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Clinical Pathology > Water Balance & Hydration Status > Flashcards

Water Balance & Hydration Status Flashcards

1
Q

How much of the body is made up of water? What are the 2 compartments?

A

TBW = 60%

  1. ECF (33%) - 25% intravascular, 75% interstitial
  2. ICF = 66%
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2
Q

What is the difference between osmolality and osmolarity?

A

OSMOLALITY - [solute] per kg of SOLVENT (plasma/serum), mOsm/kg (solution with an osmolality of 1.0 = 1 osmole of solute added to 1 kg (1 L) of water)

OSMOLARITY - [solute] per L of SOLUTION mOsm/L (solution with an osmolarity of 1.0 = 1 osmole of solute with water added to make 1 L)

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3
Q

How is osmolality measured? What equation is used to estimate it? What are the normal values in dogs, cats, horses/cows?

A

freezing point depression osmometry, which measures all of the osmoles in plasma

mOsm/kg = 2[Na + K (mmol/L)] + [glucose (mg/dL)]/18 + [BUN (mg/dL)]/3

  • DOG = 210-310 mOsm/kg
  • CAT = 290-300 mOsm/kg
  • HORSE/COW = 270-300 mOsm/kg
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4
Q

What is diuresis? What are the 2 types?

A

urine flow is greater than normal (> 1-2 mL/kg/hr in dogs and cats)

  1. OSMOTIC: increased urine flow caused by excessive amounts of impermeant solutes within the renal tubules (Na, glucose from DM, mannitol), causing the urine osmolality to approach plasma osmolality
  2. WATER: increased urine flow caused by decreased reabsorption of free water, causing urine osmolality to drop below plasma osmolality (DI)
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5
Q

What is specific gravity? What 2 things does it depend on? How does it relate to osmolality?

A

ratio of weight of a volume of liquid to the weight of an equal volume of distilled water

  1. number of particles present
  2. molecular weight of those particles

estimate of osmolality —> linear relationship

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6
Q

What are the 2 methods of maintaining blood volume and body fluid tonicity?

A
  1. ADH/vasopressin control water balance (osmolality, [Na])
  2. aldosterone controls sodium and ECF volume
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7
Q

What 4 things does water balance depend on? What does it require?

A
  1. adequate intake
  2. renal and GI function
  3. losses in sweat and respiration
  4. neural control

FUNCTIONAL KIDNEY - adequate delivery of tubular fluid (plasma flow, GFR, isoosmotic resorption of Na and water in PT), ascending loop of Henle, CDs impermeable to water

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8
Q

What physical sensors regulate water balance? How do they respond to changes in osmolality?

A

osmoreceptors in the hypothalamus

  • HYPEROSMOLALITY (low body water): osmoreceptors shrink and stimulate ADH release, which increases water reabsorption via the kidney and stimulates the thirst response
  • HYPOOSMOLALITY (excess body water): osmoreceptors swell and inhibit ADH release, which increases renal water excretion
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9
Q

How does the renin-angiotensin-aldosterone system affect ADH secretion?

A

stimulates ADH release if needed for hypovolemic state regardless of sodium concentration

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10
Q

What is the primary regulator of blood volume? How does water concentration affect the concentration of it?

A

sodium [Na] = mmol (# Na molecules in plasma) / L (water in plasma)

  • increased water = decreased [Na]
  • decreased water = increased [Na]
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11
Q

How is blood volume regulated? What 3 hormones are involved? What are the major outcomes?

A

sensing atrial stretching and renal perfusion pressure

  1. renin, angiotensin II, aldosterone
  2. sympathetic nervous system
  3. atrial natriuretic peptide (ANP)

urine Na retention or excretion

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12
Q

How does the kidney affect blood volume?

A
  • renal Na resorption = water follows = blood volume expands
  • renal Na excretion = water follows = blood volume contracts
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13
Q

What are the 4 major water components in the ECF compartment?

A
  1. intravascular (-volemia): blood volume
  2. intercellular fluid
  3. transcellular (third spaces): pleural, peritoneal, pericardial
  4. GI
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14
Q

What does TBW volume determine?

A

hydration status - controlled by water intake (thirst) and renal output

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15
Q

Where are electrolytes most commonly measured in? What organs take part in maintaining their concentrations?

A

serum —> may not completely reflect balance of electrolytes in the body, especially for intracellular electrolytes

GIT and kidneys

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16
Q

What are the most common electrolytes found in the ICF and ECF?

A

ICF: potassium*, calcium, phosphorus, magnesium

ECF: sodium (renal conservation of water), chloride (secretory fluids)

17
Q

What ions are coupled with the movement of chloride?

A

SODIUM = same direction for electroneutrality

BICARBONATE = opposite direction

18
Q

How is potassium affected by acid-base imbalances?

A

exchanged between ICF and ECF

19
Q

What solutes mostly affect osmolality of the ECF? What does it influence?

