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Renal Physiology > Regulation of Plasma Osmolarity > Flashcards

Regulation of Plasma Osmolarity Flashcards

1
Q

what is the normal range of plasma osmolarity? what are the major solute determinants?

A

280-300 mOsm/L

Na and Cl

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2
Q

what type of water transportation does not occur?

A

active

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3
Q

what are the major sources of water? what are the major methods of excretion?

A

fluids, food and metabolism (less)

urine, feces, skin, sweat and exhaled air

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4
Q

what parts of water loss are regulated? why does this occur?

A

only urine output

very small osmolarity differences can compromise CNS function

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5
Q

what does the ratio of urine to plasma osmolarity do?

A

normalizes the concentrating/diluting ability of the kidney to changes of plasma osmolarity

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6
Q

when U/P osmolarity equals 1, what does that mean? when it is greater than one? when it is less than one?

A

1- urine is isotonic to ECF
>1- urine is concentrated (retaining water in excess of solute)
<1- urine is dilute (excreting water in excess of solute)

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7
Q

what does “free water” refer to?

A

water in excess of solute that is either taken from or left in the tubular fluid (not obligated to remain with solutes in tubular fluid)

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8
Q

what hormone reflects the status of ECF and plasma osmolarity? what does it do?

A

ADH

increase in osmolarity increases ADH- retention of water in the kidney

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9
Q

what is the equation for osmolar excretion? how does it change with high and restricted water intake?

A

U osmolarity x volume

does not change although osmolarity and volume will

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10
Q

what is the average osmolar excretion per day? what is the typical osmolarity of urine?

A

600 mOsm/day

400 mOsm/L

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11
Q

how much can the kidney reduce Uosm to? what is the maximum urine volume?

A

30 mOsm/L

20L/day

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12
Q

how high can the kidney increase Uosm? what is the minimum urine volume?

A

1200 mOsm/L

.5 L/day

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13
Q

what is the equation for free water clearance?

A

C H20= V- Cosm (osmotic clearance)

Cosm= (Uosm V)/Posm

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14
Q

what is C osm if U/P osm equals 1? C H2O?

A

C osm=V
C H2O=0
kidney does not add or remove free water from plasma

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15
Q

what is C osm if U/P osm is greater than 1? C H2O?

A

C osm>V
C H2O<0
kidney retains free water to add to plasma

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16
Q

what is C osm if U/P osm is less than 1? C H2O?

A

C osm0

kidney eliminates free water taken from the plasma

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17
Q

what does ADH signal in respect to free water clearance?

A

tells kidney to decrease free water clearance to make hypertonic urine

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18
Q

what limits the magnitude of free water clearance?

A

the limits of the concentrating and diluting ability of the kidney as well as magnitude of solute consumed

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19
Q

what will occur to free water clearance with a diet low in solute?

A

increase negative FWC and decrease positive

more free water is available for reabsorption because there is less solute in the tubular fluid

20
Q

what will occur to free water balance with a diet high in solute?

A

decrease negative FWC and increase positive

amount of water osmotically obligated to remain in the tubular fluid is increased so less free water is available

21
Q

how can the volume of free water clearance be measured from the volume of urine?

A

subtract urine volume from what isosmotic urine would be (2 L)

22
Q

if solute consumption is 600 mOsm/day what is the largest volume of urine that will be excreted? the smallest? (solute balance)

A

largest-20 L (600/30) most dilute urine

smallest-0.5 L (600/1200) most concentrated urine

23
Q

if solute consumption is 1200 mOsm/day what is the largest volume of urine that will be excreted? the smallest? (solute balance)

A

largest-40 L (1200/30) most dilute urine

smallest-1 L (1200/1200) most concentrated urine

24
Q

what causes the excretion of dilute urine?

A

solute resorption in the ascending limbs (water impermeable) dilutes the fluid and then it is excreted.

25
Q

what causes the excretion of concentrated urine?

