Sodium and Water Balance Flashcards
what hormone controls water balance and where is it secreted from?
ADH (aka vasopressin)
secreted from posterior pituitary
what does ADH do?
makes you pee less by causing water to be reabsorbed from the renal tubules
increased ADH = small volume of concentrated urine
decreased ADH = large volume of dilute urine
what injury can cause a decrease in ADH secretion?
transection of pituitary stalk in head injury
diabetes insipidus
how is urine concentration measured?
urine osmolality
high osmolality = concentrated urine
low osmolality = dilute urine
…
sodium and (potassium?) pumped out of the loop of henle?
what controls sodium balance?
steroids
- aldosterone (mainly)
- other steroids (e.g cortisol)
what do steroids do to control sodium balance?
mineralocorticoid activity
- refers to Na+ reabsorption in renal tubules in exchange for K+/H+
too much mineralocorticoid activity = sodium gain
too little = sodium loss
how do you calculate sodium concentration?
mmol Na+ / 1 L water
how do intra and extracellular fluid differ/
intra = contains more fluid intra = lower Na+ concentration
can water move between all body compartments?
yes
means that if you lose water you loose it from everywhere
can sodium move between all body compartment?
no
confined to the extracellular fluid
- Na+/K+ pump in plasma membrane keeps sodium in ECF
- if sodium is lost its only lost from ECF
how does water balance relate to sodium balance?
water follows solute (sodium is the most abundant solute so water basically follows sodium)
so if you loose or gain sodium, you loose or gain water with it
what are the two possible causes for reduced concentration of sodium?
loss of sodium from the ECF (dangerous) fluid retention (too much water dilutes sodium)
what can cause increase in sodium concentration?
loss of water
increase in ECF sodium
why is loss of sodium from ECF dangerous?
little fluid in ECF anyway
if sodium lost then water will follow causing reduction in fluid volume
will have severe symptoms
what can cause sodium loss?
adrenal/kidney (adrenals not making steroid hormones so not reabsorbing sodium and water)
gut (diarrhoea and vomiting, fistulae)
skin (burns)
what can cause fluid overload (too much water)?
reduced water excretion (SIADH) increased intake (compulsive water drinking)
what can cause increased sodium?
some IV medications
near drowning (consumption of high amount of sea salt water)
malicious (child been given salt by parent)
what can cause loss of water?
diabetes insipidus (increased water loss) reduced water intake
how do you manage low sodium concentration?
if too little sodium - give sodium
if too much water - fluid restriction
how do you manage increased sodium concentration?
too much sodium - remove sodium (i.e loop diuretic)
too little water - give water (as dextrose)
what 3 fluids can be given and where does each affect?
dextrose = plasma, interstitial fluid and intracellular fluid compertment saline = plasma and interstitial fluid plasma/blood = plasma
when is sodium inbalance serious?
if concentration <120 or >160
if high or low enough to cause symptoms (confusion, vomiting, nausea etc)
if sodium level has fallen or risen rapidly to its current level
what are the 2 kinds of stimuli for ADH release?
osmotic (in health)
non-osmotic (in disease)
give 3 examples of non-osmotic stimuli?
hypovolaemia/hypotension
pain
nausea/vomiting
what is SIADH?
syndrome of inappropriate ADH
2 opposing forces in capillary which control balance of water?
hydrostatic force (push water out) oncotic force (pull water in)
what can cause reduced oncotic force/increased hydrostatic force?
low proteins in the capillary (e.g low albumin)
what is the impact of low blood volume on sodium?
causes ADH and aldosterone secretion which causes sodium and water retention leading to hyponatraemia
much of water reabsorbed ends up being pushed out into interstitial fluid and causing oedema
what does oedema signift?
effective circulating volume depletion
low blood volume
how is oedema managed?
loop diuretics
cause loss of sodium and water
what is pseudohyponatraemia?
if tests show low serum sodium
but concentration of sodium in serum water is normal
total serum volume is made up of serum water component which contains sodium, and proteins/lipoprotein component which doesn’t contain sodium
if total serum volume is made of more proteins/lipoproteins and less serum water then sodium may look low for total serum volume but normal for serum water volume
signs of pseudohyponatraemia?
if tests show very low sodium but patient has no symptoms
what is the pathophysiology of addisons disease?
adrenal insufficiency > cant make enough steroids > not enough mineralocorticoid activity > cant retain enough sodium in kidneys > loss of sodium and water from ECF > reduced ECF = patient is clinically dehydrated
what are the features of addisons disease and what causes these?
reduced ECF volume and hypotension cause dizziness
excess pigmentation in mouth and hand creases - from excess ACTH from pituitary
tanned skin - ACTH molecule contains sequence for MSH within it so when its degraded by proteases, the MSH is exposed
how does water overload present?
patient often already in hospital reduced [Na+] unremarkable volume status addisons test negative often don't have symptoms of hyponatraemia
why can volume status be clinically unremarkable in water overload?
as water retention is distributed all over body compartments
how is ADH deficiency (e.g transection of pituitary stalk) treated?
desmopressin (exogenous ADH)
