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Answers to renal physiology problems worked in class Robert G. Carroll, Ph.D. Brody School of Medicine East Carolina University View presentation using “Normal View” button on PowerPoint to read notes at beginning slide of each problem Renal Processes Problem Assume RPF = 625 ml/min, GFR = 125 ml/min [plasma] Renal Delivery Tubular Load Urinary Excretion 1 7 mg/dl 43.75 mg/min 0 mg/min 0 mg/min 2 100 mg/dl 625 mg/min 125 mg/min 0 mg/min 3 140 mEq/l 87.5 mEq/min 17.5 mEq/min 0.18 mEq/min 4 1 mg/dl 6.25 mg/min 1.25 mg/min 1.25 mg/min 5 10 units/ml 6250 units/min 1250 units/min 3000 units/min Renal Processes Problem Assume RPF = 625 ml/min, GFR = 125 ml/min 1 Not filtered, protein [plasma] Renal Delivery Tubular Load Urinary Excretion 7 mg/dl 43.75 mg/min 0 mg/min 0 mg/min 2 Filt, reabs glucose 100 mg/dl 625 mg/min 125 mg/min 0 mg/min 3 Filt, reabs Na 140 mEq/l 87.5 mEq/min 17.5 mEq/min 0.18 mEq/min 1 mg/dl 6.25 mg/min 1.25 mg/min 1.25 mg/min 10 units/ml 6250 units/min 1250 units/min 3000 units/min 4 Filtered creatinine 5 Filt, secr Penicillin Problem - GFR What is the consequence of • Afferent arteriole constriction • Afferent arteriole dilation • Efferent arteriole constriction • Efferent arteriole dilation, on Renal blood flow? Glomerular filtration? Peritubular capillary reabsorption? Renal blood flow and GFR • Aff. Constr Eff. Constr Aff. Dilate Eff. Dilate RBF GFR Peri Reabs Reasoning behind problem Aff. Arteriole Eff. Arteriole Glom cap Bowman’s Capsule Peritub cap Aff or Eff contraction decreases peritubular capillary hydrostatic pressure, and increase peritubular capillary reabsorption Aff. Arteriole Eff. Arteriole Glom cap Bowman’s Capsule Peritub cap Aff contraction decreases glom cap hydrostatic pressure, and GFR. Eff contraction increases glom cap hydrostatic pressure and increases GFR Aff. Arteriole Eff. Arteriole Glom cap Bowman’s Capsule Peritub cap Renal blood flow and GFR • Aff. Constr Eff. Constr Aff. Dilate Eff. Dilate RBF GFR Peri Reabs Proximal Tubule Problem Carbonic anhydrase can be inhibited by acetazolamide. Will this alter the tubular reabsorption of: Na Water Cl Glucose K Proximal Tubule Problem Solution Na absorption is reduced. Na absorption depends on 1) proton secretion across the apical surface, and 2) a counter ion being absorbed with the Na. Intracellular carbonic anhydrase helps generate the proton, and luminal carbonic anhydrase generates the HCO3. Blockade of carbonic anhydrase diminishes both steps. Proximal Tubule Problem Answer Water absorption reduced. Water movement is passive, dependent on an osmotic gradient. If Na and HCO3 are not absorbed, filtrate remains isotonic with plasma, and no osmotic gradient exists Proximal Tubule Problem Answer Cl absorption is decreased. Cl absorption in the proximal tubule is dependent on a Cl concentration gradient. If water is not absorbed from the filtrate, filtrate Cl concentration is identical to plasma Proximal Tubule Problem Answer Glucose reabsorption is normal. As long as there is Na available to drive the secondary active transport, proximal tubule glucose reabsorption will proceed normally Proximal Tubule Problem Answer K reabsorption is decreased. K reabsorption in the proximal tubule is dependent on a K concentration gradient. If water is not reabsorbed from the filtrate, filtrate K concentration is identical to plasma. Proximal Tubule Problem Answer Carbonic anhydrase can be inhibited by acetazolamide. Will this alter the proximal tubular reabsorption of: Na YES Water YES Cl YES Glucose NO K YES Problem • Amiloride blocks Na entry through the apical channel • What happens to luminal Na+ • What happens to luminal K+ Amiloride problem answer • What happens to luminal Na+? • Na+ entry across the apical surface of the principal cell occurs through the Na channel. If the Na channel is blocked, then Na remains in the lumen, so luminal Na content is increased (compared to normal). Amiloride Problem Answer • What happens to luminal K+? • Identical K channels exist on the apical and basolateral surfaces of the principal cells. K secretion into the lumen requires the development of a negative transepithelial potential. If Na does not enter the cell on the apical surface, then the transepithelial potential does not develop, and K does not cross the apical surface into the lumen. Amiloride Problem Answer • Potassium wasting diuretics. • If distal tubule Na delivery is increased, then Na entry on the principal cell apical surface is increased. This results in enhanced K secretion into the lumen, and enhanced K loss from the body. Any diuretic that impairs Na absorption before the distal tubule is classified as K wasting. Amiloride Problem Answer • Aldosterone and Potassium wasting • Aldosterone is required for diuretics to “waste” potassium. Aldosterone stimulates the basolateral Na/K ATPase to assist the transepithelial reabsorption of Na. • A high salt diet also increases distal tubule Na delivery, but because aldosterone is low, potassium wasting does NOT occur Amiloride Problem Answer • Potassium sparing diuretics • Only those diuretics that act on the principal cells are considered K sparing. This is because if the apical entry of Na is blocked, then the transepithelial potential does not develop, and K secretion (loss) does not occur. Potassium sparing diuretics Distal Tubule and Cortical Collecting Duct Lumen amiloride Capillary Na+ -70 mV K + K + + Na K K Transepithelial potential -5 to -70 mV + +
