63-Medullary Gradient Dilution-Notes_2024 PDF

WSUSOM Medical Physiology Rossi-Renal Physiology Page 1 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Medullary Gradient: Dilution and Concentration of Tubular Fluid Learning Objectives: 1. Loop of Henle A. Identify the unique or...

WSUSOM Medical Physiology Rossi-Renal Physiology Page 1 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Medullary Gradient: Dilution and Concentration of Tubular Fluid Learning Objectives: 1. Loop of Henle A. Identify the unique organization of components of the loop of Henle and the vasa recta B. Recognize the transport properties of the specific segments of the loop of Henle and the need for these components to function together to permit counter current multiplication C. Be able to describe the concept countercurrent multiplication and distinguish from countercurrent exchange 1) Describe in detail the way the descending and ascending limbs of the loop of Henle work in concert to establish the medullary interstitial gradient 2) Describe the flow through the vasa recta and its properties that make it uniquely important for the medullary gradient 3) List the three main factors that determine the steepness of the medullary gradient and how they influence it D. Recognize the transport mechanisms utilized by the cells of the thick ascending limb of Henle and how inhibition of transport may alter concentrating and diluting mechanisms E. Describe the transport modalities involved in urea handling by all segments of the loop of Henle and how they relate to the medullary gradient and urea recycling WSUSOM Medical Physiology Rossi-Renal Physiology Page 2 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Loop of Henle Tubular fluid at the end of the proximal tubule as it enters the descending limb of Henle is isotonic. Since 80% of glomerular filtrate is reabsorbed in the proximal tubule this leaves (125 ml/min - [0.8 * 125 ml/min]) = 25 ml/min entering the descending limb. The figure shows that 20% of the H2O and 75% of the solute is reabsorbed in the loop. Since more solute than water is reabsorbed by the loop then the reabsorbate is hypertonic, hence the ISF in the medulla is hypertonic the TF leaving the ascending limb is hypotonic The loop of Henle is important in setting up the conditions so that we can generate either a dilute or concentrated urine. For the purposes of “simplicity” we will again assume all nephrons are alike. That is NOT true and should be in the back of your mind. There are subtle differences, but the principles of function are alike. E.g.,The juxtamedullary nephrons (JM) have longer loops of Henle than the superficial ones and therefore contribute differently to the medullary gradient (see urea below). Components of the medulla: corticomedullary gradient ISF osmolality ­ progressively from cortex to inner medulla descending limb permeable to H2O and some solutes TF equilibrates with ISF TF osmolality ­ as fluid goes down limb thick ascending limb impermeable to H2O reabsorbs solute osmolality of TF thus ¯ as fluid goes up the loop 200 mosm gradient between TF in thick ascending limb and ISF WSUSOM Medical Physiology Rossi-Renal Physiology Page 3 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Properties of the loop of Henle – countercurrent multiplication 1. Descending limb is permeable to water and to some solutes 2. Thick ascending limb actively reabsorbs solute but is impermeable to water. 3. There is a gradient of 200 mosm/kgH2O between TF in the ascending limb and ISF at any horizontal level 4. TF flows in opposite directions in descending (down) and ascending (up) limbs and multiplies this small 200 mosm/kgH2O gradient to a larger corticomedullary gradient, a process called countercurrent multiplication. 5. Thus, the ISF osmolality progressively increases from the cortex (300 mosm/kgH2O) to the inner medulla (»1200-1500 mosm/kgH2O in humans). Countercurrent Multiplication – to help conceptualize outgoing water heats up the incoming water before it even reaches the heat source the small temp gradient at level is multiplied by the countercurrent flow The loop acts much like a coil of a radiator that heats water more efficiently than a straight pipe. A. On the left, the temperature of the fluid entering the pipe is 30°C. Energy is transferred from the heat source to the fluid and raises the fluid temperature to 40°C. The energy source must continually emanate heat to raise the temperature 10°C. B. On the right, the temperatures entering and exiting the coiled pipe are the same. As the fluid flows down the left