Collecting tubule and ducts Flashcards
What is the distal nephron made up of?
Early and late distal tubule + cortical collecting tubule + medullary collecting duct
What is the medullary collecting duct?
The joining of several nephrons into one
What transport occurs in the CCT and MCD?
Na absoprtion through ENAC on apical, Na/K ATPase on basal with K secretion.
H2O absorption through AQP2 on apical and AQP3 on basal.
What regulates H2O absorption in CCT and MCD?
ADH
What does ADH do?
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.
How much water can be reabsorbed a day under maximum ADH? How much urine will therefor be produced?
- 5L/day
1. 5L/day of urine
What is Na absorption linked with in the distal nephron?
K secretion
What does aldosterone do?
Stimulates ENAC and Na/K ATPase synthesis for increased absorption.
Which diuretic will block aldosterone effects?
Spironolactone - moderate diuretic of CCT and MCD. Used in heart failure to prevent Na absorption and K secretion.
How much urine is produced under normal conditions?
1.25ml/min = 1.8L/day
What is excreted in urine?
water, Na, Cl at an increased concentration, K, Hco3 at a reduced concentration, Cr, urea
What is the fuction of urea?
It is a freely filtered substance involved in raising interstitial osmolality for the countercurrent effect.
What is the plasma conc of urea?
2.5-7.5mmol.L
Where is most of urea reabsorbed?
Proximal convoluted tubule
What happens to the urea concentration in the descending limb?
Concentration RISES due to H2O reabsorption and urea moves down its concentration gradient from the interstitium to the filtrate.
What happens to the concentration of urea in the distal tubule?
The concentration RISES further as the tubule is IMpermeable to urea, but permeable to H2O which is reabsorbed.
What causes the change in permeability of urea in the MCD?
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.
What happens to the urea in the interstitium?
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.
How does ADH increase the permeability of urea?
Incorporates UT-A1 channels onto the apical and basolateral membrane of the MCD for facilitated diffusion through the uniporter.
What happens if blood volume increases?
Less ADH is produced to reduce urea permeability and reducing H2O absorption.
What is the consequence of having a less dilute filtrate in the MCD?
The smaller the gradient between the MCD and interstitium so less urea and H2O absorption.
