A. Lecture 26 - regulation of water balance

Question 1

what is the value of plasma osmolality?

Answer 1

280-290 mosmol/Kg H2O (± 3)

Question 2

what is diuresis?

Answer 2

increased production of urine.

Question 3

what does ADH increase H2O permeability of?

Answer 3

Cortical collecting duct (2/3 reabsorption)

Medullary collecting duct (1/3 reabsorption)

Question 4

how much hypotonic fluid reaches collecting ducts daily?

Answer 4

23 litres

Question 5

in the absence of ADH how much dilute urine can be produced?

Answer 5

approx 20 litres.

Question 6

in the presence of ADH how much concentrated urine can be produced?

Answer 6

300 ml

Question 7

what is ADH?

Answer 7

Vasopressin

Question 8

where is ADH produced?

Answer 8

Synthesised in the supraoptic (SON) and paraventricular nuclei (PVN)

Question 9

where are the SON and the PVN found?

Answer 9

in the hypothalamus.

Question 10

what is the half life of ADH?

Answer 10

15 minutes

Question 11

where is ADH degraded?

Answer 11

liver and kidney

Question 12

what happens to ADH after synthesis?

Answer 12

moves to neurohypophysis

Question 13

how does ADH travel to neurohypophysis?

Answer 13

in axons of hypothalamophypophyseal tract

Question 14

what happens to ADH during the movement?

Answer 14

progressively cleaved.

Question 15

how is ADH stored in nerve terminals?

Answer 15

with neurophysin.

Question 16

what causes ADH release?

Answer 16

primary stimulus is plasma osmolality.

Question 17

how is plasma osmolality sensed?

Answer 17

osmoreceptors.

Question 18

where are osmoreceptors found?

Answer 18

located near SON

Question 19

what is the activation threshold for osmoreceptors?

Answer 19

280 mosmol/kg H2O

Question 20

what happens when osmoreceptors are activated?

Answer 20

small amount of tonic ADH release

Question 21

how sensitive are osmoreceptors?

Answer 21

Very sensitive (small increase osmolality = big increase in plasma ADH)

Question 22

how can osmoreceptor thresholds be reset?

Answer 22

hypovolaemia

Question 23

what is another stimulus for ADH release?

Answer 23

haemodynamics - blood pressure and or volume

Question 24

how is haemodynamics monitored?

Answer 24

via baroreceptors

Question 25

how sensitive are baroreceptors compared to osmoreceptors?

Answer 25

less sensitive

Question 26

how does nausea impact plasma ADH?

Answer 26

plasma ADH increases 100 to 1000 fold

Question 27

what are 3 other stimuli for ADH release?

Answer 27

Hypoglycaemia (only modest changes) Hypoxia (via carotid chemoreceptors) Angiotensin (↑ osmotic response)

Question 28

what receptor does ADH interact with?

Answer 28

V2

Question 29

How does ADH activate adenylate cyclase?

Answer 29

via Gs protein

Question 30

what does the activation of adenylate cyclase produce?

Answer 30

cyclic AMP (cAMP)

Question 31

what does cAMP activate?

Answer 31

protein kinase A (PKA)

Question 32

what happens to PKA?

Answer 32

PKA phosphorylates and activates non-functional aquaporin-2 water channels

Question 33

where does active aquaporin-2 channels insert?

Answer 33

apical (luminal) membrane.

Question 34

what does the insertion of aquaporin-2 do?

Answer 34

causes water permeability to increase. water is reabsorbed and urine is concentrated.

Question 35

true or false - concentrating ability of kidney is limited?

Answer 35

true

Question 36

what is the maximum urine osmolality?

Answer 36

1400 mosmol/kg H2O

Question 37

what does volume of urine excreted depend on?

Answer 37

Level of circulating ADH | Amount of solute to be excreted

Question 38

how much solute to be excreted?

Answer 38

800 mosmol/kg H2O solute to be excreted

Question 39

how do you work out the minimum volume of urine which can be excreted?

Answer 39

solute/max urine this is over 24 hours

Question 40

what is micturition?

Answer 40

Micturition = urination (voiding of urine)

Question 41

how is urine conveyed?

Answer 41

Urine is conveyed from renal pelvis to bladder via ureters

Question 42

what do ureters do?

Answer 42

Ureters encourage urine flow by peristalsis

Question 43

how is peristalsis initiated?

Answer 43

by pacemaker cells in the renal pelvis

Question 44

what happens to the rugae as bladder initially fills?

Answer 44

rugae unfold causing little change in pressure

Question 45

what happens to pressure as volume of urine increases?

Answer 45

pressure rises

Question 46

what happens to the stretch receptors in bladder?

Answer 46

activate pelvic nerve afferents

Question 47

what do the pelvic nerve efferents do?

Answer 47

Pelvic nerve efferents relax internal urethral sphincter

Question 48

is the micturition reflex under voluntary control?

Answer 48

not in babies (0-2) but in children and adults.

Question 49

how is the micturition reflex achieved?

Answer 49

Achieved by integration with higher centres (pons) via pudendal nerves

Question 50

what happens when the pudendal nerves are tonically active?

Answer 50

keep external urethral sphincter closed

Question 51

what happens with voluntary inhibition of pudendal nerve activity?

Answer 51

relaxes external sphincter.

Question 52

what does the relaxation of sphincter allowed?

Answer 52

micturition.