Renal Physiology. April, J. Mohan, PhD. Lecturer, Physiology Unit, Faculty of Medical Sciences, U.W.I., St Augustine.

Transcription

1 Renal Physiology April, 2011 J. Mohan, PhD. Lecturer, Physiology Unit, Faculty of Medical Sciences, U.W.I., St Augustine. Office : Room 105, Physiology Unit. References: Koeppen B.E. & Stanton B.A. (2010). Berne & Levy Physiology. 6th Edition. Mosby, Elsevier. Marieb, E. & Hoehn, K. (2010). Human Anatomy & Physiology. 8th Edition, Pearson, Benjamin Cummings. Costanzo L.S. (2006) Physiology. 3rd Edition, Elsevier, Saunders. Stanfield, C.L. & Germann W.J. (2008). Principles of Human Physiology. 3rd Edition, Pearson, Benjamin Cummings. Hall, J.E. (2011). Guyton and Hall Textbook of Medical Physiology. 12th Edition, Elsevier, Saunders. 18 April,

2 Physiology Objectives 1. Derive the normal blood ph value using the Henderson- Hasselbalch equation, and give the normal range. 2. Outline the important points relating to the principal buffer systems of the body. 3. Predict the effects of altered concentrations of carbon dioxide and bicarbonate ions on the blood ph values. 4. List the common causes of acid-base imbalance. 5. Explain the physiological bases for acid-base imbalance. 6. Briefly describe the effect of carbonic anhydrase inhibitors and other diuretics on acid-base balance and bicarbonate reabsorption in the nephron. 7. Describe the renal compensation for acid-base disturbance. 8. Briefly describe the respiratory compensatory mechanisms for acid-base disturbance. Acid-Base Balance ph affects all functional proteins and biochemical reactions ; ph = - log 10 [H+] Normal ph of body fluids Arterial blood: ph 7.4 Venous blood and Interstitial Fluid: ph 7.35 ICF: ph 7.0 Alkalosis or alkalemia: arterial blood ph >7.45 Acidosis or acidemia: arterial ph < April,

3 ph of arterial blood : ph 7.4 Acid-Base Balance Can be calculated with the Henderson-Hasselbalch equation : ph = pk + log [ HCO3 - ] [ CO 2 ] = 0.03 x P CO2 where : pk = 6.1 [ HCO3 - ] = 24 mmol/l PCO 2 = 40 mmhg to convert to concentration, multiply by the solubility of CO 2 (0.03 mmol/l/mmhg) Acid-Base Balance ph = pk + log [ HCO3 - ] [ CO 2 ] = 0.03 x P CO2 ph = log 24 mmol/l x 40 ph = log 20 = April,

4 Acid-Base Balance pk = 6.1 for the reversible chemical reaction : K1 H 2CO3 H+ + HCO3 - K2 K1 = rate constant for forward reaction K2 = rate constant for backward reaction K = equilibrium constant = K1/K2 i.e. ratio of rate constants pk = - log 10 K ; characteristic value for a buffer pair e.g. weak acid : less dissociated low K; high pk strong acid : more dissociated high K; low pk Acid-Base Balance Most H + is produced by metabolism Phosphoric acid from breakdown of phosphorus-containing proteins in ECF Sulfuric acid from metabolism of sulfur containing aa s (methinione, cysteine, cystine) Lactic acid from anaerobic respiration of glucose Fatty acids and ketone bodies from fat metabolism H + liberated when CO 2 is converted to HCO 3 in blood Also from ingestion Salicylic acid (aspirin overdose) 18 April,

5 Acid-Base Balance volatile acid Acid-Base Balance Concentration of hydrogen ions is regulated sequentially by : Chemical buffer systems: rapid; first line of defense Brain stem respiratory centers: act within 13 min Renal mechanisms: most potent, but require hours to days to effect ph changes 18 April,

6 Acid-Base Balance Strong acids dissociate completely in water; can dramatically affect ph Weak acids dissociate partially in water; are efficient at preventing ph changes Strong bases dissociate easily in water; quickly tie up H + Weak bases accept H + more slowly HCI H 2 CO 3 Efficient at preventing ph changes (a) A strong acid such as HCI dissociates completely into its ions. (b) A weak acid such as H 2 CO 3 does not dissociate completely. Figure 26.11; Marieb & Hoehn, April,

7 Chemical Buffer Systems Chemical buffer: system of one or more compounds that act to resist ph changes when strong acid or base is added : mixture of weak acid & conjugate base OR weak base & conjugate acid : 1. Bicarbonate buffer system 2. Phosphate buffer system 3. Protein buffer system How? By binding to H+ when ph drops and releasing them when ph rises Bicarbonate Buffer System Mixture of H 2 CO 3 (weak acid) and salts of HCO 3 (e.g., NaHCO 3, a weak base) The most important ECF buffer (phosphate also a buffer in ECF, but not to same extent) 18 April,

