Renal Physiology: The Collecting Duct PDF

Last edited: 4/15/2022 THE COLLECTING DUCT The Collecting Duct Medical Editor: Mariel Antoinette L. Perez OUTLINE I) REVIEW VII) VAS...

Last edited: 4/15/2022 THE COLLECTING DUCT The Collecting Duct Medical Editor: Mariel Antoinette L. Perez OUTLINE I) REVIEW VII) VASA RECTA II) OVERVIEW VIII) UREA RECYCLING III) INTERCALATED A-CELL IX) APPENDIX IV) INTERCALATED B-CELL X) REVIEW QUESTIONS V) PRINCIPAL CELL XI) REFRENCES VI) REABSORPTION I) REVIEW II) OVERVIEW NaCl reabsorption in early DCT via NaCl symporters Principal Cell Calcium reabsorption dependent upon PTH o maintain mineral and water balance o PTH: parathyroid hormone Intercalated A and B cells Aldosterone can cause sodium reabsorption and o Maintain acid-base balance potassium secretion o Helps keep body within homeostatic range Presence of ADH increases aquaporin-II expression o found in the late distal tubule and collecting duct o Allows water to flow into the blood o Note: o Decreases potassium levels  Intercalated A cells: acidic condition o Increases sodium levels  Intercalated B cells: basic condition o Increases water  increases volume There are also other cells that could be secreting drugs. o Increases blood pressure Toxins, and creatinine o Also, others such as ammonia, protons, bicarbonate THE COLLECTING DUCT RENAL PHYSIOLOGY: Note #5. 1 of 5  III) INTERCALATED A-CELL Responds to acidosis o Respiratory acidosis o Metabolic acidosis Scenario: there’s CO2 in the blood o In an acidosis, there is low pH = many protons (H+) o Very little bases to counteract the protons CO2 + H2O  H2CO3  H+ + HCO3– o Carbon Dioxide (CO2)  Found in our blood; moves into the cell, and combines with water to form H2CO3  Catalyzed by enzyme carbonic anhydrase o Sodium Bicarbonate (H2CO3)  Unstable; dissociates into proton and HCO3– o Protons (H+)  H-K-ATPase ATP-dependent pathway o Both ions are moving against their concentration gradients K+ goes into the cell H+ goes out of the cell  Body needs to secrete substances it doesn’t like Ammonia (NH3) o Can be excreted out into the urine where it combines with the protons to produce ammonium (NH4+) o Bicarbonate (HCO3–)  Can be pumped out of the cell into the blood via the HCO3–/Cl– transporter IV) INTERCALATED B-CELL Responds to alkalosis o Bicarbonate (HCO3–) o Respiratory alkalosis  HCO3–/Cl– transporter o Metabolic alkalosis HCO3– goes out of the cell The same pathway as intercalated-A cell, but flipped. o pumped out of the cell into the urine o Get rid of bicarbonate instead of the proton Cl– goes into the cell o Reabsorb proton into the blood instead of bicarbonate o Cl– will exit the cell via the chloride channels on the basolateral membrane pH, H+, HCO3– o Protons (H+) CO2 + H2O  H2CO3  H+ + HCO3–  H-K-ATPase o Carbon Dioxide (CO2) ATP-dependent pathway  Found in our blood; moves into the cell, and o Both ions are moving against their combines with water to form H2CO3 concentration gradients  Catalyzed by enzyme carbonic anhydrase K+ goes into the cell o Sodium Bicarbonate (H2CO3) H+ goes out of the cell into the blood  Unstable; dissociates into proton and HCO3– 2 of 5 RENAL PHYSIOLOGY: Note #5 THE COLLECTING DUCT  V) PRINCIPAL CELL Cells that maintain mineral and water balance Hypothalamus o Collection of neurons from the supraoptic nucleus o Axons move through from the hypothalamus to the posterior pituitary o When stimulated, it will release ADH (A) ANTIDIURETIC HORMONE (ADH) / VASOPRESSIN released whenever the plasma osmolality is changing can work in the late distal tubule and collecting duct (1) Osmolality (3) Process (i) Hypertonic ADH binds to the vasopressin receptor (on the principal cell) in the collecting duct of the kidneys,  osmolality o Will activate the secondary messenger system  solutes (e.g., Na+,Cl–), H2O o Activates G-stimulatory protein  GTP  hypertonic o Becomes active and activates another effector (ii) Hypotonic