Regulation of Osmolality and Volume Flashcards
What does the final osmolality (concentration) of urine depend on?
hormonal regulation of H2O reabsorption in distal nephron (DT and CD)
What does arginine vasopressin (AVP) do? Where does it act?
regulates addition of AQP2 (aquaporins) into apical membrane of collecting duct principal cells
debatable whether AVP acts in distal convoluted tubule
Regulation of Urine Concentration
What happens when AVP is present?
- insert AQP2 into apical membrane
- water can pass through and enter cell
- completion of H2O reabsorption by AQP3 and AQP4 – always expressed on basolateral membrane, but complete reabsorption only when AQP2 is expressed on apical membrane
Regulation of Urine Concentration
What happens when AVP is absent?
- no way for H2O to enter cell
- even with basolateral transporters present, H2O has no way to cross apical membrane to access them
What is AVP? When does it get released?
hormone released by posterior pituitary gland in response to:
- high plasma osmolality – detected by osmoreceptors
- low blood pressure – detected by baroreceptors
- low blood volume – detected by baroreceptors
Why is AVP released in response to low blood pressure?
want to absorb has much H2O as possible so BP doesn’t further decrease
How does AVP affect H2O absorption?
can maximize amount of H2O absorption
- if you have high plasma osmolality, you can prevent it from getting higher faster
- if you have low blood volume or low blood pressure, by absorbing water, you can prevent it from getting lower
- but you can’t increase blood pressure or reduce osmolality – only way to do that is to drink water (kidneys can’t generate new H2O)
Relationship Between Plasma Osmolality and Plasma AVP
more AVP released as plasma osmolality increases – makes sure H2O is reabsorbed as much as possible
Relationship Between Blood Pressure/Volume and Plasma AVP
- if BP decreases, there is a steep increase in plasma AVP
Release of AVP is greater when looking at changes in what factors?
release of AVP is greater when looking at changes in reductions in blood volume or pressure, than increases in osmolality
What does increased blood volume/pressure do to osmolality?
results in decreased sensitivity to changes in osmolality
- change in AVP release is lower
- even if we have increase in osmolality by adding or retaining as much H2O as possible, there will be lots of fluid in plasma – therefore not the best way to accommodate for increase of osmolality when you are already basically full of water
What does decreased blood volume/pressure do to osmolality?
results in increased sensitivity to changes in osmolality
- change in AVP release is greater
- start having AVP release at lower osmolality
AVP Regulated Water Transport – Mechanism
see notes
AVP Regulated Water Transport
How does drinking alcohol affect AVP?
- inhibits AVP
- H2O cannot be reabsorbed – become dehydrated
- once alcohol goes down, drink more water
Describe water movement along the nephron in each segment.
PT – H2O reabsorption, no change in osmolality
DTL – H2O reabsorption, increase in osmolality
ATL and TAL – no H2O reabsorption, decrease in osmolality (hypotonic)
DCT – no H2O reabsorption
CCD & onward – AVP-dependent H2O reabsorption
- all regions are slightly permeable to H2O, and have regulated permeability
- with AVP, they become highly H2O permeable, which facilitates lots of H2O reabsorption
What is the composition of the resulting fluid when AVP is present?
- more H2O reabsorption
- concentrated urine
- hyperosmotic fluid
What is the composition of the resulting fluid when AVP is absent?
- less H2O reabsorption
- dilute urine
- hypoosmotic fluid
- reabsorbing other solutes
Urine Dilution and Concentration
What does water restriction lead to?
- increased plasma osmolality
- decreased blood volume/pressure
- stimulates release of AVP, and promotes H2O reabsorption in collecting duct (retain as much H2O as possible by reabsorbing it from kidney) – makes transport of urea out of collecting duct easier, helps maximize gradient
Urine Dilution and Concentration
What does high water intake lead to?
- decreased plasma osmolality
- increased blood volume/pressure
- AVP release will be minimal, leading to reduced H2O reabsorption in collecting duct
- no insertion of AQP2
- still have insertion of AQP3 and AQP4, BUT no way to get H2O across apical membrane
- water is stuck in nephron, which helps excrete more water
- if we have lots of water in blood, we can get rid of that to help reduce blood volume/pressure
- very dilute urine produced (hypoosmotic) – much lower osmolality than surrounding interstitium
Urine Dilution and Concentration
What does high water intake lead to?
- decreased plasma osmolality
- increased blood volume/pressure
- AVP release will be minimal, leading to reduced H2O reabsorption in collecting duct
- no insertion of AQP2
- still have insertion of AQP3 and AQP4, BUT no way to get H2O across apical membrane
- water is stuck in nephron, which helps excrete more water
- if we have lots of water in blood, we can get rid of that to help reduce blood volume/pressure
- very dilute urine produced (hypoosmotic) – much lower osmolality than surrounding interstitium
How can plasma osmolality be restored?
- thirst response
- increased H2O intake to system – reduces plasma osmolality and increases blood volume
What is the thirst response stimulated by?
- low blood volume/pressure stimulates hypovolemic thirst response
- high plasma osmolality stimulates osmotic thirst response
What increases AVP secretion (promotes renal H2O reabsorption)?
increased plasma osmolality and/or decreased blood pressure/volume
What receptors response to changes in blood volume/pressure?
stretch receptors in right atrium
baroreceptors in aortic arch and carotid sinus
