5: Renal Tubular Function

Question 1

Diabetes Insipidus

Answer 1

• These patients lack or are insensitive to ADH

- This leads to excretion of a large volume of urine (up to 18 L or 10% of the filtered load per day)

Question 2

How much water do they kidneys filter a day?

Answer 2

180 L

Question 3

What is the most dilute osmolar urine possible to be excreted?
-How much urine a day does this correspond to?

Answer 3

-Excretion of 10% of filtered water corresponds to a maximal urine excretion of 18L/day

Question 4

What is the most concentrated urine can get?

-How much urine a day does this correspond to?

Answer 4

As concentrated as urine can get: 1200 mOsm/L

• must excrete about 600 mOsm of solutes per day (50% urea)
• Since max osmolarity of urine is 1200 mOsm, minimum urine volume is 0.5 L/day

Question 5

What two factors allow for excretion of dilute urine?

Answer 5

1. Maximum osmolarity in the medulla becomes only 600 mOsm

2. Without ADH principal cells’ H20 channels are taken into the cell by endocytosis and they become impermeable to H20

Question 6

How are the kidneys able to create such a concentrated inner medullary interstitium?

Answer 6

Countercurrent Exchange:

1. Thin descending limb of loop is permeable to H20
2. Transporters in ascending limb have ability to generate a solute gradient transporting solute from the the RT lumen into the interstitium (and water can’t follow cuz ascending limb is impermeable)
3. Water will flow into the interstitium from the descending limb, but the ascending limb transporters can just generate more solute gradient
   * not a lot of water left in descending limb once it gets deep into medulla, why the inner medulla ends up being the most concentrated

Question 7

Urea, two general but important characteristics:

Answer 7

• Urea makes up 40% (500 mOsm/L) of the maximum medullary interstitial osmolarity (1200 mOsm/L)
• 50% of filtered urea is excreted.

Question 8

Summarize Urea’s journey along the renal tubule of the nephron

Answer 8

PT: Urea conc. increases cuz H20 is reabsorbed faster than urea here
Thin portions of Loop: These portions are highly permeable to urea. Urea is highly concentarted in the medullary interstitium, however, so will flow into the lumen, further concentrating the urea there.
Thick ascending loop, DT and CCD: Impermeable to urea

Question 9

Urea’s role in maintaining a highly concentrated (1200 mOsm/L) medullary interstitium under conditions where secretion of a concentrated urine is desired.

Answer 9

When ADH is high:

1. DT/CCD are reabsorbing H20 (but impermeable to urea) so urea’s concentration is further increased.
2. MCD permeable to urea, and the extra urea diffuses back into medullary interstitium, helping to maintain its high osmolarity

Question 10

Why is the medullary interstitial osmolarity 600 mOsm/L when ADH is low?

Answer 10

At low ADH the permeability of the medullary collecting duct is reduced:

1. less diffusion of urea from MCD to interstitium
2. More wash out of urea in urine
3. So urea contributes less to the osmolarity of the medullary interstitium.

Question 11

Why don’t the Vasa Recta “Wash Out” the highly concentrated medullary interstitium?

Answer 11

1. Countercurrent exchange: vasa recta highly permeable to both solutes and water
2. Blood flow through the vasa recta is very low

Question 12

What’s special about osmolarity in the DT relative to the interstitium around it?

Answer 12

DT is hypoosmolar under both high ADH and low ADH conditions because the thin and thick ascending limbs of the loop are impermeable to water but continue to reabs. Na+
-He mentioned this twice but I don’t quite understand what is so jaw-dropping about it.