enzyme, adenylate cyclase  osmolality Adenylate cyclase converts ATP  cAMP  solutes (e.g., Na+,Cl–), H2O cAMP activates Protein Kinase A o Phosphorylates the proteins on the vesicles: (iii) Isotonic  Pre-synthesized vesicles with proteins and  solutes = H2O channels (aquaporins) o Activates aquaporin-II (2) Stimulus  Fuses with the cell membrane High plasma osmolality  There’s aquaporin-III and aquaporin-IV in the o ADH wants to have more water in the blood, which basolateral membrane means that the plasma osmolality was initially high Water goes out aquaporin-II, then passes through Angiotensin-II aquaporins III & IV  goes into the blood o To increase blood pressure  increases blood volume, and increases blood pressure o Also reaches normal plasma osmolality  isotonic THE COLLECTING DUCT RENAL PHYSIOLOGY: Note #5. 3 of 5  VI) REABSORPTION (1) Calcium reabsorption (3) Sodium reabsorption Dependent on the presence of PTH 65% reabsorbed in the PCT (2) Water reabsorption 25% reabsorbed in the ascending limb of Loop of Henle 5-6% was reabsorbed in early DCT 65% reabsorbed in the PCT Remaining 4-5% is reabsorbed depending on the 15% reabsorbed in the descending limb of Loop of Henle presence of aldosterone 20% reabsorbed in DCT Water reabsorption dependent on aquaporin-II, which is dependent on ADH o ADH = more water reabsorption o ADH = less water reabsorption VII) VASA RECTA Peritubular capillary network present within the deep part of the medulla Known as the “Counter-Current Exchanger” As the ascending limb goes up, it pumps the solutes out  pulling water out of the descending limb o Solutes: Mg2+, Ca2+, K+, Cl–, Na+ Vasa Recta gets saltier as we go down o due to the Counter-Current Multiplier Mechanism o Water wants to flow out towards where it’s salty  Obligatory Water Reabsorption o NaCl is pulled into the Vasa Recta Processes reverses when vasa recta turns and goes up o Water now wants to go back inside o NaCl is being pushed back inside as we go up Two functions o Prevents rapid removal of sodium chloride  When blood enters, it’s 300 mosm  When blood leaves, it’s 325 mosm This implies that the vasa recta kept a bit of NaCl with it to prevent rapid removal o Carries Oxygen  Cells depend on oxygen  Vasa recta also delivers oxygen and nutrients VIII) UREA RECYCLING A lot of urea still gets lost in the urine, but some are recycled (1) Process (2) Purpose of Urea Urea gets reabsorbed in the last part of the collecting duct o after all the water has been reabsorbed, [urea] starts (i) To make concentrated urine increasing (ii) To contribute to medullary gradient It then moves out of the collecting duct and into the medullary interstitium via facilitated diffusion o It gets reabsorbed in the ascending limb of Loop of Henle o At the same time, urea accumulates outside  This helps makes the medulla more salty 4 of 5 RENAL PHYSIOLOGY: Note #5 THE COLLECTING DUCT  IX) APPENDIX Figure 1. Summary of The Collecting Duct X) REVIEW QUESTIONS XI) REFRENCES 1) Presence of ADH increases the expression of which of the following? a) aquaporin-I Le T. First Aid for the USMLE Step 1 2020. 30th anniversary b) aquaporin-II edition: McGraw Hill; 2020. Jameson JL, Fauci AS, Kasper DL, Hauser SL, Longo DL, c) aquaporin-III Loscalzo J. Harrison's Principles of Internal Medicine, Twentieth d) aquaporin-IV Edition (Vol.1 & Vol.2). McGraw-Hill Education / Medical; 2018 Marieb EN, Hoehn K. Anatomy & Physiology. Hoboken, NJ: 2) Which of the following is incorrectly matched? Pearson; 2020. a) Water : aquaporin-II Boron WF, Boulpaep EL. Medical Physiology.; 2017. b) Calcium : ADH Guyton and Hall Textbook of Medical Physiology. Philadelphia, PA: Elsevier; 2021. c) Sodium : aldosterone 3) Which of the following is true? a) Principal cells maintain mineral and water balance b) Intercalated-A cells respond to alkalosis c) Both a and b are true d) Neither a nor b are true THE COLLECTING DUCT RENAL PHYSIOLOGY: Note #5. 5 of 5