A

solute reabsorption in the thick ascending limb generates a hypertonic environment. tubular fluid in the collecting duct is equilibrated with this environment because it becomes water permeable

26
Q

what is the long term correction for high plasma osmolarity?

A

satisfying thirst

27
Q

what does urine osmolarity depend on?

A

whether or not absorption occurs in the collecting duct

28
Q

what is antidiuresis? diuresis?

A

anti- restricted water intake

D- excess water intake

29
Q

what are the osmolarities of the proximal tubule, descending limb of the loop of henle, ascending limb and collecting duct compared to plasma?

A

PT- isosmotic
DL- hyperosmotic
AL- hypoosmotic
CD- hyper or hypo depending up on ADH presence

30
Q

describe antidiuretic hormone.

A

9 AA peptide made in paraventricular and supraoptic nuclei of hypothalamus. secreted by posterior pituitary

31
Q

what triggers the release of ADH?

A

changes in plasma osmolarity are detected by osmoreceptors in hypothalamic neurons. send AP to posterior pituitary inducing ADH release

32
Q

what is the half life of ADH? what is the significance of this?

A

18 min

minimizes an over correction because once the plasma returns to normal, the pituitary stops releasing it

33
Q

how does ADH increase water permeability in the outer and inner medullary collecting tubules and ducts?

A

vesicles containing aquaporin fuse with apical membrane allowing H2O to flow transcellularly out of the tubule and into the interstitium

34
Q

what do high levels of ADH do elsewhere in the body?

A

cause vasoconstriction to maintain blood pressure and flow when volume is severely reduced (hemorrhage)

35
Q

what segments of the nephron are highly permeable to water? which are not?

A

permeable- PT tDLH

impermeable- tALH TAL DCT (distal convoluted tubule)

36
Q

what receptors does ADH bind to and in what cells of the collecting tubule/duct?

A

V2 receptors in the principal cells from the initial collecting tubule until the end of the duct

37
Q

how does receptor binding cause vesicle fusion in ADH signaling?

A

g linked receptor stimulates AC->cAMP-> PKA

PKA phosphorylates proteins and results in synthesis of new aquaporin and fusion of vesicles

38
Q

in the collecting tubule, where are aquaporins constitutively located? where are they added with ADH?

A

always exist on principal cells’ basolateral membrane. added to apical membrane

39
Q

describe an aquaporin.

A

4 monomers with a hollow center and many transmembrane regions. mediates diffusion of water only

40
Q

with posterior pituitary removal, what response is changed? which is kept intact?

A

response to dilution of plasma is kept intact while response to increased osmolarity of plasma is gone

41
Q

what is the threshold for posterior pituitary secretion of ADH with euvolemia? what does this mean for normal physiological conditions?

A

280 mOsm/L

there is usually a little bit of ADH in the blood

42
Q

with euvolemia, what increase in plasma osmolarity will cause detectable increase in ADH levels?

A

only 1%

43
Q

other than plasma osmolarity, what is the secretion of ADH reliant upon?

A

dependent upon volume status
hypovolemia results in more ADH secretion for same plasma osmolarity and hypervolemia results in less ADH secretion for the same osmolarity

44
Q

where are CNS osmoreceptors? how do they sense plasma osmolarity?

A

the organum vasculosum of the lamina terminalis and the subforical organ
there is a gap in the BBB and changes in plasma cause changes of stretch in the neuronal cell membranes (sense cell swelling and shrinkage)

45
Q

in the CNS osmoreceptors, what happens with increased osmolarity?

A

it opens mechanosensitive cation channels causing depolarization and increase frequency of action potentials to the hypothalamus (decrease closes them)

46
Q

how do ADH secreting neurons respond to changes in circulating volume?

A

decreased volume sensed by low pressure receptors in the left atrium. vagal input to brain induces release of ADH. renin angiotensin system also causes release

Renal Physiology (12 decks)

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