PSL301: Water 5 Flashcards
Water balance is mostly regulated by…
ADH
What are ways of water intake?
- food
- drink
- metabolism
What are the ways of water loss?
- skin
- lungs
- urine
- feces
Insensible water loss
Water loss through breathing and sweat
Why does urine volume not equal water intake?
Insensible water loss
The body regulates which aspect of water loss?
Urine output
Net water movement in body:
Intake (2.5L) - Outtake (2.5L) = 0
Difference between water & sodium regulation
Water is tightly regulated
- respond rapidly to changes
- maintain Na concentration
Sweating causes rapid reduction in ____
urine volume
Do we need to drink 8 glasses of water/day?
No, the body will just excrete any excess. There is almost no health benefits.
How is water balance regulated?
- osmoreceptor in hypothalamus detects serum Na concentration
- swell/shrink depending on stimuli -> send info to hypothalamus
- change water excretion using ADH
What happens to the osmoreceptor when there is increased intake of water?
swell
What happens to the osmoreceptor when there is decreased water?
shrink
When does the osmoreceptor swell?
diluted serum Na
When does the osmoreceptor shrink?
concentrated serum Na
What happens when osmoreceptors sense there is concentrated serum Na?
Send 2 diff signals to diff parts of hypothalamus
- Increase ADH secretion
- Increase thirst & water drinking
2 ways in which vasopressin is secreted
- osmotic - osmoreceptor initiated
2. non-osmotic
Where is ADH produced?
large cell bodies on a few hypothalamic nuclei
- PVN
- SON
Where is ADH stored?
posterior pituitary
How does ADH get from hypothalamus to posterior pituitary?
vesicle
PVN
paraventricular nucleus
SON
supraoptic nucleus
Where is the SON?
above the optic chiasm (where optic nerves cross)
OR
osmoreceptor
br
baroreceptor
baroreceptors are located at…
carotid sinus
baroreceptors react to…
- BP
- CO
baroreceptors are connected to…
What is the implication of this?
SON
Circulation also influences ADH secretion
Where is the hypothalamic osmoreceptor located?
anterior hypothalamus (OVLT)
what happens when osmoreceptors shrink?
- cation channels open
- cations enter cell (depolarize)
- AP lead to increased ADH secretion & thirst
OVLT have ___ cation channels
stretch-inhibited
When are the ion channels on osmoreceptors open/closed?
closed: diluted Na (stretched)
open: concentrated Na (shrunk)
Vasopressin has a threshold. This means….
Below plasma osmolarity of 280 mOsM, there is no effect (no ADH can be found in blood)
Relationship between ADH and plasma osmolarity
As plasma osmolarity increases, so does plasma ADH -> LINEAR relationship
What happens when plasma osmolarity gets higher than 290?
You start to feel thirsty
When vasopressin is signalled to be released, where does it get released to?
venous system
secretion of ADH is regulated by…
summation of stimulatory & inhibitory signals to SON and PVN
serum sodium ____ stimulates thirst
> 145
serum sodium ____ stimulates vasopressin
> 135
humans can excrete ___ mL of urine per hour if they wanted to (upper limit). What is the urine osmolality?
> 1000
Low osmolality: 50 - 100 mOsM/L
humans can excrete ___ mL of urine per hour if they wanted to (lower limit). What is the urine osmolality?
< 20
High osmolality: 800 - 1200 mOsM/kg
When there is water excess, vasopressin is…
suppressed
when there is water depletion, vasopressin is…
high
Urine concentration and dilution
- isotonic in proximal tubule
- very concentrated in descending limb
- very dilute in ascending limb (hyposmotic)
- depends on ADH/hormone action at collecting duct
Which transporters are found on the lumenal side of the ascending limb?
Na-K-2Cl cotransporter
Allow NaCl to be transported without H2O
Which face is always permeable to water: apical or basolateral?
basolateral
How can permeability to water be blocked at the apical membrane?
Block paracellular pathway
How does ADH increase water reabsorption?
- ADH bind to basolateral membrane receptor V2
- activate cAMP & secondary messangers
- cause vesicles w/ aquaporin 2 to move to apical membrane
Besides the presence of ADH, what else is needed for water to move out of the lumen? How is this achieved?
- osmotic gradient
- high solute concentration in ISF because of the solute absorbed out at the ascending limb
Which receptor does ADH bind to?
V2 on basolateral membrane
Where can V2 receptors be found?
distal convoluted tubule & collecting duct
Where are V1 receptors located?
in blood vessels
What does V1 receptors do?
Vasopressin binds to them
Causes constriction of blood vessels
What must be functional in order to have urine excretion?
- thick ascending limb to generate concentrated ISF
2. cortical & medullary collecting duct (ADH binding site)
How does the CD dilute urine when there is too much water?
- ADH is not secreted
- Na channels on CD & DCT still pump Na out of lumen
How does the CD prevent urine dilution?
countercurrent arrangement of vessels in medulla
countercurrent exchange
- arrangement of vasa recta in medulla
- blood becomes progressively concentrated as vessels enter the inner medulla
- prevent disturbing the concentrated ISF
- allow urine to be concentrated
Minimum urine osmolality
50 mOsM/kg
value: concentrated urine
> 300 mOsM/kg
value: dilute urine
< 300 mOsM/kg
hyponatremia
too much water
hypernatremia
too little water
polyuria
high urine flow > 3L/day
Is hyper/hyponatremia more common?
hyponatremia
hyponatremia is usually caused by…
Too much ADH; failure to suppress ADH
What causes reduced circulating volume?
- heart failure
- volume depletion
What might some reasons for too much ADH be?
- decreased circulating volume (baroreceptor stimuli)
- cancer (make ADH)
- drugs
- NOT OSMORECEPTOR
hyponatremia is a common symptom of which disease?
heart failure
rapid onset of hyponatremia causes…
swelling of brain cells
- increased intercranial pressure
- compression of brain stem in foramen magnum
- seizure, coma, death
slow onset of hyponatremia causes…
why?
no symptoms
- brain cells adapt by removing solutes (takes water out with it)
hypernatremia indicates a problem with…
- thirst/intake of fluid (thirsty, but can’t get access to water)
- water loss (less common)
consequence of acute hypernatremia
brain cells shrink
- vessels that attach brain to skill breaks -> hemorrhage
- seizure, coma, death
consequence of chronic hypernatremia
no symptoms
- brain cells adapt by adding solutes to hold water in
Causes of polyuria
- not enough ADH (central diabetes insipidus)
- kidney can’t respond to ADH (nephrogenic DI)
- excess water intake
- osmotic diuresis / hyperglycemia
central diabetes insipidus means…
central = problem with hypothalamus or posterior pituitary diabetes = too much urine insipidus = dilute urine
nephrogenic DI vs. central DI
nephrogenic = problem at kidney central = problem at brain
genetic causes of neprogenic DI
- abnormal V2 receptor gene (x-linked)
2. abnormal aquaporin 2 gene
why is polyuria commonly seen in diabetics?
- lots of glucose in blood
- filtered by glomerulus
- not enough SLGT to reabsorb all back to blood
- glucose is a solute, so pulls water into lumen
DDAVP
synthetic vasopressin
why is hypernatremia so rare?
As long as person is conscious, he/she can drink water to prevent it
How do baroreceptors stimulate ADH release?
Connected to SON, which produces ADH
