Lecture 20: Renal System 2

Question 1

How much of the water/Na/Cl/HCO3 is reabsorbed?

Answer 1

99%

Question 2

Describe Na+ and glucose reabsorption in S1 vs S2/S3

Answer 2

S1: Uses SGLT2 and GLUT 2 which are low affinity, high capacity
Na+ comes in through NHE3, goes out through Na/K ATPase. K leaves through leak channel. H+ supplied by carbonic anhydrase, HCO3- leaves via leak channel

S2/S3: Uses SGLT1 and GLUT 1 which are high affinity, low capacity
high concentration of charge allows some ions to move paracellularly
chlorine comes in, anion moves out via Cl-anion antiporter. Anion combines with H+, moves back in and dissociates. H+ moves out via NHE, this brings Na in. Na leaves via Na/K ATPase, K leaves via leak. Cl leaves via K/CL cotransporter

Question 3

Describe the differences in HCO3- handling between the PCT and the DCT/CD

Answer 3

PCT: carbonic anydrase converts CO2 and H2O to HCO3- and H. bicarbonate is absorbed back to blood via various cotransporters. H+ goes back to tubular fluid via NHE, where it attaches to any remnant HCOe not absorbed and converted to CO2 and H2O via CA, where it can diffuse into the PCT.

DCT/CD: no brush border CA therefore must rely on diffusion of CO2.

Question 4

Review how to tell how much of a substance has been reabsorbed at any distance along the proximal tubule

Answer 4

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Question 5

Discuss how contercurrent exchange works and what the purpose is. How is NaCl reabsorbed in the ascending limb?

Answer 5

separates water and solutes. Allows for production of hypertonic urine therefore need hyperosmotic ECF to pull out water from tubular fluid. Descending limb sucks out water by osmosis, ascending limb actively transports solutes out. NaCl reabsorbed by NKCC2, ROMK (renal outer medullary K+ channel), NHE
50% moves paracellularly due to build up of positive charge in lumen

Question 6

Describe what happens at the distal tubule and collecting duct

Answer 6

Early distal tubule: continued secretion/dilution of urine and ions
NaCl reabsorbed by symporter, leaves by ATPase or leak channel

Late distal tubule/CD: have principle cells (reabsorb NaCL, secrete K) and intercalated cells (secreted H+ and HCO3-)
Na reabsorbed by ENaC (epithelial Na+ channel), leaves by ATPase. Cl left, drives paracellular reabsorption
K secreted via leak channel, taken up from blood by ATPase

Question 7

How does ammonium excretion work?

Answer 7

substituted for the place of Na+ and used in the NHE to be antiported out

NH3 diffuses through, combines with H+ from carbonic anydrase to NH4+ which is not permeable

Question 8

How is water permeability regulated

Answer 8

via production of aquaporins (AQP2). this is upregulated by Gs and adenylyl cyclase activating PKA, downregulated by Gi and phosphodiesterase