8 Bicarbonate Buffer System If strong acid is added : HCO 3 ties up H + and forms H 2 CO 3 HCl + NaHCO 3 H 2 CO 3 + NaCl strong acid weak base weak acid salt ph decreases only slightly, unless all available HCO 3 (alkaline reserve) is used up HCO 3 concentration is closely regulated by the kidneys ~ 25 meq/l Bicarbonate Buffer System If strong base is added : It causes H 2 CO 3 to dissociate and donate H + H + ties up the base (e.g. OH ) NaOH + H 2 CO 3 NaHCO 3 + H 2 O strong base weak acid weak base water ph rises only slightly H 2 CO 3 supply is almost limitless (from CO 2 released by respiration) and is subject to respiratory controls 18 April,

9 Phosphate Buffer System Action is nearly identical to the bicarbonate buffer Components are sodium salts of : Dihydrogen phosphate (H 2 PO 4 ), a weak acid Monohydrogen phosphate (HPO 4 2 ), a weak base Effective buffer in urine and ICF, where phosphate concentrations are high HCl + Na 2 HPO 4 NaH 2 PO 4 + NaCl strong acid weak base weak acid salt NaOH + NaH 2 PO 4 Na 2 HPO 4 + H 2 O strong base weak acid weak base water Protein Buffer System Intracellular proteins are the most plentiful and powerful buffers; plasma proteins are also important Protein molecules are amphoteric (can function as both a weak acid and a weak base) When ph rises, organic acid or carboxyl (COOH) groups release H + R COOH R COO - + H + When ph falls, NH 2 groups bind H + R NH 2 + H + R NH 3 18 April,

10 Physiological Buffer Systems Respiratory and renal systems Act more slowly than chemical buffer systems Have more capacity than chemical buffer systems Respiratory Regulation of H + Respiratory system eliminates CO 2 A reversible equilibrium exists in the blood: CO 2 + H 2 O H 2 CO 3 H + + HCO 3 During CO 2 unloading the reaction shifts to the left (and H + is incorporated into H 2 O) During CO 2 loading the reaction shifts to the right (and H + is buffered by proteins) 18 April,

11 Respiratory Regulation of H + Hypercapnia ( arterial PCO 2) PCO 2 in cerebrospinal fluid [H+] in cerebrospinal fluid ph of cerebrospinal fluid activates medullary (central) chemoreceptors arterial PCO 2 [H+] in arterial blood activates peripheral chemoreceptors Effect of activation of both rate & depth of breathing More CO 2 is removed from the blood H + concentration is reduced See following figure Respiratory Regulation of H + Figure 22.25; Marieb & Hoehn, April,

12 Respiratory Regulation of H + Alkalosis depresses the respiratory center Respiratory rate and depth decrease CO 2 H + concentration increases returns blood ph to normal range CO 2 + H 2 O H 2 CO 3 H + + HCO 3 Respiratory system impairment causes acid-base imbalances Hypoventilation respiratory acidosis Hyperventilation respiratory alkalosis Acid-Base Balance Chemical buffers cannot eliminate excess acids or bases from the body : Lungs eliminate volatile carbonic acid by eliminating CO 2 Kidneys eliminate other fixed metabolic acids (phosphoric, uric, and lactic acids and ketones) and prevent metabolic acidosis 18 April,

13 Renal Mechanisms of Acid-Base Balance Most important renal mechanisms : Conserving (reabsorbing) or generating new HCO 3 Excreting HCO 3 CO 2 + H 2 O H 2 CO 3 H + + HCO 3 Generating or reabsorbing one HCO 3 is the same as losing one H + (pushes equation to left) Excreting one HCO 3 is the same as gaining one H + (pushes equation to right) Renal Mechanisms of Acid-Base Balance Renal regulation of acid-base balance depends on secretion of H + H + secretion occurs in the PT and in collecting duct type A intercalated cells: The H + comes from H 2 CO 3 produced in reactions catalyzed by carbonic anhydrase inside the cells See Steps 1 and 2 of the following figure 18 April,

