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MS2 Renal > 9/10- Physiology of Sodium, Water and Volume > Flashcards

9/10- Physiology of Sodium, Water and Volume Flashcards

1
Q

What is the fluid/water breakdown between the various body compartments?

A

Water ~ 60% of the total body weight

  • 50-60% in males (~40 L)
  • 45-50% in females

ICF = 2/3 total water, 26L (40% body weight)

ECF = 1/3 total water, 13L

  • Interstitial fluid = 16% (3/4 of ECF)
  • Plasma = 4% (1/4 of ICF)
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2
Q

Principles of water balance and solute distribution (what maintains gradient; osmolarity between chambers, role of albumin…)

A
  • Water is in steady state equilibrium between ECF and ICF
  • Solute distribution results from action of Na-K-ATPase (Na in ECF, K in ICF)
  • Albumin contributes to oncotic pressure and is found mostly in the serum
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3
Q

Describe ion transport of Na-K-ATPase

A

Pumps 3 Na out and 2 K into the cell

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4
Q

What changes when water is added to ECF?

A

Water moves from ECF to ICF until the osmolalities are equal

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5
Q

What changes when Na is added to ECF?

A

Water moves from ICF to ECF until the osmolarities are equal

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6
Q

How is plasma osmolality regulated?

A

Water balance

  • Na and water are independently regulated by systems designed to maintains stable EABV and serum osmolality
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7
Q

How is EABV regulated?

A

Sodium balance

  • Na and water are independently regulated by systems designed to maintains stable EABV and serum osmolality
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8
Q

Excess Na causes what? Clinically?

A

Excess Na -> expansion of ECF (edema)

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9
Q

Na deficit causes what? Clinically?

A

Na deficit -> contraction of ECF (volume depletion)

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10
Q

Excess water causes what?

A

Excess water -> hyponatremia (decreased osmolality)

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11
Q

Water deficit causes what?

A

Water deficit -> hypernatremia (increased osmolality)

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12
Q

Urine concentration can vary from what?

A

50 - 1200 mOsm/L

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13
Q

Urine volume can vary from what?

A
  • As little as 0.5 L (water deficit)
  • As much as 12 L (water excess)
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14
Q

What is body osmolality in each compartment?

A

287; equal in all compartments! (ICF = plasma = interstitium)

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15
Q

How can osmolality be measured/calculated? What is normal?

A
  • Measured either by vapor pressure or freezing point depression
  • Ca be calculated from primary solutes that contribute to it:
  • P = 2Na + [glucose]/18 + [BUN]/2.8
  • Normal plasma osmolality ~ 280-290 mOsm/kg
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16
Q

What is the osmolar gap? What is normal?

A

Osmolality: measured - calculated

  • Normal ~ 10 mosm/L
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17
Q

What does an increased osmolar gap suggests what?

A
  • Methanol
  • Ethanol
  • Isopropanol
  • Ethylene
  • Glycol
  • Mannitol
  • Contrast media
  • Glycine
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18
Q

Set-point for ADH release is lower in what populations?

A
  • Pregnant women
  • Asians (some)
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19
Q

Mechanism for (hormonally-regulated) osmoregulation?

A
  • Plasma osmolality is sensed by osmosensors in the hypothalamus
  • This induces the release of ADH and thirst sensation
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20
Q

Where does ADH act in the tubule? What does it do?

A
  • Collecting ducts (via V2 receptors)
  • Enhance water permeability (uptake) through AQP2 (aquaporin2)

(- Thirst simultaneously increases water intake)

  • The resultant water retention -> decrease in Na concentration
  • ADH shut-off decreases water flux through AQP2 -> water clearance -> increased serum Na
21
Q

What are V1 receptors? Functions?

A

Mediated by action of vasopression

  • Vasonconstriction
  • Increased PG synthesis by the kidney
  • PGs reduce the action of ADH on the collecting duct
  • Therefore, NSAIDs cause water retention!
22
Q

What are effective solutes? Examples?

A

Solutes confined to one compartment; may cause water shift between ECF and ICF

  • Hyperglycemia (in insulin deficiency)
  • Contrast media
  • Mannitol
23
Q

What are non-effective solutes? Examples?

A

Membrane permeable and equilibrate between the ICF and ECF (cause no water shift)

  • Urea
  • Ethanol
  • Methanol
  • Isopropanol
24
Q

T/F: In order for changes fluid volume (loss/addition) to be confined to ECF, it must be isotonic with plasma

A

True; must be 0.9% saline

25
Q

What ion is the primary determinant of ECF volume?

