50: Sodium Disorders Flashcards
What is the normal plasma Na concentration?
147-156 mEq/L
what is the unit of osmolality?
mOsm/kg
what is the unit of osmolarity?
mOsm/L
what is the calculation for serum osmolality?
2Na +BUN/2.8 + glucose/18
2.8 and 18 to convert BUN and glucose from mg/dL to mmol/L
What are the main effective osmoles?
Na and K
effective because do not cross cell membrane freely
What regulates the plasma osmolality?
hypothalamic osmoreceptors
induces compensation from changes in osmolality as low as 2-3 mOsm/kg
What are the 2 main compensatory systems counteracting changes in osmolality
- ADH system
- thirst
Where is ADH secreted?
by the posterior pituitary gland
What stimuli lead to ADH secretion?
- elevated plasma osmolality
- decreased effective circulating volume
Where are the osmoreceptors located?
Hypothalamus
What receptors sense decreased circulating volume and where are they located?
baroreceptors located in the aortic arch and carotid bodies
==> send neuronal impulses to the pituitary gland ==> ADH release
Explain how ADH regulates water balance
low circulating volume/high osmolality ==> ADH release by pituitary gland
* ADH activates V2 receptors on renal collecting tubular cells
* inserts aquaporin-2 molecules into luminal cell membrane
* aquaporin = channels allowing water to move across renal tubular cell membrane
* water will follow its osmotic gradient, kidneys must generate hyperosmolar medulla to achieve this
What may impair effective water reabsorption by ADH secretion
- water will follow its osmotic gradient through aquaporin channels
- kidneys must generate hyperosmolar medulla to achieve this
- impaired in disease or administration of diuretics ==> loss of osmotic gradient
What is prioritized, maintenance of effective circulating volume or osmolality?
circulating volume is always prioritized
if low effective circulating volume ==> ADH will be released regardless of osmolality ==> hyponatremia/hypoosmolality
e.g., congestive heart failure
What are the effects of Aldosterone?
- water resorption
- Na resorption
- K excretion
What are the effects of ADH?
- water resorption
What is considered severe hypernatremia?
serum Na cc > 180 mEq/L
List clinical signs of hypernatremia
- obtundation
- head pressing
- seizures
- coma
- death
What happens to cells during hypernatremia?
shink
water moves down osmotic gradient
How may hypernatremia affect the heart?
associated with decreased myocardial activity
Explain the early (minutes) neuronal adaption to hypernatremia
- neuronal water is lost to the hypernatremic circulation ==> decreased interstitial hydrostatic pressure ==> fluid drawn from CSF to interstitium of the brain
- Na and Cl move from CSF into cerebral tissue ==> increases neuronal osmolality ==> draws back water into the IC space
Explain the delayed (24h+) neuronal adaption to hypernatremia
neurons accumulate organic solutes to increase IC osmolality and retain water
how long does accumulation of idiogenic osmoles in neurons take?
begins within few hours
full compensation within 2-7 days
name 3 idiogenic osmoles
- inositol
- glutamin
- glutamate