coil, heat is transferred from the hotter fluid flowing up the right coil, thereby “pre-warming” the fluid in the descending coil. The heat source at the turn need only provide enough energy to raise the temp 5°C (use of less energy). Note that the temp at the turn of the coil (100°C) is much higher than that exiting on the right, 40°C. Penguins are warm blooded and use the same countercurrent mechanism principles to keep their blood warm after going through their feet. Think of the kidney when you watch “March of the Penguins.” Just as less energy is used to heat the water (or penguin blood), less O2 and ATP energy is used in the medulla by having a countercurrent in the loop of Henle. WSUSOM Medical Physiology Rossi-Renal Physiology Page 4 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Follow along in lecture or on stream over the next few pages…. to see how the medullary gradient is formed. Flow occurs Countercurrent Multiplication Follow the arrows in the diagram above and the “slow motion” on stream or lecture… Food for thought: Note that the longer the loop the more of a gradient one can generate. Some desert animals can generate a gradient as high as 6000+mosm/kg H2O!!! Patients who have damage to the inner medulla and long loops of Henle will have problems generating a high urine osmolality, as in sickle cell disease. WSUSOM Medical Physiology Rossi-Renal Physiology Page 5 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid 1.Assume all the fluid starts with the same Summary of the stop-flow osmolality and the flow is stopped. example. 2. Since the ascending limb actively resorbs solute but is impermeable to H2O, the solute in TF the ascending limb decreases…and water cannot follow. 3. The solute is transported into the ISF. Since the descending limb is permeable to H2O and some solutes, solute from the ISF enters the descending limb and some H2O leaves the lumen of the descending limb. 4. The TF osmolalities increase and equilibrate at »200 mosm/kg H2O higher than in the ascending limb at that level. 5. Permit flow to occur…the fluid column shifts …then stop the column again. 6. Repeat the active solute transport form the ascending limb into the ISF with equilibration with the descending limb. 7. Repeat steps 5 and 6 until the gradient goes from 300 mosm/kg H2O entering the descending limb to 100 mosm/kg H2O solute exiting the ascending limb and the tip of the loop at 1200-1500 mosm/kg H2O (This is not exactly precise as we shall see that the deepest medulla is actually more urea.) 8. As a result, the 200 mosm gradient across the descending and ascending limbs is multiplied to a gradient from cortex to deep medulla from 300 mosm to 1200 mosm or more. The process of countercurrent multiplication is more dynamic. TF flow does not stop as in the illustration. Consider also the ability of the cells in the medulla to survive and function in a milieu with such high osmotic pressures and low O2! Corticomedullary Gradient Loop of Henle makes TF hypotonic (thick ascending limb = diluting segment) makes the ISF in medulla hypertonic sets up the gradient for concentrated urine to be made Final Uosm and urine flow rate (V) depend on proper function of the loop of Henle (dilution) amount of H2O reabsorbed downstream from loop in collecting duct (concentration) steepness of the corticomedullary gradient Uosm range: 50 - 1500 mosm/L (in humans) WSUSOM Medical Physiology Rossi-Renal Physiology Page 6 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Urinary Dilution. TF is diluted in the ascending limb (~100 mosm/kg H2O). Since the distal tubule and collecting duct are able to reabsorb additional solute, the TF may become even more dilute by the time it is finally urine (~50 mosm/kg H2O). Urinary Concentration. The active reabsorption of solute by the loop without H2O reabsorption permits the formation of the corticomedullary gradient. This gradient is of key importance for the TF to become concentrated as it passes through the collecting duct under the influence of antidiuretic hormone (to be discussed shortly). The three factors that influence the steepness of the corticomedullary gradient: 1. Rate of solute reabsorption by the thick ascending limb of Henle 2. Tubular fluid flow rate through the loop of Henle 3. Rate of blood flow though the vasa recta (By “steepness” I mean the difference osmolality from cortex to medulla.) CM Gradient: Factor 1 Steepness is directly related to rate of TALH solute reabsorption ¯ solute reabsorption ® ¯ gradient ­ solute reabsorption ® ­ gradient The steepness is directly related to the