Regulation of urine volume and fluid balance

Question 1

The kidney maintains a body fluid osmolarity at a value of what?

Answer 1

290 mOsm/L

Question 2

Free water clearance is used to assess what?

Answer 2

The kidney’s ability to conserve water or excrete excess water

Question 3

Which value can be used to assess the kidney’s ability to conserve water or excrete excess water?

Answer 3

Free water clearance

Question 4

What are the 2 steps for calculating free water clearance?

Answer 4

Calculate osmolar clearance

Calculate free water clearance

Question 5

Give the equation for calculating osmolar clearance.

Answer 5

Cosm = (Uosm / Posm) x V

Osmolar clearance = (urine osmolarity / plasma osmolarity) x urine flow rate

Question 6

Give the equation for calculating free water clearance.

Answer 6

CH2O = V - Cosm

Free water clearance = urine flow rate - osmolar clearance

Question 7

Describe urine osmolarity when the free water clearance value is positive.

Answer 7

Hypoosmotic urine

Question 8

Describe urine osmolarity when the free water clearance value is negative.

Answer 8

Hyperosmotic urine

Question 9

What is the equation for calculating osmoles excreted per day?

Answer 9

Urine osmolarity x urine output

Question 10

Which term refers to the minimum volume of urine that needs to be produced each day to excrete waste solutes?

Answer 10

Obligatory urine volume

Question 11

Is ADH released from the anterior or posterior pituitary gland?

Answer 11

Posterior

Question 12

ADH is produced in which 2 nuclei of the hypothalamus?

Answer 12

Paraventricular nucleus
Supraoptic nucleus

Question 13

What is the half-life of ADH?

Answer 13

15 - 20 minutes

Question 14

What is the main stimulus for ADH release?

Answer 14

Raised plasma osmolarity

Question 15

Besides raised plasma osmolarity, list 2 things that can stimulate the release of ADH.

Answer 15

Low blood pressure (but the change in BP must be of a much larger magnitude than that of osmolarity)

Nausea / vomiting

Question 16

Which cells does ADH primarily act on?

Answer 16

Principal cells

Question 17

Which receptors on principal cells does ADH bind to?

Answer 17

V2 receptors

Question 18

When ADH binds to V2 receptors on principal cells, which mechanism does this trigger?

Answer 18

Adenylyl cyclase

Question 19

Describe how ADH increases water reabsorption in principal cells.

Answer 19

ADH binds to V2 receptors on the basolateral membrane, which activates adenylyl cyclase.

Protein kinase A stimulates vesicles containing AQP2 to fuse with the apical membrane.

Question 20

ADH increases the numbers of which channel on the apical membrane of principal cells?

Answer 20

AQP2 (aquaporin 2)

Question 21

For the formation of dilute urine, will ADH be present or absent?

Answer 21

Absent

Question 22

For the formation of concentrated urine, will ADH be present or absent?

Answer 22

Present

Question 23

ADH increases the activity of which ion transporter to generate an osmotic gradient for the reabsorption of water?

Answer 23

NKCC2

Question 24

What is the counter current multiplier mechanism?

Answer 24

The method by which the medullary interstitium is concentrated

Question 25

Which mechanism describes the method by which the medullary interstitium is concentrated?

Answer 25

Counter current multiplier mechanism

Question 26

The counter current multiplier mechanism is dependent on which part of the nephron?

Answer 26

Loop of Henle

Question 27

The interstitial osmolarity is always the same as the osmolarity in which part of the nephron? Why is this?

Answer 27

Osmolarity in the descending limb Because the descending limb is permeable to water, so there's free movement of it with solutes

Question 28

Describe the osmolarity of the ascending limb compared to the osmolarity of the interstitial fluid, with values.

Answer 28

The osmolarity of the ascending limb is about 200 mOsm/L lower than the interstitial fluid

Question 29

What is the maximum osmolarity the medullary interstitium can reach?

Answer 29

1200 mOsm/L