Collecting tubule and ducts

Question 1

What is the distal nephron made up of?

Answer 1

Early and late distal tubule + cortical collecting tubule + medullary collecting duct

Question 2

What is the medullary collecting duct?

Answer 2

The joining of several nephrons into one

Question 3

What transport occurs in the CCT and MCD?

Answer 3

Na absoprtion through ENAC on apical, Na/K ATPase on basal with K secretion.
H2O absorption through AQP2 on apical and AQP3 on basal.

Question 4

What regulates H2O absorption in CCT and MCD?

Answer 4

ADH

Question 5

What does ADH do?

Answer 5

Anti-diuretic functions to retain water for BP and volume regulation. Increases AQP2 channels on the apical membrane for increased absorption. It causes the concentration of urine and increases urea permeability in MCD.

Question 6

How much water can be reabsorbed a day under maximum ADH? How much urine will therefor be produced?

Answer 6

22. 5L/day

1. 5L/day of urine

Question 7

What is Na absorption linked with in the distal nephron?

Answer 7

K secretion

Question 8

What does aldosterone do?

Answer 8

Stimulates ENAC and Na/K ATPase synthesis for increased absorption.

Question 9

Which diuretic will block aldosterone effects?

Answer 9

Spironolactone - moderate diuretic of CCT and MCD. Used in heart failure to prevent Na absorption and K secretion.

Question 10

How much urine is produced under normal conditions?

Answer 10

1.25ml/min = 1.8L/day

Question 11

What is excreted in urine?

Answer 11

water, Na, Cl at an increased concentration, K, Hco3 at a reduced concentration, Cr, urea

Question 12

What is the fuction of urea?

Answer 12

It is a freely filtered substance involved in raising interstitial osmolality for the countercurrent effect.

Question 13

What is the plasma conc of urea?

Answer 13

2.5-7.5mmol.L

Question 14

Where is most of urea reabsorbed?

Answer 14

Proximal convoluted tubule

Question 15

What happens to the urea concentration in the descending limb?

Answer 15

Concentration RISES due to H2O reabsorption and urea moves down its concentration gradient from the interstitium to the filtrate.

Question 16

What happens to the concentration of urea in the distal tubule?

Answer 16

The concentration RISES further as the tubule is IMpermeable to urea, but permeable to H2O which is reabsorbed.

Question 17

What causes the change in permeability of urea in the MCD?

Answer 17

ADH increases permeability of urea allowing it to move down its concentration gradient into the interstitium, as it has been highly concentrated in the distal tubule.

Question 18

What happens to the urea in the interstitium?

Answer 18

Urea aids H2O reabsorption from the distal nephron to concentrate urine. It is recycled back into the descending limb by moving down its concentration gradient into the filtrate.

Question 19

How does ADH increase the permeability of urea?

Answer 19

Incorporates UT-A1 channels onto the apical and basolateral membrane of the MCD for facilitated diffusion through the uniporter.

Question 20

What happens if blood volume increases?

Answer 20

Less ADH is produced to reduce urea permeability and reducing H2O absorption.

Question 21

What is the consequence of having a less dilute filtrate in the MCD?

Answer 21

The smaller the gradient between the MCD and interstitium so less urea and H2O absorption.