A

Na

  • Sodium contributes to the tonicity of ECF
  • As sodium is primarily confined to the ECF (Na-K pump), adding sodium to the body primarily affects ECF & only secondarily affects ICF
26
Q

T/F: Addition/subtraction of pure water affects all compartments (ECF and ICF) equally?

A

True

27
Q

Ex) If 1 L of D5W is administered IV to pts, how much goes into each compartment?

A
  • 2/3 goes into ICF (666 mL)
  • 1/3 goes into ECF (333 mL)

—- Only 1/4 goes into vascular compartment (83 mL)

—- 3/4 goes into interstitial compartment (250 mL)

… Just note, for each L infused, only 83 mL makes it into blood

28
Q

Ex) If 1L of isotonic saline (0.9% saline) administered IV to pts, how much goes into each compartment?

A

As it is isotonic to plasma, all of it stays in the ECF

  • ¼ goes into the intravascular space
  • ¾ goes into the interstitial space
29
Q

Ex) If 1L of half normal saline (0.45%) is adminstered IV to pts, how much goes into each compartment?

A

Think of as 500 mL of water + 500 mL of isotonic saline

  • Water distributes 2/3 ICF , 1/3 ECF with ¼ into vascular space (42ml)
  • Normal saline portion stays in ECF but ¼ in vascular space (125 ml)
30
Q

What is effective arterial blood volume? Easily measured?

A
  • Physiologic concept (not easily measurable)
  • Proportion of intra-vascular volume in the arterial compartment & effectively perfusing the kidneys (body)
  • Adequacy of arterial volume to fill the capacity of the vascular tree
31
Q

What is effective arterial blood volume (EABV)?

A

Clinical conditions in which:

  • cardiac output is decreased, or
  • systemic arterial vasodilation exists Baro-receptors perceive underfilling Result in neurohumoral activation and renal sodium and water retention
32
Q

T/F: EABV and ECF are basically the same

A

False!

33
Q

How do ECF and EABV respond in dehydration?

A

Both are low

34
Q

How do ECF and EABV respond in cirrhosis?

A
  • ECF volume is expanded
  • EABV is diminished
35
Q

How do ECF and EABV respond in CHF?

A
  • ECF volume is expanded
  • EABV is diminished
36
Q

What are some renal indicators of EABV?

A
  1. FENA
  2. Low urine Na
  3. High urine specific gravity
37
Q

How is FENA calculated? What is pathologic?

A

FENA = (Una/Sna)/(Ucr/Scr) x 100

FENA < 1% is CW low EABV (or pre-renal state)

38
Q

What are some tests and physical signs of dehydration?

A
  • Low BP
  • Shock
  • Orthostasis (+ tilt)
  • Decreased skin turgor
  • Decreased CVP
  • Tachycardia
  • Hemoconcentration
39
Q

What happens to the EABV (up/down/same), if we give:

  • 1.2 L of water
  • 400 mL of 3% saline
  • 1 L of normal saline
A
  • Water: EABV increases
  • 3% NaCl: increases
  • NS: increases All increase!
40
Q

What happens to the serum sodium concentration (up/down/same), if we give:

  • 1.2 L of water
  • 400 mL of 3% saline
  • 1 L of normal saline
A
  • Water: Serum Na decreases
  • 3% NaCl: increases
  • NS: same
41
Q

What happens to the urine sodium concentration (up/down/same), if we give:

  • 2 L of water
  • 400 mL of 3% saline
  • 1 L of normal saline
A
  • Water: Urine Na increases
  • 3% NaCl: increases
  • NS: increases
42
Q

How does urine Na compare to EABV? Serum Na? ECF volume?

A
  • Urine reflects EABV
  • Does not necessarily reflect serum Na (ore even ECF volume!)
43
Q

All the effectors that are induced in low EABV (including ADH) increase ____ by the kidney, directly or indirectly

A

All the effectors that are induced in low EABV (including ADH) increase Na by the kidney, directly or indirectly

44
Q

Thus, low EABV is associated with increased ______ (____follows)

A

Thus, low EABV is associated with increased renal Na uptake (water follows) -

45
Q

High EABV is associated with ______ Na+ uptake

A

High EABV is associated with decreased Na+ uptake

[Na+ retentive hormones shut off (plus ANP release)]; i.e., natriuresis and diuresis.

46
Q

ANP (BNP) is produced in response to what? What does it do?

A

Stretch (volume expansion)

  • Works on the kidney to induce natriueresis
47
Q

ANP (BNP) is a marker of what?

A

ANP (BNP) is a reliable marker for heart failure,

48
Q

Is ANP (BNP) effective in the treatment of Na retention (edema)?

A

Has little usefulness in the treatment of Na retention (edema ), due to the operation of compensatory renal pathways that offset its action.

49
Q

Summary

A

MS2 Renal (25 decks)

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