Unit 7 - Loop of Henle

Question 1

general considerations of the loop of Henle

Answer 1

U-shaped and starts at the end of the PT in otuer medulla

• descends as thin descending limb (tDLH) to inner medulla
• 180 turn to ascend as thin ascending limb (tALH) back to outer medulla
• thickens to thick ascending limb (TAL)

Question 2

what do tALH and TAL impermeable to and reabsorb, and what does this cause?

Answer 2

relatively impermeable to water

• TAL actively reabsorbs 25% of Na+, and 20% of K+, to dilute tubular fluid
• this decreases tubular fluid osmolarity below 100 mOsm/L

Question 3

how does the osmolarity of medullary interstitium change from cortex to medulla?

Answer 3

progressively increases upon descending from cortex to inner medulla
-can achieve max value in inner medulla from 600 to 1200 mOsm/L (2-4x plasma)

Question 4

what is reabsorptive solute transport essential for?

Answer 4

1. diluting tubular fluid osmolarity to values less than plasma
   - excretes dilute urine when plasma volume is expanded and/or hypo-osmotic
2. concentrating tubular fluid osmolarity in collecting duct by maintaining gradient of interstitial osmolarity driving reabsoption of water from collecting duct back into circulation, and concentrating urine when plasma volume is contracted and/or hyper-osmotic

Question 5

what is the effect of ADH?

Answer 5

if increased plasma osmolarity, ADH increases water permeability of collecting duct via aquaporins, so osmotic equilibration of tubular fluid with interstitum, and peritubular vasculature around collecting duct

Question 6

what happens to the collecting duct in the absence of ADH?

Answer 6

collecting duct is impermeable to water, preventing water reabsorption, and thus osmotic equilibration of tubular fluid with interstitum and peritubular vasculature surrounding CD
-causes excretion of dilute urine

Question 7

how does the osmolarity of the tubular fluid of the tALH, TAL, and DCT compare to plasma?

Answer 7

it is always less than plasma, with or without ADH, regardless of volume expansion/contraction
-gradient is always present, but less steep in diuresis than in antidiuresis

Question 8

diuresis VS antidiuresis

Answer 8

diuresis: volume expanded with decreased plasma osmolarity
anti: volume contracted with increased plasma osmolarity

Question 9

what are the functional properties of the tDLH?

Answer 9

• low permeability to solutes (NaCl, urea)
• high permeability to water
• passive tubular fluid - interstitum equilibration of solute and water concentrates the tubular fluid as it descends

Question 10

what are the functional properties of the tALH?

Answer 10

• passive NaCl reabsorption
• relatively water impermeable
• passive urea secretion (increased urea causes increased osmolarity)

Question 11

what are the functional propertries of the TAL?

Answer 11

• active NaCl reabsorption via Na/K/2Cl symporter
• -cells have more mitochondria, thus are bigger
• relatively water impermeable
• generates and maintains a 200 mOsm/L difference in osmolarity between tubular fluid in lumen (low) and interstitum (high)

Question 12

what do loop diuretics do?

Answer 12

decrease reabsorption of Na+, K+, and Cl-, so increased in urine

Question 13

countercurrent multiplication of solute concentration difference - role of active Na+ reabsorption

Answer 13

active reabsorption of NaCl is “slute transport engine” driving and maintaining “counter current multiplication of solute concentration difference” from cortex to inner medulla in interstitium surrounding LH and collecting duct

• TAL reabsorbs solute out of the tubular fluid against 200 mOsm/L gradient, creating higher osmolarity outside TAL
• LoH creates and keeps a large cortico-medullary osmotic gradient by countercurrent multiplication to amplify up to 6x transport capacity of TAL to pump solute against 200 mOsm/L difference in transtubular osmolarity

Question 14

how is countercurrent multiplication generated?

Answer 14

begins with equivalent interstitial and tubular fluid osmolarities in descending and ascending limbs; each of 5 cycles involves 2 steps

1. pumping/reabsorption of solute out of ascending limb into interstitum to limiting osmotic gradient of 200 mOsm/L and instant osmoticequilibration with tubular fluid in opposite descending limb
2. axial advance or “shift” of fluid forward along tubule and instantaneous equilibration of tubular fluid in descending limb and interstitium

Question 15

countercurrent multiplication of solute concentration difference - role of urea recycling

Answer 15

tubule to interstitum to tubule recycling generates and maintains increased cortico-medullary gradient of hyperosmolarity

• osmolarity of inner medulla varies between 1200 mOsm/L during antidiuresis (~600 mM urea) and 500 mOsm/L during diuresis (<100 mM urea)
• in outer medulla, interstitial solute osmolarity is due to NaCl, while deeper parts of inner medulla in antidiuresis is 50% NaCl and 50% urea

Question 16

how does urea recycling in inner medulla come about?

Answer 16

1. reabsorption of urea from tubular fluid of inner medullary collecting duct into inner medullary interstitum (55% reabsorbed)
2. secretion of urea from inner medullary interstitium into tubular fluid of tDLH and tALH and TAL (50% secreted)
3. delivery of urea in flow of tubular fluid from tDLH and tALH and TAL to inner medullary collecting duct to, again, be reabsorbed

Question 17

countercurrent multiplication of solute concentration difference - role of vasa recta

Answer 17

blood supply surrounding LoH in medulla

• specialized vascular anatomy for countercurrent exchange of solutes to preserve cortico-medullary gradient and prevent “washout” of solute from medulla
• achieved by slower rate of blood flow through vasa recta

Question 18

how is the blood supply in renal cortex and renal medulla related?

Answer 18

they are segregated, so solutes reabsorbed from PT in cortex can rapidly re-enter circulation and exit kidney in renal vein, without ever having to enter medulla

Question 19

are the vasa recta passive or active countercurrent exchange?

Answer 19

passive countercurrent exchanger