Physio 8

Question 1

solute transport engine

Answer 1

active Na and Cl reabsorption occuring in the TAL. driving and maintaining a counter current multiplication of solute concentration difference in the interstitium surrounding the loop of henle and the collecting duct. urea recycling in the inner medulla also contributes to the solute concentration gradient in the interstitium. osmolarity increases going from cortex to inner medulla

Question 2

reabsorptive solute transport in the TAL is essential for:

Answer 2

1. diluting the tubular fluid osmolarity to less than plasma, excreting a dilute urine when plasma volume is high or hypo-osmotic
2. concentrating the tubular fluid osmolarity in the collecting duct by maintaining a gradient of interstitial osmolarity driving reabsorption of water and concentrating the urine when plasma volume is low or hyperosmotic

Question 3

ADH action

Answer 3

secreted in response to changes in plasma osmolarity. increases water permeability of the collecting duct allowing the osmotic equilibration of tubular fluid with the interstitium and the peritubular vasculature surrounding the collecting duct. in presence of ADH, concentrated urine due to water reabsorption. without ADH, dilute urine

Question 4

compare osmolarity of tubular fluid in diluting segment to plasma

Answer 4

ALWAYS less in the diluting segment. regardless of whether ADH is present. tALH, TAL, and DCT are insensitive to ADH

Question 5

gradient changes with diuresis and antidiuresis

Answer 5

gradient is less steep in diuresis (volume up, plasma osmolarity down)
and steeper in antidiuresis (volume down, plasma osmolarity up)

Question 6

permeability/impermeability of thin ascending limb and thick ascending limb

Answer 6

impermeable to water. TAL reabsorbs approx 25% of the filtered Na. TAL dilutes the tubular fluid by reabsorbing Na and Cl without reabsorbing water. decreases the tubular fluid osmolarity below 100

Question 7

functional properties of the thin descending loop of henle

Answer 7

low permeability to solutes, high permeability to water. passive tubular fluid-interstitium equilibration concentrates the tubular fluid as it ascends

Question 8

functional properties of thin ascending limb

Answer 8

passive NaCl reabsorption. relatively water impermeable. passive urea secretion

Question 9

functional properties of the thick ascending limb

Answer 9

active NaCl reabsorption via Na/K/2Cl symporter. relatively water impermeable. generates and maintains a 200 mOsm/L diff in osmolarity between tubular fluid in lumen (low) and interstitium (high)

Question 10

transport mechanisms of NaCl reabsorption in the TAL

Answer 10

membrane specific transporter at the luminal (tubular fluid) and basolateral (vascular) side of the TAL cell. luminal: Na-K-2Cl cotransporter. Basolateral: ion specific channels do Cl and K, and Na/K ATPase does Na.

Question 11

role of Active Na reabsorption

Answer 11

“solute transport engine” driving and maintaining a counter current multiplication of solute concentration difference from the cortex to medulla. TAL reabsorbs or “pushes” solute out of tubular fluid against a gradient, creating a higher osmolarity outside of the TAL in the interstitium.

Question 12

steps of generating the high interstitial osmolarity by countercurrent multiplication

Answer 12

1. a pumping of solute out of the ascending limb into the interstitium to a limiting osmotic gradient of 200 mOsm/L and instant osmotic equilibration with the tubular fluid in the opposing descending limb.
2. an axial advance or shift of fluid forward along the tubule and instantaneous equilibration of tubular fluid in the descending limb and the interstitium.

Pump -> equilibrate -> shift -> equilibrate

Question 13

role of urea recycling in the countercurrent multiplication

Answer 13

tubule to interstitium and interstitium to tubule recycling. contributes to the generation and maintenance of the gradient. in the deep parts of the inner medulla, urea accounts for 50% of the gradient

Question 14

recycling of urea in the inner medulla results from:

Answer 14

1. reabsorption of urea from the tubular fluid of the inner medullary collecting duct into the inner medullary interstitium
2. secretion of urea from the inner medullary interstitium into the tubular fluid of the tDLH and the tALH
3. delivery of urea in the tubular fluid from the tDLH and tALH to the inner medullary collecting duct, where urea is, again, reabsorbed into the interstitium

Question 15

renal handling of urea in antidiuresis

Answer 15

increased ADH increase water and urea permeability of the inner medullary collecting duct. Reabsorption of water concentrates urea in the tubular fluid, creating an outward urea concentration gradient driving increased urea into the interstituim, where it concentrates.

Question 16

renal handling of urea in diuresis

Answer 16

decreased ADH decrease water and urea permeability. this leads to increased excretion of water and urea in the urine.

Question 17

role of the vasa recta in the countercurrent multiplication

Answer 17

blood supply surrounding the loop of henle in the medulla. specialized to accommodate the exchange of solutes between the vasa recta and the interstitium to prevent washout of solute from the medulla. this is achieved by a slower rate of blood flow

Question 18

lumenal and interstitial osmotic forces and water and solute transport during anti-diuresis

Answer 18

dehydration. ADH is present, so inner collecting duct is more permeable to urea. increases urea recycling, which leads to medullary interstitium urea concentration increases

Question 19

lumenal and interstitial osmotic forces and water and solute transport during diuresis

Answer 19

no ADH, so no urea recycling. medullary interstitium concentration drops. urea is essentially absent from the inner medullary interstitium. movement of urea from the interstitium into the tubular fluid of the tDLH and the tALH prevents recycling and promotes excretion of urea