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Renal Week 1 > Proximal Tubular Dysfunctions and Disorders of Water Balance II > Flashcards

Proximal Tubular Dysfunctions and Disorders of Water Balance II Flashcards

1
Q

What determines fluid distribution between body compartments?

A

Osmotic pressure

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2
Q

What are ‘effective’ osmoses? Ineffective?

A

(main are Na and K) that determine body fluid osmolality and

“ineffective” such as urea and glucose (the latter two cross easily between body compartments)

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3
Q

What is the eqn for plasma osmolality (Posm)?

A
  • Posm= 2 x [Na]+ [glucose]/18 + [urea]/2.8

- Simply Effective Posm = 2 x [Na]

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4
Q

What is the eqn for total body osmolality?

A

Total body solutes (extra- and intracellular)/

Total body water

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5
Q

What is the estimated eqn for plasma Na?

A

Total body exchangeable Na and K (Nae + Ke)/
Total body water

Plasma Na directly correlates with changes in Nae + Ke and indirectly with changes in total body water. However, the numerator is hard to change (although not impossible), therefore, changes in plasma sodium mainly determined by changes in total body water*

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6
Q

What are Osmoreceptors?

A

osmo-sensing neurons with Ca2+-permeable cation channels serving as stretch receptors and regulating water balance.

When stimulated by high osmolarity, these promote thirst (because water has been lost) and AVP secretion

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7
Q

What else regulates water balance?

A

Neurons in suproptic and paraventricular nuclei secrete AVP/ADH and oxytocin (stimulates AVP receptors, but has weaker action than AVP)

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8
Q

What are the 2 types of ADH receptors in the kidney?

A
  • V2R: the principle cells, the connective tubule cells, distal convoluted tubule cells, thick ascending limb tubule cells, regular water absorption
  • V1a: vascular cells of medulla, regulate renal blood flow
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9
Q

Overview of mechanisms maintaining serum osmolality.

A
  • increase in Posm or a decrease in effective circulating volume (hypotension), nausea, pain, drugs corticosteroid deficiency etc.
  • activation of supraoptic and paraventricular nuclei in the hypothalamus
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10
Q

What does activation of supraoptic and paraventricular nuclei in the hypothalamus stimulate?

A
  • thirst
  • ADH secretion (regulate aquaporin 2 in the renal CD)

both cause increased water reabsorption, and thus decreased Posm

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11
Q

What is the normal Posm?

A

285-290 mOsm/kg

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12
Q

What is the normal osmotic threshold for ADH release?

A

280-290 mOsm/kg

NOTE: In absence of ADH, Urinary osmolality (Uosm)
can be lowered to 40-60 mOsm/kg H2O

Maximal Uosm ~1200 mOsm/kg H2O

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13
Q

What is the normal osmotic threshold for thirst?

A

290-295 mOsm/kg

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14
Q

Change in Posm as little as __% stimulates ADH secretion

A

1% (relationship is concave up). Similarly, a decrease in

blood volume >7% stimulates ADH secretion

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15
Q

How does ADH act in the collecting duct?

A

regulates apical AQP-2 in the collecting duct

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16
Q

How does ADH regulate AQP-2?

A

two ways:

  • short-term: “shuttle hypothesis”-within minutes rapid and reversible increase in AQP-2 in luminal surface stimulating water reabsorption
  • long term: increase synthesis of AQP-2 via gene regulation, occurs >24hours, not readily reversible
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17
Q

What is the eqn for estimating total body water from plasma sodium?

A

1) Water exc.→ ↓ PNa
Water excess = 0.6 TBW (weight) x (1-Na(observed)/140)

2) Water deficit→ ↑PNa
Water deficit= 0.6 TBW (weight) x (Na(observed)/140-1)

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18
Q

What is hypoateremia defined as?

A

[Na+] less than 135 meq/L

most common electrolyte disorder

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19
Q

What is increased osmolality (hyperosmolar) hyponatremia?

A

due to presence of other osmotically active substances that cause water movement out of cells - “dilutional” hyponatremia (glucose in absence of insulin, mannitol, glycine)

20
Q

What is pseudohyponatremia?

A
  • Normal osmolality and hyponatremia (pseudohyponatremia): occurs due to limitation of some Na assays when Na is measured in the whole plasma while solid phase of plasma (usually 6-8%) is greatly increased, such as with hypertriglycerdiemia or paraproteinemia
21
Q

What is Decreased osmolality (hypoosmolar) hyponatremia?

A

true hyponatremia - always due to impaired urinary dilution mechanisms

22
Q

What is the cause of true hypoatremia?

A

Occur as a result of increase in Total body water (TBW), which can be absolute or relative to total Na

Changes in TBW can co-exist with total Na changes (decrease or increase in Na); however, TBW increase predominates

23
Q

T or F. Appropriate or inappropriate increase in ADH is usually present in majority of cases of true hyponatremia

A

T.

24
Q

What are some causes of true hypoatremia?

