Renal Tubules and Osmoregulation

Question 1

Isosthenuria

Answer 1

(SG 1.008-1.012) refers to urine with a SG neither greater nor less than that of protein-free plasma

Question 2

Hyposthenuria

Answer 2

(SG <1.008) is urine with a SG less than the specific gravity of protein free plasma (dilute)

Question 3

Hypersthenuria

Answer 3

(SG > 1.012) is urine with a specific gravity greater than protein free plasma (concentrated)

Question 4

urine specific gravity

Answer 4

comparison of density of solution to water that is affected by the presence of solutes

Question 5

describe the medullary osmotic gradient

Answer 5

a difference in solute concentration in different regions of the medulla -> solute concentration increases deeper into the medulla

Question 6

what 2 things are required for the kidney to be able to concentrate urine?

Answer 6

• high concentration of ADH
• high osmolarity of the renal medullary interstitium

Question 7

describe the countercurrent multiplier

Answer 7

• occurs in the nephron loop and create a concentration gradient down the medulla
• descending limb (permeable to water) secretes water in the interstitium, making urine more concentrated as it moves down the loop
• ascending limb (impermeable to water) secretes salts (NaCl) into the interstitium

Question 8

describe how dehydration leads to concentration of urine

Answer 8

during dehydration the corticopapillary osmotic gradient increases (more concentrated urine)

Question 9

countercurrent exchange __ countercurrent interstitium the countercurrent multiplier __countercurrent interstitium

Answer 9

maintains

creates

Question 10

how is urea recycling involved in urine concentration?

Answer 10

Urea contributes to the maintenance of the hyperosmotic medulla
- DT/Cortical CD impermeable to urea
- ADH -> medullary CD to increase urea permeability (facilitated diffusion through urea transporters)

Question 11

how is the vasa recta involved in urine concentration?

Answer 11

slow flow into the vasa recta allow time for exchange and maintenance of the hyperosmolar interstitium

• blood flow in the vasa recta flow opposite the direction of tubular fluid in the nephron loop
• in the descending vasa recta, water diffuses into the interstitium
• in the ascending vasa recta, electrolytes diffuse out into the intersititium
• final result is hypoosmotic urine in DCT
• this allows for retention of salts in the interstitium (maintenance of the concentrated interstitium)

Question 12

vasa recta

Answer 12

capillary network that supply blood to the medulla

Question 13

explain how the collecting duct determines the final urine osmolarity, including the response of the tubule to ADH.

Answer 13

• ADH inserts aquaporins into the DCT, making it permeable to water
• as fluid moves down the collecting duct, water exits the tubule to dilute the increasingly concentrated interstitium
• urine as a result becomes more concentrated

Question 14

most diuretics __ urinary excretion of solutes, especially sodium and chloride

Answer 14

increase

Question 15

loop diuretics

Answer 15

work at the ascending thick loop of henle by inhibiting the Na+/K+/2Cl- cotransporter

ex. furosemide

Question 16

thiazide diuretics

Answer 16

inhibits sodium reabsorption in the distal renal tubules by blocking the Na-Cl cotransporter
- potassium wasting

ex. hydrochlorothiazide

Question 17

potassium sparing diuretics

Answer 17

result in natriuresis/diuresis and increased serum K+ at the distal tubule and collecting duct

Question 18

osmotic diuretics

Answer 18

-mannitol
- freely filtered at the glomerulus, poorly absorbed
- decreases reabsorption of water

Question 19

medullary washout

Answer 19

the hypertonic renal medullary interstitium is diluted and corticopapillary gradient destroyed causing severe PU/PD
- it becomes impossible to concentrate the urine as it passes through the medulla on the way to the renal pelvis

can’t create a concentrated urine without a concentration gradient