Fluid & Electrolytes Flashcards
Causes of hypokalemia
increased diuresis
GI losses (NG suction, diarrhea, vomiting, laxatives )
insulin (activates Na/K pump)
metabolic/respiratory alkalosis
B2 agonists
hyperaldosteronism
erythropoiesis, leukocytosis, thrombocytosis (K+ stored inside cells)
Consequences of hypokalemia
muscle paralysis
respiratory arrest
Cell membrane permeability
freely permeable to water
not permeable to electrolytes
Where is the thirst center located
In the hypothalamus
*very powerful autonomic reflex –> dehydration is rare unless person is physically or cognitively impaired
What stimulates the thirst center
osmoreceptors (detect increased plasma osmolality)
angiotensin II
neurons in the mouth that detect dryness
Fluid Intake
ingested fluids
foods –> absorbed in GI tract
metabolic water (~200 mL/daily)
Fluid output
kidneys –> urine
GI tract –> feces
skin –> sweat
lungs –> expired air
Diuresis
increased water excretion
Natriuresis
increased sodium excretion
What stimulates renin release
decreased blood pressure
decreased blood volume
stress (B1 adrenergic receptors)
macula densa cells (detect low sodium in the DCT)
Hormones regulating fluid/electrolyte balance
Angiotensin II Aldosterone* ADH* Natriuretic pepties (ANP & BNP)* Cortisol (mild mineralocorticoid fx)
What stimulates ADH release
increased serum osmolality
decreased blood volume/blood pressure
ATII stimulation
stress
Aldosterone mechanism
increases insertion of Na+/K+ pumps in the DCT increasing sodium reabsorption
increases potassium excretion
ADH mechanism
increases insertion of aquaporin 2 channels in the DCT/collecting duct
causes systemic vasoconstriction
Natriuretic hormone mechanism
increase sodium excretion
decrease SNS activity –> inhibit renin release
afferent arteriole vasodilation –> increase GFR (increase diuresis)
vasodilation
Water intoxication
decreased osmolality of plasma causes a fluid shift from ECF –> ICF
cellular swelling –> cellular lysis
cerebral edema –> convulsions, coma, death
Insulin & Potassium
insulin increases insertion of Na+/K+ gates –> increases K+ movement into cells
Fluid compartments
ICF
ECF
Divisions of ECF
intravascular (blood + lymph)
extravascular (interstitial, serous membranes, aqueous humor)
Percentage of K+ stored intracellulary
98%
ECV imbalances
fluid volume excess –> hypervolemia
fluid volume deficit –> hypovolemia
does not cause a osmotic shift between fluid compartments
Osmolality imbalances
hypernatremia –> cellular dehydration
hyponatremia –> cellular swelling
causes osmotic shift between fluid compartments
Common post-op fluid loss
estimated blood loss vomiting diarrhea decreased intake (NPO, N/V, paralytic ileus) fever drainage (NG tube, chest tube, hemovac) intra-op insensible loss (open cavity surgery) new ileostomy
Common post-op fluid gain
IV fluid therapy
feeding tubes
fluid retention d/t PSR (ADH & aldosterone rls)
Common intracellular cations
potassium
magnesium
Common intracellular anions
phosphate –> usually attached to organic molecules like ATP
proteins
Common extracellular cations
sodium
hydrogen
Common extracellular anions
bicarbonate
chloride
Sources of sodium intake
processed food seasoning medication canned food condiments canned food
Routes of sodium loss
kidneys –> urine
GI –> feces
skin –> sweat (hypotonic)
Na+/K+ pump
pumps out 3 sodium from ICF –> ECF
pumps in 2 potassium from ECF –> ICF
*maintains resting membrane potential
Types of hyponatremia
hypovolemia –> water lost in excess of sodium
euvolemia –> water and sodium lost in equal amounts but water restored from ICF or d/t aldosterone/ADH rls
hypervolemia –> sodium lost in excess of water. dilutional hyponatremia
Normal sodium range (ECF)
135-145 mmol/L
Normal potassium range (ICF)
3.5-5.0 mmol/L
Causes of hyponatremia
sodium restricted diet diuretics (thiazide/loop) vomiting, diarrhea, NG suction excess intake of pure water --> dilutional fx conditions causing increased ADH rls --> CHF, cirrhosis, PSR excessive hypotonic IV fluids hyperglycemia (fluid shift) inadequate aldosterone
Causes of hypernatremia
excessive administration of sodium IV fluids (normal saline)
excessive dietary intake
primary hyperaldosteronism
insufficient water intake (dehydration)
increased hypotonic fluid losses –> sweating, RR, watery diarrhea, osmotic diuresis
ADH deficiency, diabetes insipidus
Consequences of hyponatremia
cerebral edema –> neurologic symptoms (headache, impaired LOC, nerve/motor function, seizures)
peripheral edema
Hyponatremia & cellular excitiability
hyponatremia makes depolarization slower = reduced excitability
Na+ needed to cause depolarization
Excitable cells
neurons
skeletal muscle fiber
cardiac cells
smooth muscle cells
Resting membrane potential
-70 mV
Threshold
- 55 mV
* at this point voltage-gated sodium channels open –> depolarization
Depolarization mV
+30 mV
Action potential physiology
1) membrane depolarizes to threshold (-55 mV) causing opening of voltage-gated sodium channels
2) sodium rushes into cell causing cell to reach 0 –> +30 mV
3) repolarization –> potassium exits cell to reestablish RMP
Factors impacting clinical manifestation of hyponatremia
underlying cause
acute vs. chronic onset
severity
assoc S/S of fluid gain/loss
Hyponatremia CNS symptoms
d/t cerebral edema
fatigue headache confusion (altered LOC) seizures coma