rate of solute reabsorption in the thick ascending limb (TALH). The figure is a simplified rendition. It shows Na+ and Cl- transported into the medulla. Urea is particularly important at the very tip of the loop (inner medulla), but is also present in smaller quantities in the outer medulla. The main transporter actually transports 1 + - Na+:1 K :2 Cl but the NET effect is NaCl reabsorption since the K leaks back out into the TF (the ICF [K+] is >> TF [K+] and the Nernst potential for K+ still applies here. K+ is present in the the medullary ISF and in higher concentrations than in ISF elsewhere where Kisf ~ Kplasma. Drugs that inhibit the NaK2Cl transporter (also known as NKCC2) will prevent the formation of an optimum gradient…among these are commonly used diuretics, such as furosemide. WSUSOM Medical Physiology Rossi-Renal Physiology Page 7 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid CM Gradient: Factor 2 Steepness inversely related to TF flow rate ¯ flow rate ® ­ corticomedullary gradient ­ flow rate ® ¯ corticomedullary gradient PARADOX: ¯¯¯ flow rate ® ¯ gradient due to insufficient solute delivery to TALH to be transported to ISF to make it hypertonic enough The steepness of the corticomedullary gradient is inversely related to TF flow rate through the loop of Henle. Consider that if the TF flow is slower, this permits more time for the transporters to reabsorb the solute. If the flow is speeding past, it would be like fishing in Niagra Falls…the transporters cannot get a “bite” on the solute. The paradox is that at REALLY SLOW flow rates eventually leads to the driving force for secondary active transport to dissipate. Not enough solute is delivered to the transporters (fishing in Lake Huron with only 5 fish in the whole lake). Thus, the gradient actually DECREASES with very slow flow. (This is seen in some diseases such as hepatorenal syndrome.) WSUSOM Medical Physiology Rossi-Renal Physiology Page 8 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid CM Gradient: Factor 3 Simplified rendition of vasa recta. Real latex casts of renal vasculature including vasa recta of various species. Remember that the tubules of the loop of Henle are surrounded by the vasa recta which are spatially organized to help maintain the gradient the tubules set up. Steepness inversely related to vasa recta blood flow ¯ flow ® ­ corticomedullary gradient ­ flow ® ¯ corticomedullary gradient ¯¯¯ flow ® ¯ gradient due to lack of nutrients to support active solute transport Countercurrent Exchange vessels arranged to facilitate exchange of nutrients etc. and preserve the gradient The steepness of the corticomedullary gradient is inversely related to the blood flow rate through the vasa recta, which are the peritubular capillaries in the medulla. Again, decreased flow through these vessels increases the steepness of the gradient UNLESS the flow is so slow that nutrients and oxygen are limiting for the tubular cells that blood vessels supply. The reabsorption of Na is secondary active transport and requires the maintenance of low Na+ in the ICV by the Na,K ATPase…lots of energy (ATP) is needed. Recall that the blood that flows through the vasa recta has already passed through the glomerulus and given oxygen and nutrients to sustain more proximal portions of the nephron. Characteristics of the blood in the vasa recta/fluid in the interstitium: - the hematocrit may be as high as 70-75% - pO2 is ~20-40 mmHg (normal is 100 mmHg) - the osmolality is 1200-1500 mosm/kg H2O at the tip WSUSOM Medical Physiology Rossi-Renal Physiology Page 9 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid This is a very inhospitable environment for the cells of the tubules and for the red blood cells (RBCs) going through the vasa recta. RBCs shrink to about 70% normal size going down into the medulla and then re-expand to 100% size going up due to urea transport in the RBC. Think what happens to an RBC with sickle hemoglobin? High osmolality and low oxygen cause polymerization of sickle hemoglobin ® sickling ® deformed RBCs ® clogged capillaries ® decreased blood flow ® inability to maintain a corticomedullary gradient ® infarction of then medulla. Think of the renal medulla as a parfait that is carefully constructed to have layers. This takes energy. Disruption of the energy supply to the medulla, damage of the tubules, or changes in the blood flow through the vasa recta can disrupt the gradient. Depending on the degree of changes or injury the steepness of the gradient may be decreased or it may be obliterated (so that the osmolality will be