5. Control of Water Balance

Question 1

What is plasma osmolarity?

Answer 1

285-295 mOsmol/L

Urine can vary a lot

Question 2

Increased volume means what?

Answer 2

Increased salt and water

Question 3

Decreased volume means what?

Answer 3

Decreased salt and water

Question 4

On an average day what percentage more water and salt must be consumed to replace that is lost?

Answer 4

20-25%

Question 5

What must the kidneys get rid of?

Answer 5

Excess volume = excess water and salt

Question 6

What effect on osmolarity does getting rid of excess water have?

Answer 6

It keeps osmolarity up

Question 7

What effect on osmolarity does getting rid of excess salt have?

Answer 7

It stops the osmolarity from going too high

Question 8

What is the most abundant component of plasma?

Answer 8

Water

Question 9

What is the most prevalent solute in the plasma?

Answer 9

sodium - 140mmol/L

Question 10

What is water used to balance?

Answer 10

It is used to regulate plasma osmolarity

Question 11

What is salt level used?

Answer 11

It is used to determine the ECF volume

Question 12

What percentage of water makes up intracellular compartment fluid?

Answer 12

65%

Question 13

What percentage of water makes up extracellular compartment fluid?

Answer 13

35%

Question 14

How do we get rid of water?

Answer 14

450mls/day from sweat and skin
100mls/day from faeces
350mls/day from respiration

1500mls/day from urine output
All of these are variables

Question 15

How much water will reach the loop of Henle?

Answer 15

~30%

Question 16

How much water will reach the DCT?

Answer 16

~20%

Question 17

Where is water reabsorption regulated?

Answer 17

In the collecting duct

Question 18

What does the loop of Henle produce?

Answer 18

It produces a region of hyperosmolar interstitial fluid

Question 19

As you move from the cortex to the inner medulla what changes?

Answer 19

The osmolarity of the interstitial fluid increases. This gradient exists so water can be drawn out of the lumen. There are no pumps for water

Question 20

What happens when fluid travels down the descending lumen?

Answer 20

Water leaves

Question 21

In the thicker ascending limb what happens at the lumen?

Answer 21

There are large amounts of Na+/Cl- pumps which move these ions into the interstitial fluid around it. Triple transporter

Question 22

What is the bottom of the loop of Henle permeable to?

Answer 22

Urea

Question 23

How is the gradient of the loop of henle established?

Answer 23

Look at diagram in lecture (think about osmolarity and ions move out of the lumen)

Question 24

What are the parts of the nephron that is very permeable to urea?

Answer 24

Bottom of the collecting duct and the bottom of the loop of henle

Question 25

As the fluid moves down the descending limb what happens to the concentration of urea?

Answer 25

It increases because water is more water is leaving as you go down

Question 26

Describe the concentration of the urea by the time it reaches the bottom of the collecting duct?

Answer 26

The concentration of urea exceeds the concentration in the interstitial fluid, as a result urea moves in by diffusion. Eventually the concentration of urea will be higher in the interstitial fluid than in the loop of henle so it will move into the loop. (see diagram) This means that the urea can be washed out if the permeability of it decreases in the collecting duct

Question 27

What urea transporters are there in the collecting duct?

Answer 27

UT-A1 and 3

Question 28

What are the urea transporters in the descending limb of the loop of henle?

Answer 28

UT-A2

Question 29

What are the urea transporters in the vasa recta?

Answer 29

UT-B1

Question 30

How important are UT-A1/3 urea transporters?

Answer 30

Lack of these transporters causes: Severe reduction in the ability to concentrate urine Increased water intake by 20% No ability to reduce urine output it water restricted for 24 hours HOWEVER THERE ARE NO MUTATIONS FOR THESE TRANSPORTERS IN HUMANS

Question 31

How important are UT-A2 urea transporters?

Answer 31

Lack of this transporter causes: Very mild phenotype only observable on a low protein diet Point mutations - Reduced Blood Pressure

Question 32

How important are UT-B urea transporters?

Answer 32

Lack of this transporter causes: Increased urine production Reduced urine concentrating ability Weight loss Loss of function - reduction in urine concentrating ability

Question 33

Describe the blood flow in the vasa recta?

Answer 33

Counter-current. Water diffuses out of the descending limb and solute in. In the ascending limb the reverse happens. The gradient is maintained why glucose and oxygen is delivered.

Question 34

Describe the permeability of the vasa recta?

Answer 34

Permeable to solute and water

Question 35

Where is vasopressin synthesised?

Answer 35

In the hypothalamous and packaged into granules

Question 36

Where is vasopressin secreted from?

Answer 36

The posterior pituitary

Question 37

How long is vasopressin?

Answer 37

9 amino acids long

Question 38

What receptors does vasopressin bind to?

Answer 38

V2 receptors on the basolateral membrane of principal cells in the collecting ducts

Question 39

What is vasopressin secreted in response to?

Answer 39

Increased osmolarity or reduced volume

Question 40

What does vasopressin cause?

Answer 40

It causes the movement of aquaporins (AQP2) to the luminal membrane Also urea transport (UTA1/3) in the collecting duct --> Increases the concentration gradient even more

Question 41

What triggers a release in ADH?

Answer 41

Regulated by osmoreceptors in the hypothalamus which trigger a release when it detects a rise above 300mOs Also stimulated by a fall in blood pressure detected by baroreceptors

Question 42

What inhibits ADH release?

Answer 42

Ethanol

Question 43

What happens when plasma osmolarity decreases?

Answer 43

Hypothalamic osmoreceptors are not triggered resulting in a reduction in ADH release causing a reduction CD water permeability causing an increase in urine flow rate. Increased fluid loss tend to increase osmolarity

Question 44

What happens when plasma osmolarity increases?

Answer 44

Opposite of when it decreases. Also triggers thirst

Question 45

What are 3 disorders of water balance?

Answer 45

No/insufficient production of ADH No detection of ADH (mutant receptor) No response to ADH signal (mutant aquaporin)

Question 46

What do any of the 3 disorders of water balance cause?

Answer 46

Diabetes insipidus