Flashcards in PSL301: Water 5 Deck (81):
Water balance is mostly regulated by...
What are ways of water intake?
What are the ways of water loss?
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?
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 ____
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?
What happens to the osmoreceptor when there is decreased water?
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
1. Increase ADH secretion
2. Increase thirst & water drinking
2 ways in which vasopressin is secreted
1. osmotic - osmoreceptor initiated
Where is ADH produced?
large cell bodies on a few hypothalamic nuclei
Where is ADH stored?
How does ADH get from hypothalamus to posterior pituitary?
Where is the SON?
above the optic chiasm (where optic nerves cross)
baroreceptors are located at...
baroreceptors react to...
baroreceptors are connected to...
What is the implication of this?
Circulation also influences ADH secretion
Where is the hypothalamic osmoreceptor located?
anterior hypothalamus (OVLT)
what happens when osmoreceptors shrink?
1. cation channels open
2. cations enter cell (depolarize)
3. AP lead to increased ADH secretion & thirst
OVLT have ___ cation channels
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?
secretion of ADH is regulated by...
summation of stimulatory & inhibitory signals to SON and PVN
serum sodium ____ stimulates thirst
serum sodium ____ stimulates vasopressin
humans can excrete ___ mL of urine per hour if they wanted to (upper limit). What is the urine osmolality?
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?
High osmolality: 800 - 1200 mOsM/kg
When there is water excess, vasopressin is...
when there is water depletion, vasopressin is...
Urine concentration and dilution
1. isotonic in proximal tubule
2. very concentrated in descending limb
3. very dilute in ascending limb (hyposmotic)
4. depends on ADH/hormone action at collecting duct
Which transporters are found on the lumenal side of the ascending limb?
Allow NaCl to be transported without H2O
Which face is always permeable to water: apical or basolateral?
How can permeability to water be blocked at the apical membrane?
Block paracellular pathway
How does ADH increase water reabsorption?
1. ADH bind to basolateral membrane receptor V2
2. activate cAMP & secondary messangers
3. 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?
1. 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
- 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
value: concentrated urine
> 300 mOsM/kg
value: dilute urine
< 300 mOsM/kg
too much water
too little water
high urine flow > 3L/day
Is hyper/hyponatremia more common?
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)
- NOT OSMORECEPTOR
hyponatremia is a common symptom of which disease?
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...
- 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
- brain cells adapt by adding solutes to hold water in
Causes of polyuria
1. not enough ADH (central diabetes insipidus)
2. kidney can't respond to ADH (nephrogenic DI)
3. excess water intake
4. 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
1. 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
why is hypernatremia so rare?
As long as person is conscious, he/she can drink water to prevent it