similar to plasma throughout). Your parfait becomes a slurpy! Vasa Recta Relationship to Tubules When we talk about the medullary interstitial gradient… note how close the blood vessels and tubules are in the medulla. There is scant interstitial space; densely packed tubules and blood vessels in a specific 3D arrangement to facilitate the generation AND the maintenance of the gradient. (See Blackboard for colors). Clinical tidbit: The 3D tubular arrangement can become deranged in diseases that infiltrate the medullary interstitium, such as interstitial nephritis (e.g., as a result of an allergic reaction to drugs like penicillin, ibuprofen, etc.). In these cases the tubules are not only subject to inflammatory damage but are “pushed apart” by cells such as lymphocytes and macrophages that invade the interstitium. The ability to concentrate the urine and other Normal Interstitial Nephritis tubular functions can be severely impaired. WSUSOM Medical Physiology Rossi-Renal Physiology Page 10 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid The vasa recta are not really capillaries. The vasa recta in the outer medulla have what are called pericytes composing their walls. These actually have musculature and can contract. The vasa recta deeper in the medulla (inner medulla; bottom of slide) are thin and have fenestrations that permit easy exchange of materials from the interstitium to the vessel lumen. Solute Reabsorption by TALH Na,K ATPase on basolateral membrane KEY impermeable to H2O (IMPORTANT) Na and Cl move down their electrochemical gradient from TF to ICF K moves by secondary active transport into ICF K leaks back out into TF (or ISF, not shown) via a K channel Cl is transported to ISF Na moves to ISF via Na,K ATPase Na is also reabsorbed in exchange for H (minimal carbonic anhydrase on apical membrane) The lumen Å voltage also drives cations from TF to ISF (Na, K, Ca, Mg) between the cells The figure shows the cell of the TALH (thick ascending limb). Transporters are shown on one cell only, but are on both. The key elements are listed in the slide. Some HCO3- that escapes the proximal tubule is reabsorbed here: H+ generated within the cell is secreted in exchange for Na+ at the apical membrane and the HCO3- is transported into ISF. The H+ that is secreted reacts with HCO3- in tubular fluid, but unlike the proximal tubule there is much less carbonic anhydrase on the apical membrane. WSUSOM Medical Physiology Rossi-Renal Physiology Page 11 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid “Loop diuretics” inhibit TF dilution AND concentration (furosemide, bumetanide, etc.) inhibiting solute reabsorption in TALH - ¯ dilution of urine inhibiting solute reabsorption in TALH - ¯ gradient inhibiting solute reabsorption in TALH - ¯ concentration (as we shall see) Loop Diuretics. Called this because they act on the loop of Henle by inhibiting the Na,K,2Cl co-transporter (furosemide, bumetanide). These drugs are filtered and secreted by the proximal tubule organic base transporter so very high concentrations of these diuretics are present in the TF at the TALH (20-30 x >>> than in plasma). By inhibiting solute reabsorption, these diuretics permit the solute to remain in tubule lumen, hence the dilution of the tubular fluid as it moves up the TALH is impaired. Since less solute is being transported into the cell and thence to the interstitial fluid, the gradient is less steep. Decreasing the steepness of the corticomedullary gradient will impair (not necessarily obliterate) the ability to concentrate the urine. Thus, loop diuretics prevent both the maximum dilution and maximum concentration of urine. Diseases. The NaK,2Cl co-transporter, the K+ channel on the apical membrane and the Cl- channel on the basolateral membrane are sites where mutations cause a group of hereditary diseases called Bartter’s syndrome. These individuals have multiple electrolyte abnormalities (K+, HCO3-), volume issues leading to low blood pressure, and inability to concentrate the urine. Learn more about these next year. WSUSOM Medical Physiology Rossi-Renal Physiology Page 12 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Just a reminder that not all tubules are created equal. The deep juxtamedullary nephrons have LONG loops of Henle whereas the superficial nephrons have loops that only extend to the outer medulla. The thick ascending limb is only in the outer medulla, not the deepest part or inner medulla. We will see how the LONG loops generate even a greater medullary gradient with UREA. Urea: What is its role? Urea excretion is necessary