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Life support > Renal salt and water balance > Flashcards

Renal salt and water balance Flashcards

1
Q

What is osmoregulation?

A

Regulation of the amount of water in the body to maintain constant ECF osmolarity

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2
Q

What is volume regulation?

A

The regulation of blood volume and pressure to ensure effective circulating volume

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3
Q

How is volume regulation accomplished?

A

Accomplished by regulating total amount (not concentration) of sodium in the ECF

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4
Q

What determines the volume of ECF?

A

The total amount of sodium in the ECF
determines its volume

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5
Q

How is blood volume linked to blood pressure?

A
  1. Increase in ECF volume leads to increase in venous return
  2. This results in increased filling of ventricles which leads to an increase in cardiac output
  3. This overall results in increased blood pressure
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6
Q

How is ECF volume sensed and what effects occur?

A
  1. Change in ECF volume detected by the following sensors:
    -Atrial stretch receptors
    -Arterial baroreceptors
    -Afferent arteriole
    -NACl delivery to DT
  2. This causes effectors to act such as :
    -RAAS which is sodium retaining
    -ANP which is sodium excreting
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7
Q

What 2 factors activate RAAS and what do both of these factors interpret?

A
  1. Reduced renal perfusion
  2. Increased sympathetic activity
    -Both factors interpreted as a fall in blood volume
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8
Q

What is aldosterone secretion increased by?

A
  1. RAAS
  2. Increased plasma [K+]
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9
Q

Where is most of the filtered salt and water reabsorbed and what activity increases this fraction?

A

Most of the filtered salt and water is reabsorbed in the proximal tubule(PT) – this fraction increases with RAAS activity

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10
Q

Where else, other than the proximal tubule, is water and salt reabsorbed and at what fraction?

A

Much smaller, variable fraction reabsorbed from DT and CD

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11
Q

How does ECF volume increase due to reabsorption of water and salts from DT and CD?

A
  • Reabsorption of Na and solute-free water are separated
  • Aldosterone-mediated Na reabsorption increases plasma osmolarity, which is then adjusted by pure water reabsorption via ADH system
  • Result is increased Na and water in ECF with little or no change in plasma [Na] or osmolarity
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12
Q

What cells does aldosterone act on in collecting duct?

A
  1. Acts on principal cells lining collecting duct
  2. Acts on intercalated cells on collecting duct
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13
Q

What happens when aldosterone acts on principal cells lining collecting duct and what does this result in?

A
  • Increases Na/K ATPase
  • Increases expression of ENaC channels on luminal membrane
    Resulting in:
  • Increased Na+ reabsorption
  • Increased K+ secretion
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14
Q

What happens when aldosterone acts on intercalated cells on collecting duct and what does this result in?

A
  • Increases H+ ATPase
    Resulting in
  • Increased H+ secretion
  • Increased HCO3- reabsorption
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15
Q

What is the principal volume regulation system?

A

RAAS is the principal volume regulation system

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16
Q

What is the last line defence volume depletion?

A

ADH

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17
Q

When will non-osmotic ADH secretion occur?

A

Non-osmotic ADH secretion if BP drops by ≈ 10-15%

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18
Q

At what rate is the RAAS system working at when ADH is secreted?

A

RAAS is already fully activated

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19
Q

What does the osmotic force determine in ECF and ICF compartments?

A

Osmotic forces determine the distribution
of water between ECF and ICF
compartments

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20
Q

What are the 2 ways to change the concentration of a solution?

A
  • Add/remove solute
  • Add/ remove water
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21
Q

How is the osmolarity of the ECF adjusted?

A

The osmolarity of the ECF is adjusted by
adding or removing water, not solute

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22
Q

What is the only site of regulated water loss?

A

The renal tubule is the only site of regulated
water loss

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23
Q

What does water excess excreted by kidney mean?

A

Large volume of dilute urine

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24
Q

What does water deficit excreted by kidney mean?

A

Small volume of concentrated urine

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25
Q

What ability does the renal tubule mechanism of regulation have?

A

Ability to vary amount of solute-free water in the urine
-Concentration of interstitial fluid in medulla
-Dilution of urine in ascending limb and distal tubule

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26
Q

What is ADH?

A

ADH is the osmoregulatory hormone

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27
Q

What is the urine like when entering the collecting duct?

A

Urine entering the collecting duct is
maximally dilute

28
Q

What would happen in the absence of ADH?

A

Osmotic gradient for water
reabsorption is large,
but in the absence of ADH the
collecting duct is impermeable to water

29
Q

Why is ADH required?

A

ADH is required to unlock water
permeability of collecting duct

30
Q

What happens with low levels of ADH?

A

With low levels of ADH some water will
be reabsorbed

31
Q

What happens at maximum ADH level?

A

-Limit of water reabsorption is set by
osmolality of medullary interstitial fluid
-This is the maximum urine concentration (and hence minimum
water excretion) that can be achieved

32
Q

What does net water loss increase in ECF?

A

Net water loss increases ECF osmolarity

33
Q

What is the normal range for ECF osmolarity?

A

Normal range 285-295 mOsm/kg

34
Q

How does ADH maintain osmolality?

