Urine Concentration and Dilution

Question 1

How does the kidney concentrate/dilute urine?

Answer 1

• generate medullary solute gradient
• maintain medullary solute gradient
• concentrate/dilute urine via collecting ducts and ADH

Question 2

Name the 2 medullary solutes and how they get there

Answer 2

• salt and urea

- get there by countercurrent multiplication and urea recycling

Question 3

What are the 3 major components of the medullary osmotic gradient?

Answer 3

• active reabsorption of Na+ from the ascending limb into the medullary interstitium
• recycling of urea between the MCD and deep medullary portions of the loop of Henle
• vasa recta structure

Question 4

What is the countercurrent multiplication?

Answer 4

• occurs in loop of Henle

- builds up osmolality gradient in interstitial fluid

Question 5

Name the two steps of the countercurrent multiplication

Answer 5

1. “single effect”

2. flow of tubular fluid

Question 6

What is the “single effect” step?

Answer 6

• removes salt from the thick ascending limb and delivers it to the interstitium, water cannot follow

Question 7

Which of the following passes through cell membranes with the least resistance?
A. Sodium
B. Chloride
C. Potassium
D. Water

Answer 7

D. water

Question 8

How does the “single effect” work?

Answer 8

• release of antidiuretic hormone (ADH) increases activity of Na-K-2Cl cotransporter

Question 9

What is the “flow of tubular fluid” step?

Answer 9

continuous flow of tubular fluid from cortex to bend in loop of Henle results in extrusion of salt and equilibration with water

Question 10

What can cause increased urea recycling?

Answer 10

increased ADH activity

in absence of ADH, urea accumulation in interstitium is abolished

Question 11

What is countercurrent exchange?

Answer 11

passive process that helps maintain the concentration gradient

(carried out by vasa recta)

Question 12

Characteristics of vasa recta

Answer 12

• freely permeable to water and solutes
• flow rate very slow
• blood leaving has higher osmolality

Question 13

What happens to maximum urine concentrating ability if flow through the vasa recta increases?
A. Maximum urine concentration increases
B. Maximum urine concentration declines
C. Maximum urine concentration will be unchanged

Answer 13

B. maximum urine concentration declines

Question 14

Name the actions of ADH

Answer 14

• increases water permeability of the principal cells of the LDT and CD
• increases activity of the Na-K-2Cl cotransporter of the TAL
• increases urea permeability of the inner medullary collecting ducts

Question 15

The “first step” or “single effect” in generating urine concentration ability is:
A. Movement of water from the descending loop of Henle tubular fluid into the interstitium
B. Movement of sodium from the thin ascending loop of Henle tubular fluid into the interstitium
C. Movement of sodium from the thick ascending loop of Henle tubular fluid into the interstitium
D. Generation of the medullary concentration gradient

Answer 15

C. movement of sodium from the thick ascending loop of Henle tubular fluid into the interstitium