Sodium and Water Transport

Question 0

Osmotic diuresis

Answer 0

Increased urine flow due to extra amount of non-reabsorbed Na+ in the PT

*Occurs with administration of mannitol salt and diabetes mellitus

Question 1

Na+ reabsorption in the PT

Answer 1

Crosses the luminal membrane with a “downhill” gradient via many different mechanims

*Always reabsorbs ~65% of filtered Na+

Question 2

Na+ reabsorption in the Loop of Henle

Answer 2

Filters ~25% of filtered Na+ and 15% of Cl-

-Occurs via Na+/K+/Cl- symporter; K+ will be resecreted

Question 3

Bartter’s Syndrome

Answer 3

Mutations in genes encoding Na+/Cl- transporters in the thick ascending limb

*Characterize by large urinary losses of NaCl, hypokalemia, and Ca+ wasting

Question 4

Reabsorption of Na+ in the DCT and CD

Answer 4

DCT= Na+/Cl- cotransport
CD= Na+ channels

*Regulated by aldosterone

Question 5

Gitelman’s Syndrome

Answer 5

Mutation in gene that encodes for Na+/Cl- cotranspoter in the DCT

*Excess secretion of ions

Question 6

Liddle’s Syndrome

Answer 6

Mutations in the genes that code for Na+ channels in the CD in the principal cells

• Causes excess Na+ REABSORPTION
• Severe hypertension, metabolic alkalosis (hypokalemia), and hypoaldosteronism

Question 7

Action of ADH

Answer 7

Acts on the principal cells in the CD by binding to V2 receptors leading to activation of AC and increased cAMP; activation of PKA leads to increased mRNA synthesis and production of AQP2

*CD very impermeable to water w/o aquaporins

Question 8

AQP 3 and 4

Answer 8

Mediate water movement across the basolateral membranes of the principal cells

Question 9

AQP1

Answer 9

Primary facilitator of H20 reabsorption in the PT and descending limb

Question 10

Obligatory Water Loss

Answer 10

The average mass of solute excreted per day divided by the maximal urinary conc.

Question 11

Countercurrent multiplier system

Answer 11

Solute is transported out of the ascending limb at a gradient that gradually decreases as it approaches the cortex; in response to the increased osmolarity of the ISF, H20 will leave the descending limb to balance the difference

*Net result is the reabsorption of both solutes and H20 => concentration of urine and increased osmolarity of ISF

Question 12

Renal Handling of Urea

Answer 12

Moves passively w/ Na+ thru the PT and is secreted into the loop of Henle due to the high concentration in the medulla; diffuses out of CD under influence of ADH via UT-AI transporters

*This urea diffuses back into the thin loop of Henle
Net result= Urea trapped in medulla contributing to the osmotic gradient

Question 13

Vasa Recta fnxn

Answer 13

Blood flow will gain solutes and lose water in medulla; gain water and lose solutes in the cortex

*Maintains medullary gradient thru a passive process

Question 14

Free Water Clearance

Answer 14

Compares the rate of solute excretion with the rate of water excretion

Ch20= V x [1- (Uosm/Posm)

Ch20>0 => Water being excreted; plasma concentrated

Ch20 Water being conserved

Ch2o = 0 => Water is isotonic to plasma

Question 15

Cl- reabsorption in DCT and CD

Answer 15

DCT: Paracellular diffusion with lumen-negativity acting as driving force; secondary active Na+/Cl- cotransport

CD: Paracellular diffusion w/ lumen-negativity acting as driving force; secondary active transport via HCO3-/Cl- cotransporter

Question 16

Na+ Balance

Answer 16

Balance b/w total input of Na+ and output

Question 17

Driving force of H2O reabsorption in CD

Answer 17

Medullary hyper osmolarity formed by the countercurrent multiplier system

*Still requires ADH to cross apical membrane tho

Question 18

Overall highest osmolarity of loop of Henle

Answer 18

Bend of the loop; the descending limb will also be slightly more concentrated than the ascending

Question 19

Water diuresis

Answer 19

Increased urine flow due to decreased reabsorption of water

*Excess fluid consumption, diabetes insipidus