glomerulus. o One is that there are extensions coming off the podocytes, these are called pedicles which are finger-like projections and they interdigitate with one another and form a size-selective filter. o The other part of the filter is that the endothelium that lines the capillaries is discontinuous or fenestrated or leaky. There are gaps between the endothelium cells. So we call this a fenestrated endothelium. This facilitates the movement of fluid and solute into the Bowman’s space or urinary space by having this leaky capillary. o These two together acts like a size-selective filter. Nothing larger than 14 nanometers can pass through this filter. So everything in the capillary can be filtered out with the exceptions of blood cells and plasma proteins. o Ions, sugars, amino acids, water can pass through this filter easily.  There is also a charge-selective filter in the middle which is the glomerular basement membrane. This is a layer of glycoproteins which are negative charged so they repel the negatively charged components of the blood plasma. One of the biggest negatively charged components of the blood plasma are plasma proteins. Many proteins in general are negatively charged.  This negative charge repels other negative charges and minimizes the protein components going out of the capillaries. o Negatively-charged glycoproteins repel plasma proteins  Ultimately, by the time hydrostatic pressure gradient is applied which overwhelms the oncontic pressure, the ultrafiltrate formed at the glomerulus contains everything found in the blood with the exception of blood cells and protiens.  The concentration of these things is the same as that you find in the blood, meaning the primary urine that’s formed and comes into the urinary space from the blood, is identical to blood in every respect except it doesn’t contain blood cells and proteins. It contains all the ions, the glucose, amino acids at the exact same concentrations you find in the blood, but the only thing you’ll not find is the blood cells and proteins.  If you do find blood in your urine, there’s a problem because it’s supposed to be kept out by the size selective urine so it suggests there’s something wrong with this filter.  The end result of glomerular filtration is primary urine, which is isomotic to blood and has similar [ ]s of ions and organic solutes (e.g., glucose) SLIDE 8 Mammalian Kidney : Mechanism of Urine Formation – Tubular Reabsorption  So in the glomerulus we’ve filtered out water, glucose, amino acids, vitamins etc. but we don’t want these things out. If you go to the doctor and there’s glucose in your urine there’s a problem.  We don’t want to be excreting these ions and water.  What occurs along the rest of the length of the nephron is reabsorption of what we filtered.  This occurs in the region of the nephron known as the proximal convoluted tubule.  Any blue arrows you see are reabsorbed by active transport. Any red arrows you see are reabsorbed passively thanks to active reabsorption of sodium.  The composition of primary urine is initially isosmotic to the blood so there is no favorable gradient for reabsorption and it can’t be done passively.  Generally, the main component of the active transport mechanism is that sodium is actively pumped out of the urine and back in the blood. The chloride, water, amino acids, glucose and vitamins are passively taken back up in a sodium linked manner.  The tubular fluid is the urine. The single layer of cells making up the wall of the tubule has sodium potassium ATPase. Sodium is actively pumped into the blood plasma which depletes the tubular cell of sodium. This permits the sodium in the tubular fluid to move into the cell by diffusion which gets pumped into the blood after.  There are many transporters present in the tubule cells which permit uptake of sodium and couple it with uptake of other things. Example sodium glucose co-transporter uses the gradient to allow sodium to enter the cell and couples it with the entry of glucose from the tubular fluid. After that the glucose diffuses back into the blood through glucose transporters.  Same mechanism occurs for amino acids. Similarly, the active transport of sodium drives the passive reuptake of chloride which passes between the cells to get into the blood. This occurs because of electrostatic interaction.  At the same time, you have ions moving into the blood plasma which creates an osmotic gradient, so water reuptake is favored. SLIDE 9 Mammalian Kidney: Mechanism of Urine Concentration  Now we’ve reabsorbed most of what we filtered out.  Now how does the urine get concentrated?  Terrestrial animals try to minimize water loss, so they want to take the urine that’s formed and concentrate it so you take back much of the water from the urine and leave behind a salty solution, so there’s a net loss of ions without a net loss of water.  The remaining urine after reabsorption may be isosmotic or hyposmotic. After urine has flowed through the proximal tubule you more or less have a diluted blood solution which might be isosmotic or hyposmotic, but it’s still not concentrated. o Once primary urine flows through proximal tubule, most water and solutes* have been reabsorbed; the urine that remains is isosmotic (or hyposomitc) relative to blood  The part of the nephron responsible for concentrated urine is the Loop of Henle. The loop of henle establishes the conditions necessary to produce concentrated urine.  It’s this component of the nephron only seen in MAMMALS means the other animals like fish, amphibians, and reptiles and some birds are unable to produce concentrated urine.  Example marine animals use salt glands, like the sea turtle and penguins.  The longer the loops of Henle are, the greater the concentration of the urine will be. In the figure on the right you see osmolarity of urine plotted against relative medullary thickness which is relative to the size of the kidneys so it takes into account the body size.  The little red line is showing plasma osmolarity of most mammals which is around 300-350 mOsM. The osmolarity of the final urine that’s produce is plotted in black dots. Some animals produce urine that’s not concentrated at all such as the beavers which produce urine that’s not much more concentrated than the blood. Why? Because they don’t have to worry about minimizing water loss because they’re surrounded by water.  The desert rat doesn’t have a lot of water in the environment and have very long loops of Henle and can concentrate urine 20x higher than the blood plasma. This means they can get rid of a lot of ions without losing a lot of water. SLIDE 10 Mammalian Kidney : Mechanism of Urine Concentration  How are loops of Henle (structure that makes up medulla of the kidney) able to set up these conditions?  The descending limb is different than the ascending limb in two regards. o The aquaporins that permit water to move through the cell are present only in the descending limb. There are none present in the ascending limb. There’s no capacity for water to escape or move in at the ascending side. o At the same time, transporters for sodium and chloride are only present in the ascending limb. This means we can only actively transport sodium out of the urine at the
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