Lecture 5: Urinary Concentration and Dilution

Question 1

how do diuretics reduce water reabsorption?

Answer 1

by preventing sodium absorption (on apical membrane)

Question 2

how do calcium anhydrase (CA) inhibitor diuretics prevent Na reabsorption in the PT

Answer 2

by blocking Na/H exchanger

Question 3

Loop diuretics target what to inhibit Na uptake in the LoH

Answer 3

NKCC2 in the thick ascending limb of the loop of henle

Question 4

How do thiazide diuretics inhibit Na uptake in the distal tubule

Answer 4

by inhibiting Na-Cl transporter (NCC) in the distal tubule

Question 5

how do potassium-sparing diuretics work in the CD to reduce Na uptake and K secretion

Answer 5

by blocking ENaC

Question 6

a plasma concentration of glucose that exceeds the tubular maximum causes ?

Answer 6

osmotic diuresis

Question 7

what is osmotic diuresis

Answer 7

occurs when reabsorption of water is prevented by osmotically active substances in the urinary filtrate

causes a loss of water and electrolytes in urine

Question 8

what is the average number of osmolality for dogs/cats?

Answer 8

about 300 mOsm/kg

Question 9

what factors can cause fluid deficiency?

Answer 9

• volume depletion (hypovolemia) caused by hemorrhage for example
• dehydration
• circulatory shock, neurological dysfunction

Question 10

what factors can cause fluid excess?

Answer 10

• volume excess (Na and H20 retention)
• hypotonic hydration
• pulmonary or cerebral edema

Question 11

What 2 mechanisms maintain medullary hypertonicity which draws water out of the descending limb

Answer 11

1. urea accumulation in the inner medulla & NaCl accumulation in outer medulla
2. variable water and salt permeability in LoH
   
   • The descending limb of LoH reabsorbs water
   • The ascending limb is impermeable to water but NaCl is reabsorbed

Question 12

what is the amount of urea that enters into the proximal tubule

Answer 12

= to GFR x plasma concentration
b/c urea is freely filtered

Question 13

between what parts of the tubular system does urea recycling occur

Answer 13

b/w the inner medullary CD and the descending limb of LoH

Question 14

how is filtered urea reabsorbed into the inner medullary CD?

Answer 14

via urea transporters (UT-A1, UT-A3)

Question 15

in the outer medulla, it is mostly _____ that creates the ‘medullary osmotic gradient’

Answer 15

NaCl

Question 16

the ascending limb LoH is impermeable to _______ but permeable to _______

Answer 16

impermeable to water, permeable to Na

Question 17

what causes Na to be drawn into the medullary interstitium

Answer 17

Na concentration gradient
secondary active transport

Question 18

what transporter is used for NaCl reabsorption in the ascending limb LoH

Answer 18

NKCC

Question 19

what causes the tubule osmolarity in the ascending limb of LoH to decrease

Answer 19

b/c NaCl is leaving the tubule fluid via NKCC

Question 20

what prevents dilution of the medullary interstitial space

Answer 20

rapid reabsorption of water by the vasa recta

Question 21

what segment of the tubule system determines whether urine will be dilute or concentrated

Answer 21

the collecting duct

Question 22

what determines how permeable the collecting duct is to water

Answer 22

ADH (anti diuretic hormone)

Question 23

The level of ADH in the CD determines the ______ of excreted urine

Answer 23

osmolality

Question 24

what is the effect of hypertension (water overload) on ADH and urine concentration

Answer 24

ADH absent, urine becomes diluted

Question 25

what is the effect of dehydration, hypotension and volume depletion on ADH and urine concentration?

Answer 25

ADH levels rise, urine becomes more concentrated

Question 26

what determines the maximum concentration of urine

Answer 26

tonicity in inner medulla
amount of urea stored in the interstitial space