14 1 CO 2 combines with water within the tubule cell, forming H 2 CO 3. 2 H2 CO 3 is quickly split, forming H + and bicarbonate ion (HCO 3 ). 3a H + is secreted into the filtrate. 3b For each H+ secreted, a HCO 3 enters the peritubular capillary blood either via symport with Na + or via antiport with CI. 4 Secreted H+ combines with HCO 3 in the filtrate, forming carbonic acid (H 2 CO 3 ). HCO 3 disappears from the filtrate at the same rate that HCO 3 (formed within the tubule cell) enters the peritubular capillary blood. Filtrate in tubule lumen HCO 3 + Na + HCO 3 4 H2 CO 3 5 * H 2 O CO 2 Tight junction Nucleus PCT cell H 2 CO 3 CO 2 + H 2 O 2K + 3Na + Cl H + 3a H + HCO 2 3 ATPase 6 1 CA 3b 2K + ATPase 3Na + Na + Na + Peritubular capillary Cl HCO 3 HCO 3 CO 2 5 The H2 CO 3 formed in the filtrate dissociates to release CO 2 and H 2 O. 6 CO2 diffuses into the tubule cell, where it triggers further H + secretion. Primary active transport Secondary active transport Simple diffusion Transport protein Carbonic anhydrase CA- inhibitor e.g. acetazolamide Figure 26.12; Marieb & Hoehn, 2010 Na+ Reabsorption in 1 st half of PT Figure 33.1, Koeppen & Stanton, April,

15 Reabsorption of Bicarbonate Tubule cell luminal membranes are impermeable to HCO 3 CO 2 combines with water in PT cells, forming H 2 CO 3 H 2 CO 3 dissociates H + is secreted, and HCO 3 is reabsorbed into capillary blood Secreted H + unites with HCO 3 to form H 2 CO 3 in filtrate, which generates CO 2 and H 2 O HCO 3 disappears from filtrate at the same rate that it enters the peritubular capillary blood one- for- one exchange 1 CO 2 combines with water within the tubule cell, forming H 2 CO 3. 2 H2 CO 3 is quickly split, forming H + and bicarbonate ion (HCO 3 ). 3a H + is secreted into the filtrate. 3b For each H+ secreted, a HCO 3 enters the peritubular capillary blood either via symport with Na + or via antiport with CI. 4 Secreted H+ combines with HCO 3 in the filtrate, forming carbonic acid (H 2 CO 3 ). HCO 3 disappears from the filtrate at the same rate that HCO 3 (formed within the tubule cell) enters the peritubular capillary blood. Filtrate in tubule lumen HCO 3 + Na + HCO 3 4 H2 CO 3 5 * H 2 O CO 2 Tight junction Nucleus PCT cell H 2 CO 3 CO 2 + H 2 O 2K + 3Na + Cl H + 3a H + HCO 2 3 ATPase 6 1 CA 3b 2K + ATPase 3Na + Na + Na + Peritubular capillary Cl HCO 3 HCO 3 CO 2 5 The H2 CO 3 formed in the filtrate dissociates to release CO 2 and H 2 O. 6 CO2 diffuses into the tubule cell, where it triggers further H + secretion. Primary active transport Secondary active transport Simple diffusion Transport protein Carbonic anhydrase Figure 26.12; Marieb & Hoehn, April,

16 Generating New Bicarbonate Ions Two mechanisms in PT and type A intercalated cells: Generate new HCO 3 to be added to the alkaline reserve Both involve renal excretion of acid (via secretion and excretion of H + or NH 4 + ) Excretion of Buffered H + Reclaiming filtered HCO 3 restores existing plasma [HCO 3 ] Normal diet introduces new H + into body ; dietary H + must be balanced by generating new HCO 3 (vs filtered HCO3 ) Most filtered HCO 3 is used up before filtrate reaches the collecting duct 18 April,

17 Excretion of Buffered H + Intercalated cells actively secrete H + into urine, which is buffered by phosphates and excreted Generated new HCO 3 moves into the interstitial space via a cotransport system and then moves passively into peritubular capillary blood Figure 26.13; Marieb & Hoehn, April,

18 Mechanisms of transport in the late DT & CD Cl- Figure 33.9, Koeppen & Stanton, 2010 Ammonium Ion Excretion Involves metabolism of glutamine in PT cells Each glutamine produces 2 NH 4+ and 2 new HCO 3 HCO 3 moves to the blood and NH 4+ is excreted in urine 18 April,

19 1 PCT cells metabolize glutamine to NH 4+ and HCO 3. 2a This weak acid NH 4 + (ammonium) is secreted into the filtrate, taking the place of H + on a Na + - H + antiport carrier. 2b For each NH4 + secreted, a bicarbonate ion (HCO 3 ) enters the peritubular capillary blood via a symport carrier. 3 The NH4 + is excreted in the urine. Filtrate in tubule lumen Nucleus PCT tubule cells Peritubular capillary Glutamine Glutamine Glutamine Deamination, 1 oxidation, and acidification (+H + ) 2a 2b NH + 4 2NH + 4 2HCO 3 HCO 3 HCO 3 (new) 3 Na + Na + Na + Na + NH 4 + out in urine Tight junction ATPase 2K + 3Na + 2K + 3Na + Na + Primary active transport Secondary active transport Simple diffusion Transport protein Figure 26.14; Marieb & Hoehn, 2010 Bicarbonate Ion Secretion When the body is in alkalosis, type B intercalated cells Secrete HCO 3 Reclaim H + and acidify the blood 18 April,

