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+ CAPS 301: Respiratory, Renal, GI, Endocrinology, Reproduction > Regulation of Osmolality and Volume > Flashcards

Regulation of Osmolality and Volume Flashcards

1
Q

What does the final osmolality (concentration) of urine depend on?

A

hormonal regulation of H2O reabsorption in distal nephron (DT and CD)

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2
Q

What does arginine vasopressin (AVP) do? Where does it act?

A

regulates addition of AQP2 (aquaporins) into apical membrane of collecting duct principal cells

debatable whether AVP acts in distal convoluted tubule

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3
Q

Regulation of Urine Concentration

What happens when AVP is present?

A
  1. insert AQP2 into apical membrane
  2. water can pass through and enter cell
  3. completion of H2O reabsorption by AQP3 and AQP4 – always expressed on basolateral membrane, but complete reabsorption only when AQP2 is expressed on apical membrane
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4
Q

Regulation of Urine Concentration

What happens when AVP is absent?

A
  • no way for H2O to enter cell

- even with basolateral transporters present, H2O has no way to cross apical membrane to access them

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5
Q

What is AVP? When does it get released?

A

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
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6
Q

Why is AVP released in response to low blood pressure?

A

want to absorb has much H2O as possible so BP doesn’t further decrease

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7
Q

How does AVP affect H2O absorption?

A

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)
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8
Q

Relationship Between Plasma Osmolality and Plasma AVP

A

more AVP released as plasma osmolality increases – makes sure H2O is reabsorbed as much as possible

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9
Q

Relationship Between Blood Pressure/Volume and Plasma AVP

A
  • if BP decreases, there is a steep increase in plasma AVP
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10
Q

Release of AVP is greater when looking at changes in what factors?

A

release of AVP is greater when looking at changes in reductions in blood volume or pressure, than increases in osmolality

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11
Q

What does increased blood volume/pressure do to osmolality?

A

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
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12
Q

What does decreased blood volume/pressure do to osmolality?

A

results in increased sensitivity to changes in osmolality

  • change in AVP release is greater
  • start having AVP release at lower osmolality
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13
Q

AVP Regulated Water Transport – Mechanism

A

see notes

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14
Q

AVP Regulated Water Transport

How does drinking alcohol affect AVP?

A
  • inhibits AVP
  • H2O cannot be reabsorbed – become dehydrated
  • once alcohol goes down, drink more water
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15
Q

Describe water movement along the nephron in each segment.

A

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
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16
Q

What is the composition of the resulting fluid when AVP is present?

A
  • more H2O reabsorption
  • concentrated urine
  • hyperosmotic fluid
17
Q

What is the composition of the resulting fluid when AVP is absent?

A
  • less H2O reabsorption
  • dilute urine
  • hypoosmotic fluid
  • reabsorbing other solutes
18
Q

Urine Dilution and Concentration

What does water restriction lead to?

A
  • 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
19
Q

Urine Dilution and Concentration

What does high water intake lead to?

A
  • 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
20
Q

Urine Dilution and Concentration

What does high water intake lead to?

A
  • 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
21
Q

How can plasma osmolality be restored?

A
  • thirst response

- increased H2O intake to system – reduces plasma osmolality and increases blood volume

22
Q

What is the thirst response stimulated by?

A
  • low blood volume/pressure stimulates hypovolemic thirst response
  • high plasma osmolality stimulates osmotic thirst response
23
Q

What increases AVP secretion (promotes renal H2O reabsorption)?

A

increased plasma osmolality and/or decreased blood pressure/volume

24
Q

What receptors response to changes in blood volume/pressure?

A

stretch receptors in right atrium

baroreceptors in aortic arch and carotid sinus

25
Q

What structure of the nephron is involved in response to low blood pressure? What does it do?

A

juxtaglomerular apparatus (JGA)

  • secretes renin, which ultimately leads to increase in plasma angiotensin II
  • increases blood pressure
26
Q

How do we respond to low blood pressure?

A

baroreceptors communicate to brain that we need to get more liquid into the system

will take in water AND salt
- increasing salt makes sure we do not dilute plasma as we take in lots of water, and that water we take in will stay in circulation

27
Q

What is the juxtaglomerular apparatus (JGA)?

A

interface between glomerulus and macula densa

28
Q

What are the two regulatory roles of the juxtaglomerular apparatus (JGA)?

A
  • regulation of GFR mediated by TGF response

- regulation of blood pressure mediated by renin-angiotensin-aldosterone system

29
Q

What are granular cells?

A

cells of of afferent arteriole secrete renin in response to:

reduced GFR

  • detected by reduced Na+ at macula densa
  • activity changes in NKCC2 leads to loss of renin inhibition

reduced blood pressure

  • detected by atrial stretch receptors and carotid and aortic baroreceptors
  • leads to increased activation of sympathetic nerve fibers innervating afferent arterioles

reduced blood pressure

  • detected directly by afferent arterioles
  • reduced stretch of afferent arteriole is stimulus for renin secretion by itself
30
Q

What do macula densa cells detect?

A

changes in tubular Na+ – has impact on vascular tone of afferent arteriole in response to that

31
Q

Regulation of Renal Responses to Changes in Fluid Balance

Increased Blood Volume – Mechanism

A

see notes

32
Q

Regulation of Renal Responses to Changes in Fluid Balance

Decreased Blood Volume – Mechanism

A

see notes

33
Q

Renin-Angiotensin-Aldosterone System

What does this system do?

A

counteracts low BP and aims to restore normal BP through its many targets

34
Q

Renin-Angiotensin-Aldosterone System

Is this a strong response?

A

YES

  • lots of different mechanisms that will be used because it’s so important for us to make sure we can get blood pressure volume up
  • ie. if you had hemorrhage and were losing blood rapidly, this would help with minimizing blood loss – make sure BP is staying as high as possible
35
Q

Renin-Angiotensin-Aldosterone System

Mechanism

A

see notes

+ CAPS 301: Respiratory, Renal, GI, Endocrinology, Reproduction (32 decks)

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