Lecture 5: Urinary Concentration and Dilution Flashcards
how do diuretics reduce water reabsorption?
by preventing sodium absorption (on apical membrane)
how do calcium anhydrase (CA) inhibitor diuretics prevent Na reabsorption in the PT
by blocking Na/H exchanger
Loop diuretics target what to inhibit Na uptake in the LoH
NKCC2 in the thick ascending limb of the loop of henle
How do thiazide diuretics inhibit Na uptake in the distal tubule
by inhibiting Na-Cl transporter (NCC) in the distal tubule
how do potassium-sparing diuretics work in the CD to reduce Na uptake and K secretion
by blocking ENaC
a plasma concentration of glucose that exceeds the tubular maximum causes ?
osmotic diuresis
what is osmotic diuresis
occurs when reabsorption of water is prevented by osmotically active substances in the urinary filtrate
causes a loss of water and electrolytes in urine
what is the average number of osmolality for dogs/cats?
about 300 mOsm/kg
what factors can cause fluid deficiency?
- volume depletion (hypovolemia) caused by hemorrhage for example
- dehydration
- circulatory shock, neurological dysfunction
what factors can cause fluid excess?
- volume excess (Na and H20 retention)
- hypotonic hydration
- pulmonary or cerebral edema
What 2 mechanisms maintain medullary hypertonicity which draws water out of the descending limb
- urea accumulation in the inner medulla & NaCl accumulation in outer medulla
- 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
what is the amount of urea that enters into the proximal tubule
= to GFR x plasma concentration
b/c urea is freely filtered
between what parts of the tubular system does urea recycling occur
b/w the inner medullary CD and the descending limb of LoH
how is filtered urea reabsorbed into the inner medullary CD?
via urea transporters (UT-A1, UT-A3)
in the outer medulla, it is mostly _____ that creates the ‘medullary osmotic gradient’
NaCl
the ascending limb LoH is impermeable to _______ but permeable to _______
impermeable to water, permeable to Na
what causes Na to be drawn into the medullary interstitium
Na concentration gradient
secondary active transport
what transporter is used for NaCl reabsorption in the ascending limb LoH
NKCC
what causes the tubule osmolarity in the ascending limb of LoH to decrease
b/c NaCl is leaving the tubule fluid via NKCC
what prevents dilution of the medullary interstitial space
rapid reabsorption of water by the vasa recta
what segment of the tubule system determines whether urine will be dilute or concentrated
the collecting duct
what determines how permeable the collecting duct is to water
ADH (anti diuretic hormone)
The level of ADH in the CD determines the ______ of excreted urine
osmolality
what is the effect of hypertension (water overload) on ADH and urine concentration
ADH absent, urine becomes diluted
what is the effect of dehydration, hypotension and volume depletion on ADH and urine concentration?
ADH levels rise, urine becomes more concentrated
what determines the maximum concentration of urine
tonicity in inner medulla
amount of urea stored in the interstitial space
