loop of henle

Question 1

2 functions of the loop of Henle

Answer 1

• 25% reabsorption of NaCl, H2O, HCO₃^-, K⁺
• Concentrates or dilutes urine

Question 2

Are there more cortical or juxtamedullary nephrons? Which one has the longer loop of Henle?

Answer 2

More cortical nephrons, but juxtamedullary ones have the longer loop.

Question 3

Characterise the 3 parts of the loop of Henle

Answer 3

1. Thin descending limb:
   
   1. ​No active NaCl transport
   2. Permeable to water, urea, or solutes (NaCl)
2. Thin ascending limb: diluter
   
   1. ​No active NaCl transport
   2. Permeable to to urea or solutes (NaCl)
   3. NOT permeable to water
3. Thick ascending limb: diluter
   
   1. NaCl reabsorption!
   2. ​NOT permeable to H₂O

Question 4

Answer 4

A & B only

Question 5

Answer 5

Everything but glucose

Question 6

What are teh permeability and transport activityof the thin ascending limb? (Select all that apply)

• Impermeable to urea
• Permeable to NaCl
• Little if any active NaCl transport
• Impermeable to H₂O

Answer 6

Permeable to NaCl

Little if any active NaCl transport

Impermeable to H₂O

Question 7

What are the permeability adn ransport activity of the thick ascending limb ( selecta all that apply)?

• Totally impermeable to water
• Permeable to solutes
• Active NaCl transport
• Active urea reabsorption

Answer 7

Totally impermeable ot water

Permeable to solutes

Active NaCl transport

Question 8

Active transport in the thick ascending limb is performed by what transporter?

What drives it?

Answer 8

Na, K, 2Cl-cotransporter brings in the Na+ after the Na,K-ATPase pumped Na+ out of the cell to decrease [Na]_{intracellular}

Question 9

Furosemide (Lasix)

Answer 9

High-ceiling diuretic that inhibits the apical Na, K, 2Cl-cotransporter

–> large increases in urine flow & natriuresis

Question 10

What happens to K+ in the thick ascending limb?

Answer 10

1. K+ enters the cell both via the Na,K, 2Cl-contransporter and the Na,K-ATPase
2. Some of the K+ is secreted back into the tubular fluid via apical membrane K+ channels, generating a lumen-positive transepithelial gradient

Question 11

What is the driving force for passive Na+ reabsorption along the paracellular pathway in the thick ascending limb?

Answer 11

The recycling of K+ from the cell back into the lumen generates a positive lumen that drives Na+ out paracellularly towards the blood.

This is ~50% of Na+ reabsorption in the thick ascending limb

Question 12

For every ATP used by the Na,K-ATPase, what happens to Na+?

Answer 12

1 Na+ is sent out transcellularly by the ATPase

Another Na+ is sent out paracellularly because the K+ put inside the cell by the ATPase is secreted into the lumen, where it generates the electrical gradient for the Na+ to leave. (50%)

Question 13

Why is the thick ascending limb the “diluting segment”?

Answer 13

It reabsorbs NaCl but not water

Question 14

Corticomedullary interstitial osmotic gradient

Answer 14

As you go down from the cortex into the medulla, the interstitium becomes more concentrated because the ascending limbs are sending NaCl into it

Question 15

Going down the descending limb, the tube gets more ____

Going up the ascending limb, the tube gets more ___

Answer 15

Going down = concentrated

Going up = diluted

Question 16

Why does the tubular fluid at the hairpin turn have the same osmolality as the surrounding interstitium?

Answer 16

The corticomedullary interstitial gradient (becuase of the ascending limbs) makes the interstitium more concentrated as you go down.

This causes water to get reabsorbed in the descending limb.

Question 17

If there is no ADH, then what happens to tubular fluid after the loop of Henle?

Answer 17

It gets a bit more dilute as the collecting duct reabsorbs some Na+.

Very diluted for excretion

Question 18

How do you concentrate urine (countercurrent mechanism)?

Answer 18

ADH makes the late distal tubule & collecting duct permeable to water.

This causes the dilute distal tubule fluid to passively send out water into the interstitium (which, due to the corticomedually interstitial gradient, is very concentated)

Question 19

Single effect

Answer 19

Thick ascending limb pumps out NaCl –> hyperosmotic interstitium & diluted tubular fluid in ascending limb

Max = 200 mOsm/kg

Question 20

Describe countercurrent multiplication after “single effect”

Answer 20

1. Flow: Descending limb fluid (concentrated by equilibrating w the more hyperosmotic interstitium) enters the ascending limb.
2. Concentrate: Now impermeable to water, fluid in the ascending limb continues pumping NaCl out into the interstitium to dilute itself.
3. Flow: The interstitium is now even more concentrated, which promotes passive reabsorption of water in the descending limb.

Question 21

The thin descending limb is highly permeable to water. Why is there still a high interstitial osmolality in the medulla?

Answer 21

• The thick ascending limb pumps NaCl into the interstitium
• Urea is reabsorbed by the inner medullary collecting duct
• Countercurrent exchange by the vasa recta

Question 22

Vasa recta

Answer 22

Blood vessels that course down with the loops of Henle

Question 23

At the vasa recta, all movement of water and solute is ___

Answer 23

PASSIVE ONLY

this is blood

Question 24

Explain countercurrent exchange in vasa recta

Answer 24

Descending vasa recta is surrounded by increasingly concentrated interstitium as it goes down–> send out water –> vasa recta fluid concentrates

Ascending vasa rect is surrounded by increasingly dilute interstitium as it goes up –> take in water –> vasa recta fluid dilutes, interstitium concentrates even more

Question 25

What effect does ADH have on the late distal tubule vs the collecting duct?

Answer 25

Late distal tubule: Increase permeability to H₂O, but not urea –> [urea]_lumen increases as water leaves

Collecting duct: Increase permeability to both –> all that concentrated urea in the fluid is reabsorbed and [urea]_lumen decreases

Question 26

Passive countercurrent multiplication

Answer 26

Urea is reabsorbed by ADH at the inner medullary collecting duct, constributing to the osmolality of the interstitium

–> driving passive reabsorption of NaCl at the ascending limb (diluting segment).