A
  • Changes detected by osmoreceptors in anterior hypothalamus
  • Project to magnocellular neurons of paraventricular and supraoptic nuclei of hypothalamus
  • PVN and SON neurons release ADH from their axon terminals in posterior pituitary
35
Q

What is threshold for ADH release?

A

Threshold for ADH release is 280-285 mOsm/kg

36
Q

What happens to ADH secretion if the mOsm/kg is above the range of 280-285 mOsm/kg?

A

Above this range small changes in osmolality produce large changes in ADH secretion

37
Q

What signifies the presence of ADH?

A

Urine osmolality >100 mOsmol/kg is
considered to signify presence of ADH
(albeit at low level)

38
Q

What happens at maximal ADH levels?

A

-Limit of water reabsorption is set by
osmolality of medullary interstitial fluid
-This is the maximum urine
concentration (and hence minimum
water excretion) that can be achieved

39
Q

What is the normal range of ECF osmolality?

A

Normal range 285-295 mOsm/kg

40
Q

What happens when a change in ECF osmolality is detected?

A
  • Changes detected by osmoreceptors in anterior hypothalamus
  • Project to magnocellular neurons of paraventricular and supraoptic nuclei of hypothalamus
  • PVN and SON neurons release ADH from their axon terminals in posterior pituitary
41
Q

What is the ECF osmolality threshold for ADH release?

A

Threshold for ADH release is 280-285 mOsm/kg

42
Q

What does a small change in osmolality, above the range of 280-285 mOsm/kg do to ADH secretion?

A

Above this range small changes in osmolality produce large changes in ADH secretion

43
Q

What type of change in BP stimulates ADH?

A

ADH also stimulated by large (10-15%) decreases in blood volume/pressure

44
Q

At what plasma osmolality is there a strong desire to drink?

A

Strong desire to drink when plasma osmolality ≥295 mOsm/kg

45
Q

What happens to signal thirst when a change in plasma osmolality is detected?

A

-Changes detected by osmoreceptors in anterior hypothalamus
-Project to centres mediating thirst, drinking

46
Q

What receptors reduce thirst on drinking?

A

Oropharyngeal and upper gastrointestinal receptors reduce thirst on drinking

47
Q

What factors stimulate thirst other than plasma osmolality?

A
  • Large (10-15%) drops in blood volume/pressure
  • Angiotensin 2 acting on hypothalamus
48
Q

What cation is the main determinant of ECF osmolality?

A

Plasma [Na+] is the main determinant of
ECF osmolality

49
Q

What is the contribution of Na+ to ECF osmolality?

A

Contribution of Na+ to ECF osmolality is 2 x plasma [Na+]

50
Q

What can plasma osmolarity be estimated from?

A

Plasma osmolarity in mOsm L-1 can be estimated from:
* 2[Na+] + 2[K+] + [glucose] + [urea]

51
Q

What happens to ECF osmolality when theres a water deficit and what is this called?

A

Water deficit
* ECF osmolality increases (hyperosmolality)
* Hypernatremia (Na > 145)

52
Q

What happens to ECF osmolality when theres an excess of water and what is this called?

A

Water excess
* ECF osmolality decreases (hypoosmolality)
* Hyponatremia (Na < 135)

53
Q

What does hypernatremia mean?

A

hypernatremia does not mean too much Na; it means too little water!

54
Q

What are the causes of hypernatremia?

A
  • Gain of sodium (rare)
  • Loss of water (common)
55
Q

What are the causes of hypernatremia in terms of loss of water?

A

Extra-renal losses
* Dehydration
* Infection (increased losses via skin and lungs)
Renal losses
* Osmotic diuresis
* Diabetes insipidus

56
Q

What is diabetes insipidus?

A

Renal water loss (inability to concentrate the urine)

57
Q

What happens to ADH in diabetes insipidus?

A

Lack of effective ADH, either
* Central (failure of secretion)
* Nephrogenic (lack of renal response)

58
Q

What does diabetes insipidus present with?

A

Presents with polydipsia and polyuria

59
Q

What is pseudo hyponatremia?

A

‘Pseudo’ hypontremia occurs when some other solute is present in
sufficient quantity that the proportional contribution of sodium to
plasma osmolality is reduced

60
Q

What is true hyponatremia?

A

True hyponatremia is associated with hypoosmolality: it signifies water
excess

61
Q

What happens if ADH secretion is not reduced with continued ingestion of water?

A

Continued ingestion of water without reducing ADH secretion will
always lead to hyponatremia

62
Q

What is SIADH?

A

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a condition in which the body makes too much antidiuretic hormone (ADH)

63
Q

What are the causes of SIADH?

A
  • CNS damage/disease
  • Ectopic ADH production by tumour
64
Q

How can ADH be involved in blood pressure?

A

-Under normal conditions the
function of ADH is osmoregulation
-However, a large drop in arterial
pressure is also a powerful
stimulus for release
-In the hypovolemic state maximal
renal water retention will dilute
the ECF

65
Q

When can hyponatremia and hypervolemia occur?

A

Can occur when total sodium is increased, but total water increased
more

66
Q

What is an example of hyponatremia and hypervolemia occuring at the same time and how does it work?

A

Life support (38 decks)

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