50 Producing a concentrated or dilute urine

Question 1

What are the vasa recta?

Answer 1

Capillaries that go deep into the medullar from the cortex

Only 5 % of blood flows here

Question 2

What is the definition of osmolarity?

Answer 2

No. of solute particles per litre (mOsm/L)

Question 3

What is the normal osmolarity of the ECF when entering the kidney?

Answer 3

300 mOsm/L

Question 4

What is the osmolarity by the time the fluid enters the distal convoluted tubule? And why?

Answer 4

Reduced to ~100 mOsm/L

Due to reabsorption of ions

Question 5

Why has the osmolarity further reduced in DCT and collecting ducts?

Answer 5

Additional reabsorption of NaCl

Question 6

In the absence of ADH, this portion of DCT is also impermeable to water and the tubular fluid becomes even more dilute to as low as ____ mOsm/L

Answer 6

50 mOsm/L

Question 7

What leads to a large volume of dilute urine in DCT?

Answer 7

Failure to reabsorb water

Continued reabsorption of ions

Question 8

What is the descending loop of henle permeable and impermeable to?

Answer 8

Permeable to H2O

Impermeable to NaCl

Question 9

What is the ascending loop of henle permeable and impermeable to?

Answer 9

Permeable to NaCl

Impermeable to H2O

Question 10

What is the osmolarity of loop of henle?

Answer 10

600 mOsm/L

Question 11

How does kidney produce a dilute urine?

Answer 11

• Descending loop of henle is permeable to H2O but impermeable to NaCl
• Osmolarity rises from 300 to 600 mOsm/L
• Ascending loop of henle is impermeable to water but permeable to NaCl
• Thin ascending loop of henle - passive diffusion of NaCl out
• Thick ascending loop of henle - actively pumps NaCl out
• Osmolarity decreases to 100 mOsm/L in DCT where more NaCl pumped out
• Tubular fluid becomes as low as 50 mOsm/L
• Failure to reabsorb water and continued reabsorption of ions leads to a large volume of dilute urine

Question 12

What 2 factors do you need to make a concentrated urine?

Answer 12

1. High level of ADH - allows distal tubules and collecting ducts to become permeable to water and so reabsorbed
2. A gradient to pull this water out so there needs to be a high osmolarity of the renal medullary interstitial fluid

Question 13

How is the osmolarity in the medulla built up?

Answer 13

• Passive absorption of ions across the epithelia of the thin ascending limb of the Loop of Henle
• Active transport of sodium ions and co-transport of potassium, chloride and other ions out of the thick portion of the ascending limb of the loop of Henle.
• Active transport of ions from the collecting duct.
• Facilitate diffusion of urea from the medullary portion of the collection ducts into the medullary interstitium.
• Diffusion of only small amounts of water from the medullary tubules into the medullary interstitium and far less than the reabsorption of ions that occurs there. This sets up an osmotic imbalance and gradient

Question 14

Where does passive absorption of ions occur?

Answer 14

Epithelia of thin ascending limb of Loop of Henle

Question 15

Where does active transport of ions occur?

Answer 15

• Thick ascending limb of Loop of Henle - Na ions
• Distal tubule
• Collecting duct

Question 16

What main ions are co-transported out of thick portion of ascending limb of Loop of Henle?

Answer 16

• Potassium

* Chloride

Question 17

Facilitated diffusion of urea occurs where?

Answer 17

Medullary portion of collecting ducts into medullary interstitium

Question 18

Difference in permeability between thin descending and thin ascending limb?

Answer 18

TDL:
• H20 ++
• No NaCl
• Urea +

TAL:
• No H20
• NaCl ++
• Urea +

Question 19

Permeability to ions of thick ascending limb?

Answer 19

• Active transport of NaCl ++

* No H2O, NaCl and Urea

Question 20

With the presence of ADH, which parts of the nephron become permeable to H2O?

Answer 20

• DT (Distal tubule)
• CCT (Cortical collecting duct)
• IMCD (Inner medullary collecting duct)

Question 21

With the presence of ADH, which parts of the nephron become permeable to urea?

Answer 21

Inner medullary collecting duct

Question 22

Renal tubular transport in a dehydrated subject, stages?

Answer 22

1. Descending limb: 
• impermeable to salt, water 
• moves passively
2. Interstitial salt conc. increases towards hairpin bend as it's actively pumped out of  tubule
3. Thin ascending:
• Only found in long loops
• Passive Na+ movement out of tubule
• No H2O movement
4. Thick ascending:
• Active pumping against Na+ gradient
• No H2O movement
5. DCT: 
 • Removes ions due to active transport of NaCl and so the osmolarity of the tubule fluid continues to fall
5. Collecting tubule: 
• H2O can move only if ADH present

Question 23

The longer the Loop of Henle, the ___ concentrated

Answer 23

More

Question 24

What does the loop of Henle tubular counter-current multiplier actively generate?

Answer 24

A concentration gradient being greater the deeper into the medulla

Can rise to 1200-1400 mOsm/L

Question 25

How does the counter current mechanisms in the Loop of Henle occur?

Answer 25

Step 1. Fluid enters the loop of Henle from the PCT at 300 mOsm/L. The same as plasma.

Step 2. Active transport of ions from the thick ascending limb establishes a 200 mOsm/L gradient between the tubular fluid and the interstitial fluid. 400 mOsm/L occurs in the medullary interstitium compared to 200 mOsm/L in the ascending tubule fluid.

Step 3. The tubular fluid in the descending limb now equilibrates with the interstitial fluid as water moves out of the descending limb into the medullary interstitial fluid. Continued transport of ions but not water in the ascending limb maintains the gradient

Step 4. Flow of fluid into the loop of Henle from the PCT moves the fluid in the limbs on. The hyperosmotic fluid in the descending limb moves on into the ascending limb.

Step 5. Additional ions are pumped out of the fluid from the ascending limb until a 200 mOsm/L gradient is again established between the ascending limb tubule fluid and the medullary interstitium. This time the intersitial osmolarity rises to 500 mOsm/L and the ascending tubule falls to 300 mOsm/L.

Step 6. There is again a movement of water out of the descending limb of the loop of Henle to reach osmotic equilibrium with the medullary interstitial fluid. This increases the osmolarity of the tubule fluid in the descending limb up to 500 mOsm/L which moves on into the ascending limb for the processes of sodium and other ions movement to continue.

These steps are repeated over and over until the net effect is that the osmolarity of the deepest part of the medulla rises to 1200-1400 mOsm/L

Question 26

What is a benefit of the H20 reabsorption in the collecting ducts occurring in the cortex vs medulla?

Answer 26

In the presence of ADH the absorbed water is rapidly transported out of the kidney by the large blood flow through the kidney cortical peritubular capillaries.
This water absorption in the kidney cortex rather than the medulla helps preserve the osmotic gradient in the medulla.