Tubular Reabsorption & Counter-Current Concentration

Question 1

what is the second step of urine production?

Answer 1

tubular reabsorption

Question 2

where does tubular reabsorption occur?

Answer 2

PCT

Question 3

what percent of water and ions are reabsorbed in the PCT?

Answer 3

65% (cannot be altered)

Question 4

what percent of glucose and amino acids are reabsorbed in the PCT?

Answer 4

100%

Question 5

describe the cells of the PCT

Answer 5

columnar epithelial with microvilli

Question 6

what is reabsorbed in the PCT?

Answer 6

• 65% of all water
• Na, Cl, K, HCO3, all small molecules
• 100% of glucose & AAs
• 30-40% urea

Question 7

what are the transport systems for glucose, amino acids, water, and sodium?

Answer 7

• glucose & AAs: secondary active Na co-transport (Na symport)
• H2O: osmosis (with small ions)
• Na: diffusion at luminal surface; primary active transport at anti-luminal surface

Question 8

what can reach a transport maximum?

Answer 8

glucose
AAs
sulfate
phosphate

Question 9

what is transport threshold?

Answer 9

some nephrons can no longer reabsorb and some molecules end up in the urine

Question 10

what is transport maximum?

Answer 10

all nephrons can no longer reabsorb any more

Question 11

what is the equation for excretion?

Answer 11

filtered load - transport maximum

Question 12

what is the third step of urine production?

Answer 12

counter-current concentration

Question 13

where does counter-current concentration occur?

Answer 13

loop of henle

Question 14

what is the descending loop permeable to?

Answer 14

water only

Question 15

what is the ascending loop permeable to?

Answer 15

salt only

Question 16

what transport systems are used for NaCl at the thin and thick ascending loop?

Answer 16

thin: passive
thick: active

Question 17

what is the thick ascending loop also permeable to?

Answer 17

Ca and Mg

Question 18

what is the osmolarity at the bottom of the loop of henle?

Answer 18

1200 mmol

Question 19

what is the osmolarity at the thick ascending loop of henle?

Answer 19

100 mmol

Question 20

what is the collecting duct permeable to?

Answer 20

water with ADH only
(distal medullary part permeable to urea)

Question 21

what is the approximate rate of urinary glucose excretion:
- GFR = 150mL/min
- plasma glucose = 300mg/dL
- normal renal transport maximum for glucose

Answer 21

filtered load = 150 x 300 = 450mg/min

excretion = FL - trans max.
450 - 300 = 150mg/min

Question 22

what is the rate of glucose excretion:
- GFR = 50mL/min
- urine flow rate = 2.0mL/min
- plasma glucose concentration = 200mg/100mL
- kidney glucose trans max = 150mL/min

Answer 22

filtration = 50 x (200/100) = 100mg/min

trans max > filtration, all filtered glucose reabsorbed, renal excretion rate = 0

Question 23

when dehydrated, assuming ADH levels are high, in what part of the renal tubule is the most water reabsorbed?

Answer 23

PCT

Question 24

in a person on a very low potassium diet, which part of the nephron would be expected to reabsorb the most potassium?

Answer 24

PCT

Question 25

which part of the nephron reabsorbs the most water?

Answer 25

PCT

Question 26

what is the rate of glucose excretion:
- GFR = 100mL/min
- urine flow rate = 2.0mL/min
- plasma glucose = 200mg/100mL
- glucose transport max = 250mg/min

Answer 26

FL = 100 x 200/100 = 200mg/min
excretion = 0 (all filtered glucose reabsorbed)

Question 26

which part of the tubule has the lowest permeability to water during antidiuresis?

Answer 26

thick ascending loop of henle is relatively impermeable to H2O even under conditions of max antidiuresis

Question 26

what is the rate of glucose reabsorption:
- glucose transport max = 350mg/min
- GFR = 100mL/min
- plasma glucose = 150mL/dL
- urine flow rate = 2mL/min
- no glucose in urine

Answer 26

FL = 100mL/min x 150mL/dL = 100 x 1.5 = 150mg/min

Question 26

which nephron segment is the primary site of magnesium reabsorption under normal conditions?

Answer 26

ascending loop of henle

Question 27

what is the rate of urinary glucose excretion:
- GFR = 150mL/min
- plasma glucose = 400mg/100mL
- glucose transport max = 300mg/min

Answer 27

FL = 150 x 400/100 = 600
excretion = 600 - 300 = 300mg/min