Loop Of Henle

Question 1

What is reabsorbed in loop of henle

Answer 1

25% of NaCl and water

Small amount of HCO3

Question 2

Major function of loop of henle

Answer 2

Enable kidney to produce either a dilute or concentrated urine

Question 3

Thin descending limb

Answer 3

Highly permeable to water

Permeable to NaCl and urea

Little to any active NaCl transport

Question 4

Thin ascending limb

Answer 4

Totally permeable to water

Permeable to solutes

Little if any active NaCl transport

Question 5

Thick ascending limb

Answer 5

Totally IMPERMEABLE to water

Permeable to water

Active NaCl transport

Question 6

NaCl active transport mechanism in thick ascending limb

And passive

Answer 6

1. Basolateral membrane Na/K-ATPase pumps Na out of cell…creates gradient for Na to enter cell through apical membrane
2. Na/K/2Cl cotransporter in apical membrane
3. Some IC K+ is recycled back into lumen via apical membrane K channels —> generates a (+) lumen potential gradient…which is important for providing a chemical gradient for passive Na reabsorption through paracellular pathways (50% of reabsorption in thick ascending)

Question 7

The diluting segment of the nephron

Answer 7

Thick ascending limb

Since permeable to solute but NOT water

So this is how you can dilute urine

Question 8

Furosemide

Answer 8

High ceiling diuretic

Targets Na/K/2Cl cotransporter in apical membrane of thick ascending limb

Question 9

Cl- after the cotransporter in apical membrane

Answer 9

Passively moves into interstitium via Cl channels of K/Cl cotransporter

…which replenished interstitial K+ to keep Na/K pump working

Question 10

Mechanism for Na+ apical entry into cell in early distal tubule compared to thick ascending limb

Answer 10

Na/Cl cotransporter (not Na/K/2Cl cotransporter)

Question 11

How can a concentrated urine be produced?

Answer 11

Countercurrent mechanism system

Question 12

Countercurrent mechanism

General ….

Answer 12

1. Tubular fluid passes through thick ascending limb…
2. Salt reabsorbed (not water) —> dilute tubular fluid + hyperosmotic interstitial fluid in the outer and inner medulla
3. Dilute Tubular fluid enters distal tubular and collecting duct

(With presence of ADH in plasma…which makes DT and CDs permeable to water)

4. Water leaves tubular fluid into interstitium down its gradient (since hyperosmotic outer and inner medulla)
5. As tubular fluid passes through late distal tubule and collecting duct…it is concentrated until osmotic equilibrium with the surrounding interstitial fluid is reached
6. Concentrated urine is excreted

Question 13

Countercurrent exchange system of the vasa recta

Answer 13

Prevents undue concentration of the plasma as it leaves the kidney

1. Blood enters the descending vasa recta at 300 mosmol/kg
2. As it flows along the descending through medullary interstitium of increasing osmalality…water diffuses out…and vasa recta becomes more concentrated
3. Once reaches bend and starts ascending…the reverse happens and exits kidney at 325

Thus…any blood flow of vasa recta tends to deplete medullary intersitium of solute…significant increases in vasa recta blood flow —> marked depletino fo solute in inner medullar …

This impairing ability of the kidney to produce a concentrated urine

Question 14

ADH and urea

Answer 14

ADH increases permeability of inner medullary collecting duct to urea

But in cortex and outer medulla —> no affect on urea permeability…so as water is reabsorbed…[urea] in tubular fluid increases

Then when it reaches the inner medullary collecting duct…the urea is also absorbed (passively since gradient created after cortex and outer medulla water reabsorption)…and urine constitutes about 40% of papillary osmolality

Question 15

Fate of reabsorbed urea from collecting duct

Answer 15

Urea recycling

Can be secreted into loop of henle

Important for urea to be in the intestitium to maintain osmolality levels, thus aids in the conservation of water