Renal VII: Renal Concentration & Dilution of Urine

Question 1

What happens during antidiuresis?

Answer 1

Antidiuresis happens during conditions of dehydration and results in production of low volume, high concentration urine. With antidiuresis, there is a high concentration of ADH in the plasma, causing increased reabsoprtion of water and urea.

Question 2

Is ADH present or absent in diuresis?

Answer 2

absent!

Question 3

Is ADH present or absent in antidiuresis?

Answer 3

present (in high concentrations)!

Question 4

Which part of the kidney has the highest osmolality?

Answer 4

inner medulla (~1200 mOsm/L)

Question 5

What are the two major contributors to the corticomedullary osmotic gradient?

Answer 5

• NaCl (50% of gradient)

- Urea (other 50% of gradient)

Question 6

What are the 3 mechanisms that regulate medullary hyperosmolality, and what role do each of the 3 mechanisms play in hyperosmolality?

Answer 6

• countercurrent multiplier: establishes hyperosmotic gradient
• urea cycle: strengthens gradient
• countercurrent exchanger: maintains gradient

Question 7

Where in the nephron is urea permeable?

Answer 7

It is permeable only in the lower collecting duct, and only if ADH is present.

Question 8

What happens in the descending vs. ascending vasa recta?

Answer 8

• descending: water moves out of capillary down osmotic gradient and salt moves in capillary down concentration gradient
• ascending: water moves into capillary down osmotic gradient and salt moves out of capillary down concentration gradient

Question 9

What is responsible for urea moving into the ascending vasa recta capillaries?

Answer 9

water pulls urea into the capillaries (water leaves tubule through aquaporins and pulls urea with it, so both are reabsorbed)

Question 10

What are the purposes of a high medullary urea concentration?

Answer 10

• protects vasa recta RBCs from crenation in a hyperosmotic environment
• sets up a gradient for urea to be excreted in low volume urine
• does NOT set up an osmotic gradient for the reabsorption of H2O

Question 11

What happens to urea recycling vs. urea clearance when urinary flow rate exceeds 10 mL/min?

Answer 11

• no urea recycling can occur due to a high flow rate

- urea clearance plateaus, meaning that it can estimate GFR

Question 12

How do urine and plasma osmolarities compare in negative free-water clearance?

Answer 12

urine osmolarity > plasma osmolarity (dark amber urine)

Question 13

How do urine and plasma osmolarities compare in positive free-water clearance?

Answer 13

urine osmolarity < plasma osmolarity (pale yellow urine)

Question 14

What is the range of possible urine concentrations?

Answer 14

50-1200 mOsm

Question 15

What is the osmolality of the renal cortex?

Answer 15

isotonic with plasma (300 mOsm/L)

Question 16

What is the formula for osmolar clearance?

Answer 16

Cosm (mL/min) = (Usom x urinary flow) / Posm

Question 17

Describes the amount of pure (solute-free) water the kidney adds to the urine per unit of time, diluting it below the osmolality of blood.

Answer 17

positive free water clearance

Question 18

Describes the amount of pure (solute-free) water the kidney subtracts from the urine per unit of time, concentrating the urine above the osmolality of blood.

Answer 18

negative free water clearance

Question 19

Is the flow rate in the vasa recta high or low? How does this affect the deep medullary gradient?

Answer 19

The flow rate is low, allowing the deep medullary gradient to be maintained. The vasa recta would NOT be able to establish these gradients with high flow rates.

Question 20

Why is urea picked up in the vasa recta deep in the inner medulla?

Answer 20

The inner medulla has a high hyperosmolarity, which could make the RBCs crenate do to being in a hypertonic environment. When urea enters, it brings water with it, thus creating a condition in which the RBCs are sitting in a hypotonic environment. This prevents them from crenating.

Question 21

What are some interesting things to note about urea that reveals why it moves the way it does in the nephron?

Answer 21

-it is an uncharged solute
-it can easily diffuse across most membranes via simple diffusion
-there are no membrane pumps for it
(all similarities shared with water)

Question 22

How much of our urea is excreted in the urine? What happens with the remainder of it?

Answer 22

50% excreted in urine and the rest is recycled

Question 23

What is the point at which urea clearance estimates GFR?

Answer 23

When urine flow rate exceeds 10 mL/min, as no urea recycling can happen due to too fast of a flow rate

Question 24

Describe a situation in which urea moves back into the collecting duct.

Answer 24

This occurs during water diuresis when there is no ADH present and therefore no aquaporins. Since water is not leaving, there as not as high of a concentration of urea building up in the lower CD. Thus, urea moves from the medullary interstitium and back into the CD to balance out its gradient.