5. Regulation of water balance

Question 1

Describe how number of dissolved particles effects osmolarity

Answer 1

The greater the number of dissolved particles, the greater the osmolarity

Question 2

What would happen if we didn’t get rid of the excess volume, excess water and excess salt?

Answer 2

Excess Volume: oedema + increase in BP
Excess Water: cells swell
Excess Salt: cells shrink

Question 3

What are the normal ranges for plasma osmolarity and urine osmolarity?

Answer 3

Plasma: 285-295 mosmol/L
Urine: 50-1200 mosmol/L

Question 4

What is the most abundant component of plasma and ECF?

Answer 4

Water

Question 5

What is the most prevalent solute of plasma and ECF?

Answer 5

Na+

Question 6

What is used to regulate plasma osmolarity?

Answer 6

Water balance

Question 7

What determines ECF volume?

Answer 7

Salt concentration (because water follows salt)

Question 8

What is the main fluid compartment?

Answer 8

Intracellular: 65%

Question 9

What are the different routes of getting rid of water? How much is removed via each of these methods per day?

Answer 9

Skin/sweat: 450 mL
Faeces: 100 ml
Respiration: 350 ml
URINE: 1500 ml = variable and regulatable

Question 10

Describe the movement of water

Answer 10

Water flows across a semi permeable membrane from a region of low osmolarity to a region of high osmolarity

Question 11

What is the consequence of regulation of water and salt balance being interrelated?

Answer 11

If we increase salt we have to increase water, which increases volume
If we decrease salt we have to decrease water, which decreases the volume

Question 12

How much excess water and salt do we consume per day?

Answer 12

20-25%

Question 13

Which region of the nephron is impermeable to water?

Answer 13

Ascending limb of loop of henle

Question 14

How much water is reabsorbed?

Answer 14

99%

Question 15

What determines an animals ability to produce concentrated urine?

Answer 15

Medulla to cortex ratio

Large medulla to cortex ratio can make highly concentrated urine

Question 16

What needs to be created for water to be drawn out of the tubules and into the interstitium?

Answer 16

Hyperosmolar Region of interstitial fluid

Question 17

Describe how much water is reabsorbed in each of the different parts of the nephron.

Answer 17

70% in the PCT
10% in the loop of Henle
Amount reabsorbed in the collecting duct varies

Question 18

Describe the osmolarity gradient in the interstitium.

Answer 18

Lowest osmolarity (around the same as plasma) in the cortex
Highest osmolarity in the inner medulla

Question 19

What does the shape of the loop of henle allow?

Answer 19

Countercurrent system
Descending limb: water exits
Ascending limb: Na+ exits

Question 20

What is the approximate gradient between the ascending loop of Henle and the interstitium?

Answer 20

200 mosmol/L

Question 21

What else accounts for the hyperosmolarity of the interstitial space?

Answer 21

UREA

Question 22

Which parts of the nephron are permeable to urea? How does this set up a circulation of urea?

Answer 22

Bottom of the CD (inner medullary collecting duct)
Bottom of the descending limb of the loop of Henle
The upper part of the CD is permeable to water so concentration of urea increases as it passes down the CD.
The urea moves out into interstitial space (down the concentration gradient) and then passes into the loop of Henle and recirculates.

Question 23

Where are the urea transporters: A1, A2, A3 and B1 found?

Answer 23

UT-A1: tubular side of the CD cells
UT-A2: thin descending limb of the loop of Henle
UT-A3: basolateral membrane of CD cells
UT-B1: vasa recta

Question 24

What do Point mutations in UT-A2 result in?

Answer 24

Reduced BP

Question 25

What does loss of function mutations in UT-B result in?

Answer 25

Reduction in urine concentrating ability

Question 26

Does length of loop of henle determine how concentrated urine can be made?

Answer 26

No. | The extent of activity of the transport system does.

Question 27

What supplies blood to the cells in the loop of henle and CD?

Answer 27

Vasa recta

Question 28

Describe the permeability of vasa recta

Answer 28

Permeable to water and solutes.

Question 29

How do you prevent the vasa recta from washing out the countercurrent gradient?

Answer 29

Vasa recta passes along the same path as the loop of Henle As blood descends it loses water and gains salt As blood ascends it absorbs water and loses salt Allows delivery of O2 and nutrients without disrupting the countercurrent gradient.

Question 30

What does ADH bind to?

Answer 30

V2 receptors on basolateral membrane of Principal cells in the CD

Question 31

What is ADH also known as?

Answer 31

Vasopressin | Antidiuretic hormone

Question 32

Where is ADH synthesised and what kind of hormone is it?

Answer 32

Synthesised in hypothalamus Peptide hormone Packaged into granules

Question 33

Where is ADH secreted from?

Answer 33

Posterior pituitary (neurohypophysis)

Question 34

Which transport channels does ADH stimulate?

Answer 34

Aquaporin 2: more move to the apical surface, increasing water permeability and increasing water reabsorption UT-A1 and UT-A3: increases membrane localisation of these in CCD, this stimulates urea transport from the IMCD into the interstitial tissue and thin ascending limb of the loop of Henle.

Question 35

What triggers ADH release?

Answer 35

Increase in plasma osmolarity detected by osmoreceptors in hypothalamus (>300mosmol/L) Large drop in BP detected by baroreceptors/ stretch receptors

Question 36

What inhibits ADH release? and what does this lead to?

Answer 36

Ethanol | leads to dehydration as urine volume increases

Question 37

Describe the response to high water load

Answer 37

``` Decrease plasma osmolarity Detected by hypothalamic osmoreceptors Decreases ADH release Decreases permeability of CD to water Increases urine flow rate Increased fluid loss raises plasma osmolarity ```

Question 38

Describe the response to dehydration

Answer 38

Increase plasma osmolarity Detected by hypothalamic osmoreceptors Increases ADH release Increases permeability of CD to water Decreases urine flow rate Decreased fluid loss lowers plasma osmolarity Increased thirst and water intake lowers plasma osmolarity

Question 39

What keeps plasma osmolarity in a normal range and determines urine output and water balance?

Answer 39

Feedback control via ADH

Question 40

State 3 issues that can cause problems with water balance. Which disease involves these 3 issues and what are the symptoms?

Answer 40

Absent ADH No detection of ADH (mutant receptor) No response to the ADH signal (mutant aquaporin 2) Diabetes Insipidus: polyuria and polydipsia