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 3000 units/min 1250 units/min 6250 units/min 10 units/ml mg/min 6.25 mg/min 1 mg/dl mEq/min 17.5 mEq/min 87.5 mEq/min 140 mEq/l3 0 mg/min125 mg/min 625 mg/min 100 mg/dl2 0 mg/min mg/min 7 mg/dl1 Urinary Excretion Tubular Load Renal Delivery [plasma] Assume RPF = 625 ml/min, GFR = 125 ml/min

Renal Processes Problem 3000 units/min 1250 units/min 6250 units/min 10 units/ml mg/min 6.25 mg/min 1 mg/dl mEq/min 17.5 mEq/min 87.5 mEq/min 140 mEq/l3 0 mg/min125 mg/min 625 mg/min 100 mg/dl2 0 mg/min mg/min 7 mg/dl1 Urinary Excretion Tubular Load Renal Delivery [plasma] Assume RPF = 625 ml/min, GFR = 125 ml/min Not filtered, protein Filt, reabs glucose Filt, reabs Na Filtered creatinine 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 RBFGFR Peri Reabs Aff. Constr Eff. Constr Aff. Dilate Eff. Dilate

Reasoning behind problem Glom cap Peritub cap Bowman’s Capsule Aff. Arteriole Eff. Arteriole

Aff or Eff contraction decreases peritubular capillary hydrostatic pressure, and increase peritubular capillary reabsorption Glom cap Peritub cap Bowman’s Capsule Aff. Arteriole Eff. Arteriole

Aff contraction decreases glom cap hydrostatic pressure, and GFR. Eff contraction increases glom cap hydrostatic pressure and increases GFR Glom cap Peritub cap Bowman’s Capsule Aff. Arteriole Eff. Arteriole

Renal blood flow and GFR RBFGFR Peri Reabs Aff. Constr Eff. Constr Aff. Dilate Eff. Dilate

Carbonic anhydrase can be inhibited by acetazolamide. Will this alter the tubular reabsorption of: Na Water Cl Glucose K Proximal Tubule Problem

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 Solution

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 Proximal Tubule Problem Answer

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.

Distal Tubule and Cortical Collecting Duct Na + K + Lumen Capillary Na + K + K mV K + Transepithelial potential -5 to -70 mV - amiloride Potassium sparing diuretics