A

electrolytes (Na+, K+), proteins, glucose, urea

changes in ECF osmolality affects water shifts between ECF and ICF

20
Q

How do measured and calculated osmolality compare?

A

MEASURED = accounts for all osmoles in the blood

CALCULATED = represents osmoles reported in the chemistry panel

21
Q

What are the most common causes of water shifts between ICF and ECF? How do they cause this?

A
  • hypernatremia
  • hyperglycemia
  • ketoacidosis
  • uremia
  • exogenous toxins (ethylene glycol)

changes plasma osmolality

22
Q

What causes plasma hyperosmolality without fluid shifts? What is an example? What clinical signs are associated?

A

increased plasma concentration of ineffective solutes causing the intracellular osmolality = extracellular osmolality

uremia - urea is freely filterable across cell membranes

no clinical signs

23
Q

What does plasma hyperosmolality cause? What are 2 common causes? What clinical signs are associated?

A

increased plasma concentration of effective solutes, where the intracellular osmolality is less, so the cell becomes dehydrated and loses water

hypernatremia, severe hyperglycemia (DM)

those manifested of cerebral cellular volume - neurologic changes associated to cellular dehydration, depression, stupor, coma, CNS deficits, seizures

24
Q

What does plasma hypoosmolality cause? What is a common cause? What clinical signs are associated?

A

increased plasma concentration of water, where intracellular osmolality is higher, so the cell swells with water

hyponatremia

cerebral edema and cell lysis - lethargy, weakness, altered mentation, obtundation, seizures, death

25
Q

What is the osmolar gap? What is the normal value? What is happening when it is elevated?

A

measured osmolality (ALL osmoles) compared to calculated osmolality (osmoles in BIOCHEM)

10 mOsm/kg

> 10 mOsm/kg = presence of osmolar substances NOT included in the osmolality calculation, like ethylene glycol, mannitol, ethanol, methanol, or radiographic contrast

26
Q

What is dehydration? What 4 clinical signs are associated?

A

fluid loss exceeds fluid intake, where TBW decreases

  1. increased skin tent
  2. increased CRT
  3. tacky MM
  4. retracted eyes
27
Q

What 5 clinicopathological parameters are observed in dehydrated patients?

A
  1. erythrocytosis
  2. (pre-renal) azotemia
  3. hyperproteinemia
  4. hypersthenuria
  5. electrolyte abnormalities
28
Q

What is overhydration? How can patients be hypovolemic at the same time?

A

increased TBW with accumulation of fluid in extracellular or third spaces

effusions, in GIT (rumen)

29
Q

How is dehydration classified? What are the 3 types?

A

type of fluid loss and remaining tonicity of fluids (confirm dehydration, then look at plasma concentration)

  1. hypertonic - water loss > Na loss = Na increased
  2. isotonic - water loss = Na loss = Na normal
  3. hypotonic - water loss < Na loss = Na loss
30
Q

What is hypertonic dehydration? What are the 5 main differentials?

A

water loss is in excess of electrolyte loss, causing plasma [Na] and osmolality to increase —> water shifts from ICF to ECF

  1. diabetes insipidus
  2. diabetes mellitus
  3. osmotic diuresis
  4. osmotic diarrhea
  5. water deprivation
31
Q

What is isotonic dehydration? What are the 2 main differentials?

A

water loss is equivalent to electrolyte loss, causing plasma [Na] and osmolality to remain the same —> no shift between compartments, but ECF volume decreases

  1. renal disease
  2. diarrhea
32
Q

What is hypotonic dehydration? What are the 4 main differentials?

A

electrolyte loss is greater than water loss, causing plasma [Na] and osmolality to decrease —> water shifts from ECF to ICF, causing volume depletion

  1. secretory diarrhea
  2. vomiting
  3. 3rd space loss
  4. heat stress and sweating in horses - Cl- losses are greater than Na+ losses
33
Q

What 3 problems are associated with hypotonic dehydration?

A
  1. fluid shifts from vasculatory into cells, causing vascular volume to decrease and cells swell —> edema when Na < 115-120 mEq/L
  2. osmoreceptors are not stimulated and ADH is not released (stimulated by increased osmolality and decreased plasma volume)
  3. medullary washout - increased urine flow rate resulting in impaired reabsorption of Na, Cl and urea
34
Q

What is another cause of medullary washout other than hypotonic dehydration?

A

increased medullary blood flow in the vasa recta flushes out solutes accumulating and creatin hypertonicity in medulla caused by hypokalemia and hypercalcemia

35
Q

What are common causes of overhydration? What 3 conditions does this typically lead to?

A
  • iatrogenic fluid overload from IV administration with inappropriate elimination
  • heart failure
  • renal obstruction
  • oliguria/anuria from renal failure
  1. cardiovascular overload
  2. pulmonary edema
  3. generalized edema

Clinical Pathology (33 decks)

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