A
  • Hyponatremia with volume depletion
  • Hyponatremia with normal volume status
  • Hyponatremia with volume overload
25
Q

When does Hyponatremia with volume depletion occur? Both decrease, but water is retained more compared to the osmoles due to ADH secretion

A
  • renal losses (diuretics, aldosterone deficit)
  • gastrointestinal losses (diarrhea, vomiting, bleeding)
  • skin losses (excessive sweating while ingesting some free water)
  • third spacing (pancreatitis, bowel obstruction, burns)
26
Q

When does Hyponatremia with normal volume occur? water increases and Na remains the same

A
  • Syndrome of inappropriate ADH release (drugs, tumors, pain, any lung or brain diseases)
  • Glucocorticoid deficiency (loss of negative feedback stimulates corticotropin releasing hormone which in turn stimulates ADH)
  • Hypothyroidism (increase ADH possibly due to low cardiac output state and impaired urine dilution from decreased GFR)
27
Q

When does Hyponatremia with volume overload occur? Both osmoses and water increase but water increases more

A

(however, effective circulating volume is usually decreased):

  • Congestive heart failure
  • Acute and Chronic kidney failure
  • Cirrhosis
  • Nephrotic syndrome
28
Q

What are some other causes of true hyponatremia?

A
  • Reset osmostat: lowered set point for ADH release
  • Psychogenic polydipsia: overwhelming of urinary diluting ability; urine osmolality will easily differentiate between inappropriate ADH action and polydypsia
29
Q

What is hyperatremia defined as?

A

serum [Na+} above 145 meq/L

30
Q

When does hyperatremia occur?

A

as a result of decrease in TBW, which can be absolute or relative to total Na

NOTE: Changes in TBW can co-exist with total Na changes (decrease or increase in Na); however, TBW decrease predominates

31
Q

What is the main defense mechanism against hyperatremia?

A

Thirst

32
Q

When is thirst impaired?

A

hypernatremia usually develops if thirst is impaired (aging, mental status changes) or there is limitation in access to free water (ex: intubated patient in ICU)

33
Q

The causes of hyperatremia are either sodium gains or increased water losses. How can sodium gains occur?

A

primary hyperaldosteronism,

Cushing’s disease,

hypertonic hemodialysis,

administration of hypertonic NaCl or NaHCO3

34
Q

How can water loss (unreplaced) occur?

A
  • Renal losses: central or nephrogenic diabetes insipidus, osmotic dieresis (glucose, mannitol), postobstruction dieresis, chronic kidney disease
  • Extrarenal losses: excessive sweating, insensible losses (respiratory, dermal), burns, diarrhea
35
Q

Diabetes insipidius (DI) is divided into two parts. What are they?

A
  • Central DI due to lack of ADH (can be acquired or congenital)
  • Nephrogenic DI due to resistance to ADH
36
Q

What are the causes of congenital Nephrogenic DI?

A

congenital forms (V2 receptor or AQP-2 mutations)

37
Q

What are the causes of acquired Nephrogenic DI?

A
  • defect in generation of medullary interstitial tonicity (chronic kidney disease-CKD, hypokalemia, hypercalcemia, sickle cell disease, protein malnutrition)
  • defect in cAMP generation (CKD, hypokalemia, hypercalcemia, demeclocycline, lithium)
  • AQP-2 downregulation (CKD, hypokalemia, protein malnutrition, lithium)
  • pregnancy (placental synthesis of vasopresinase which cleaves AVP)
38
Q

What is Cosm?

A

osmolar clearance: water needed to excrete solutes at the concentration of solutes in plasma (obligatory water excretion)

39
Q

Estimation of renal water excretion.

A

Cwater – free water clearance

V (urine volume) = Cosm + Cwater; therefore, Cwater = V - Cosm

Cosm = (Uosm/Posm) x V. Thus, Cwater = V x (1-Uosm/Posm)- to tell us what is happening with ADH in a situation of Na disorder

40
Q

T or F. In hypotonic urine Cwater is positive as Uosm is less than Posm

A

T.

  • in isotonic there is no Cwater as Uosm = Posm
  • in hypertonic urine Cwater is negative as Uosm is greater than Posm
41
Q

What is polyuria?

A

Increase in urine volume > 3L/day

42
Q

What things can cause urine volume (V) to increase?

Remember: V= Cosm+ Cwater

A
  • Cosm increases (diuretic use, renal salt wasting, excess salt ingestion, bicarbonaturia due to vomiting, osmotic dieresis, hyperalimentation)
  • Cwater increases (DI, primary polydipsia)
43
Q

How can you distinguish DI (central or nephrogenic) vs Primary polydipsia?

A

all will have polyuria and low Uosm (below 100mOsm/kg H2O), but:

Posm and Pna will be high in DI and low in PP.

44
Q

How will patients with DI respond to water deprivation?

A

Uosm shows no change while in PP Uosm increases over 500

45
Q

How will patients with DI respond to DDAVP?

A

Central DI- increase

Nephrogenic DI- little/no change

PP- little no change

Renal Week 1 (20 decks)

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