for removal of nitrogenous waste. Urea is freely filtered at the glomerulus reabsorbed by the proximal tubule secreted by o proximal straight tubule o descending limb of Henle o thin ascending limb of Henle reabsorbed by the inner medullary collecting duct (deepest part) This occurs by urea transporters (UT1, UT2, UT4. Urea is synthesized in the liver and freely filtered at the glomerulus. Excretion of urea is necessary to get rid of nitrogenous waste, the end product of protein breakdown. Urea reabsorption in the proximal tubule is thought to be via paracellular pathways. In other tubule segments, urea moves by facilitated transport. This means the transport is passive but it requires a transporter. Much of the urea in the kidney recycles. This permits it to contribute to the concentration gradient in the deepest part of the medulla. WSUSOM Medical Physiology Rossi-Renal Physiology Page 13 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid The important take home message of this slide is that the medullary gradient is NOT uniform in composition. A common misconception is that as the medullary gradient becomes more concentrated in the deeper areas there is a proportional change in all this solutes. This is NOT the case. Two points are important: 1. The osmolality in the outer medullary interstitium has proportionately more ionic solutes. 2. The greater osmolality in the deepest part (inner medullary interstitium) is made up of nearly equal parts urea and ionic solutes. DO NOT memorize numbers! DO remember that the outer medullary gradient is mostly ionic whereas the inner medullary gradient is about 50:50 ions and urea. (Note: Boron and Boulpaep made the boxes for urine PINK and the interstitium YELLOW…should have been the other way around!) WSUSOM Medical Physiology Rossi-Renal Physiology Page 14 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Distal Tubule and Collecting Duct Uosm and V depend on amount H2O reabsorbed downstream from TALH H2O reabsorption controlled by vasopressin (= ADH “antidiuretic hormone”) action on principal cells of the collecting tubules/ducts PC = principal cell (light cells): handles water & NaCl IC = intercalated cell (dark cells): handles H+ and HCO3- We will come back to the distal convoluted tubule later. Now we will skip to the collecting tubules and ducts that contain the principal cells that have receptors for antidiuretic hormone, also known as vasopressin. When vasopressin is present, the collecting tubules/ducts become permeable to H2O and H2O can be reabsorbed from the TF to form concentrated urine. When vasopressin is absent, the collecting tubules/ducts are impermeable to H2O, and H2O remains in the TF to form a dilute urine. Minimum ADH (Vasopressin) = minimum Uosm, maximum V distal tubule and collecting duct IMPERMEABLE to H2O no H2O reabsorbed in distal tubule or collecting duct solute reabsorption/secretion in distal tubules and collecting duct further decreases TF osmolality since usually solute reabsorption > secretion Minimal Uosm < 100 mosm/L Maximal V = 20 ml/min (28 L/day) When antidiuretic hormone (ADH) is very low or absent, the principal cells of the collecting tubules and ducts are impermeable to water. Since some solute is also reabsorbed in the collecting tubule the osmolality of TF which emerges from the TAHL and distal tubule can be diluted to as low as ~40-50 mosm/kg H2O. WSUSOM Medical Physiology Rossi-Renal Physiology Page 15 of 15 Medullary Gradient: Dilution and Concentration of Tubular Fluid Maximum ADH (Vasopressin) = maximum Uosm, minimum V distal tubule and collecting duct PERMEABLE to H2O H2O reabsorbed in distal tubule o TF flow ¯ from 20 ml/min to » 6 ml/min o TF osmolality ­ from 100 to 300 mosm/L H2O reabsorbed in collecting duct as TF equilibrates with ISF (hypertonic medullary gradient) TF flow ¯ from 6 ml/min to 0.35 ml/min (500 ml/day) Max Uosm » 1200 - 1500 mosm/L Minimal V ~0.5 L/day In the presence of ADH, the principal cells are permeable to H2O due to the insertion of aquaporin 2 into the apical membrane. Average Ranges for Humans V averages 1 ml/min or 1.5 L/day Minimum V » 0.35 ml/min or 500 ml/day if ECV contracted and GFR ¯ can be as low as 0.2 ml/min or 300 ml/day (but there are consequences) Maximum V » 20 ml/min or 28 L/day if ECV expanded and GFR ­ can be as high as 40 ml/min or 58 L/day Uosm ranges from 50 to 1200 mosm/L It is readily apparent that the range of urine flow (V) is very broad. This permits the loss of large volumes of excess fluid that is ingested but also conservation of water during severely dehydrated states.