2. Sodium and Water I Flashcards
how should we think of volume regulation and osmoregulation?
they are separate: linked by ADH, but separate processes
what is the difference between osmolarity and osmolality?
for the purposes of this course, they can be used interchangeably. they refer to concentration of solute per L or Kg of solvent
what formula describes the movement of water via osmotic pressure?
osmotic pressure = nCRT n = dissociable particles per molecule C = concentration of solute in solvent R = constant (0.082) T = temp in K
what are some common isotonic fluids?
normal saline, Ringer’s Lactate
what are some common hypotonic fluids?
1/2 normal saline, 5% dextrose in water (rapidly metabolized to 0 mosm/L)
what is a common hypertonic fluid?
3% NaCl
hypo/hypernatremia is a problem of what?
water
hypo/hypervolemia is a problem of what?
sodium
sodium determines volume where?
extracellular volume
usual concentration of sodium in extracellular space?
140 mmol/L
what is the primary extracellular osmole?
sodium
what is a quality of an effective osmole?
in order to exert osmotic pressure, a particle cannot cross membrane
how does water move between the 3 body compartments?
moves freely
how are changes in osmolar content balanced?
by redistribution of water
is urea an effective osmole?
no
is glucose an effective osmole?
yes unless insulin is present
body water is what % of males body wt? what % of females body wt?
60% males
50% females
what is normal plasma osmolarity?
280-290 mosm/L
how can we estimate plasma osmolarity (formula)?
formula: 2 x ([Na] + [glucose]/18 + [BUN/2.8])
if BUN is normal: 2 x ([Na] + [glucose]/20
if calculated osmolarity and measured osmolarity are different by more than 10 mosm/L (measured will be higher), what is probable?
presence of a foreign substance: alcohols, poisons
what is the primary determinant of plasma osmotic pressure? why?
albumin: confined to plasma
high serum Na will do what? low serum Na will do what?
high serum Na will draw water out of cells. low serum Na will send water into cells
overall, what will a change in serum Na accomplish?
a redistribution of total body water. not necessarily a change in absolute total body water.
low serum albumin can be associated with what?
edema, as fluid leaves intravascular space for the interstitium
adding K to serum will accomplish what?
water migrates to from extracellular space -> intracellular space;
changes to cell volume will affect what organ in particular?
brain. look here first for evidence of swelling/shrinking
hyponatremia (low serum sodium) will promote water movement in what direction?
into cells, increasing their volume
how does the brain defend itself against changes in osmolarity?
can resist change by varying concentration of solutes other than Na and K such as inositol or AAs. glial cells can create and secrete these molecules and blunt effect of systemic change. can also remove these small molecules if needed.
what are idiogenic osmoles?
the solutes that the brain can create (inositol or AAs) in order to protect itself rom systemic changes in concentration.
how long does the production or removal of idiogenic osmoles take?
2-3 days
osmolarity is monitored by what receptors, and where?
osmoreceptors in the hypothalamus
increases in osmolarity as sensed by the osmoreceptors in the hypothalamus triggers what?
ADH release, thirst
what does ADH stimulate?
water retention, peripheral vasoconstriction
at what plasma osmolarity does ADH level begin to rise quickly?
right at 280, and thirst increases accordingly
ADH released more dramatically in response to blood volume depletion or in response to hyper-osmolarity?
blood vol depletion trumps osmoregulation
how is water handled in the PT?
water passively follows solutes
how are water and NaCl handled in the LoH?
both are reabsorbed independently of each other
how is water handled in the CT?
water can be reabsorbed independently of solutes (via aquaporins)
what is the range of average urine output?
0.5 to 12L per day
in the absence of ADH, will the urine be more or less dilute?
more dilute: in the absence of ADH, relatively dilute urine from the DCT becomes further diluted by Na reabsorbtion in the collecting tubule
what will increase osmolarity of the renal medulla?
NaCl reabsorption in the LoH
volume regulation: what senses changes in the Na flow rate?
Macula densa senses increased Na load (indication of incr tubular pressure) and feeds back to the afferent arteriole, which constricts in response
volume is monitored by what receptors, and where?
baroreceptors in the carotid sinus sense decr arterial pressure
atrial stretch receptors sense volume expansion
when baroreceptors in the carotid sinus sense decr arterial pressure, how do they respond?
stimulate the adrenergic system, release ADH
when atrial stretch receptors sense volume expansion, how do they respond?
stimulate ANP release
what does Effective Circulating Volume refer to?
the perfusing, bioavailable blood volume (plasma/intravascular blood: not interstitial blood)
what compartment of the total body volume is available to stimulate baroreceptors?
only the plasma/effective circ volume
hyperaldosteronism can lead to what state?
abnormal fluid retention, hypervolemia
what is the typical response to low ECV (effective circ volume)?
incr cardiac output, incr peripheral resistance, incr intravascular volume via Na and water retention
What does renin do?
cleaves angiotensinogen to angiotensin I
what does ACE do?
converts angiotensin I to angiotensin II
what are 5 actions of angiotensin II?
- direct vasoconstriction of the efferent arteriole
- incr Na and HCO3- reab in the prox tubule
- stimulates aldosterone release
- systemic vasoconstriction
- increase thirst (though ADH is the primary stimulator of thirst)
when Na and HCO3- are reabsorbed in the PT (as a result of incr amounts of AtII), what happens to the water?
follows passively, resulting in isotonic fluid retention
where is aldosterone synthesized?
adrenal cortex
where does aldosterone primarily act?
collecting duct
what does aldosterone do to Na and K?
stimulates Na channels in principle cells of collecting duct, increases Na absorption and K secretion
what does aldosterone do to H+?
stimulates H+ secretion in intercalated cells
what does ANP do to vessels?
1) incr. Na secretion in medullary CD
2) direct systemic vasodilator, lowers BP.
3) vasodilation of the afferent arteriole leads to increased GFR.
4) inhibits renin, aldosterone, and ADH
what does ANP do to Na?
stimulates Na excretion in medullary CD
what effect does an elevated BP have in normal patients?
triggers Na diuresis: increased renal blood flow causes suppression of renin release, and ANP is released. intravascular volume falls, reducing BP
