Urinary concentration

Question 1

Osmolarity along the nephron

Answer 1

PCT = iso-osmotic
Descending loop = increase in tubular osmolarity, extraction of water
Ascending loop = osmolarity decreases, more dilute
DCT/ CD regulate with ADH

Question 2

Urine osmolarity

Answer 2

Diuresis: 20L/day at 50 mOsm/L

Anti-diuresis: 0.5L/day at 1200 mOsm/L

Question 3

ADH release

Answer 3

Hypothalamus and posterior pituitary

Detected by osmoreceptors in hypothalamus, when water volume decreases (osmolarity increases)

Question 4

ADH action

Answer 4

Principal cells of collecting duct
V2 receptors, cAMP, PKA
Insertion of AQP II into apical membrane of principal cels
AQP III/IV into basolateral to produce route across the cells
More water reabsorbed from collecting duct

Question 5

Permeabilities in loop

Answer 5

Descending: high water, low solutes (med. urea)
tALH: High Na+, no water
TALH: Active Na+, no water
CD: water and urea increased by ADH

Question 6

Other actions of ADH

Answer 6

Activation of NKCC channel in thick ascending loop
Activation or urea transporters in CD (more hypertonic interstitium)
Slow of blood in vasa recta

Question 7

Single effect

Answer 7

Pumping of Na+ out of ThAL into blood creates gradient for absorption of water
Accumulation of K+ and Cl- from NKCC on apical, due to Na+ gradient from ATPase
K+ recycles across apical
Cl- and Na+ diffuse from apical to basolateral and into intersititum
makes interstitium hypertonic

Question 8

Countercurrent multiplier

Answer 8

Different permeabilities of loop sections
Pumping of Na+ out of TAL, creates hypertonic intersititum
Water follows out of descending, makes lumen hypertonic
At the base the tubular fluid has high osmolarity (up to 1200)
Passive Na+ movement out of tALH
Progressive increase in gradients as fluid passes round the loop

Question 9

Urea absorption

Answer 9

Important for medulla hypertonicity, especially for water in the CD
Regulated by ADH
Urea that is reabsorbed can passively enter back into the loop = urea recycling
More urea absorbed = more hypertonic intersititum = more water reabsorbed

Question 10

Vasa recta countercurrent

Answer 10

Also a hairpin system but not multiplier
Permeabilities are the same
Important to not dissipate hypertonicity of the medulla
Enter in descending limb and leave in ascending
ADH slows flow

Question 11

NKCC

Answer 11

very important for loop absorption
causes removal of NaCl from lumen into intersitiial fluid to create gradient
stimulated by ADH
inhibited by loop diuretics