20 Bicarbonate Ion Secretion Mechanism is the opposite of the bicarbonate ion reabsorption process by type A intercalated cells Even during alkalosis, the nephrons and collecting ducts excrete fewer HCO 3 than they conserve Abnormalities of Acid-Base Balance Simple acid- base disorders i.e. only 1 acid-base disorder present Respiratory acidosis and alkalosis failure of respiratory system to perform normal ph balancing role indicated by P CO2 outside normal range mm Hg (primary disturbance) Metabolic acidosis and alkalosis any ph imbalance not caused by abnormal blood CO 2 levels indicated by [HCO3 - ] outside normal range meq/l (primary disturbance) 18 April,

21 Respiratory Acidosis The most important indicator of adequacy of respiratory function is P CO2 level (normally 3545 mm Hg) P CO2 above 45 mm Hg respiratory acidosis Most common cause of acid-base imbalances Due to decrease in ventilation (shallow breathing) or gas exchange (diseases e.g. pheumonia, emphysema, cystic fibrosis) P CO2 Characterized by falling blood ph and rising P CO2 See H-H equation : ph = pk + log [ HCO3 - ] [ CO 2 ] Respiratory Alkalosis P CO2 below 35 mm Hg respiratory alkalosis A common result of hyperventilation due to stress or pain See H-H equation : ph = pk + log [ HCO3 - ] [ CO 2 ] 18 April,

22 Metabolic Acidosis Causes of metabolic acidosis Gain of fixed acids Accumulation of organic acids e.g. lactic acidosis, ketoacidosis in diabetics, starvation Ingestion of fixed acids e.g. alcohol ( acetic acid) Decreased excretion of fixed acid (e.g. kidney disorders) Excessive loss of HCO 3 (e.g., persistent diarrhea) See H-H equation : ph = pk + log [ HCO3 - ] [ CO 2 ] Metabolic Alkalosis Metabolic alkalosis is much less common than metabolic acidosis Indicated by rising blood ph and HCO 3 Caused by vomiting of the acid contents of the stomach or by intake of excess base (e.g., antacids) See H-H equation : ph = pk + log [ HCO3 - ] [ CO 2 ] 18 April,

23 Effects of Acidosis and Alkalosis Blood ph below 7 depression of CNS coma death Blood ph above 7.8 excitation of nervous system muscle tetany, extreme nervousness, convulsions, respiratory arrest Respiratory Compensation If acid-base imbalance is due to malfunction of a physiological buffer system, the other one compensates Respiratory system attempts to correct metabolic acidbase imbalances i.e. if there is a disturbance in HCO3-, then the compensatory response is respiratory : to adjust the PCO2 the compensatory response always in the same direction as the original disturbance E.g. metabolic acidosis Primary disturbance = [ HCO3- ] Respiratory compensation = PCO2 (by hyperventilating) 18 April,

24 Respiratory Compensation In metabolic acidosis High H + levels stimulate the respiratory centers Rate and depth of breathing are elevated Blood ph is below 7.35 and HCO 3 level is low As CO 2 is eliminated by the respiratory system, P CO2 falls below normal Respiratory Compensation Respiratory compensation for metabolic alkalosis is revealed by: Slow, shallow breathing, allowing CO 2 accumulation in the blood High ph (over 7.45) and elevated HCO 3 levels 18 April,

25 Renal Compensation If acid-base imbalance is due to malfunction of a physiological buffer system, the other one compensates Kidneys attempt to correct respiratory acid-base imbalances i.e. if there is a disturbance in CO2, then the compensatory response is renal (metabolic) : to adjust the [ HCO3- ] the compensatory response always in the same direction as the original disturbance E.g. respiratory acidosis Primary disturbance = PCO2 Renal compensation = [ HCO3- ] (by conserving HCO3-) Renal Compensation Hypoventilation causes elevated P CO2 (respiratory acidosis) Renal compensation is indicated by high HCO 3 levels (kidneys conserving HCO 3 ) Respiratory alkalosis exhibits low P CO2 and high ph Renal compensation is indicated by decreasing HCO 3 levels (kidneys fail to conserve HCO 3 or actively secrete it) 18 April,

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