Sodium and Water Balance

Question 1

what hormone controls water balance and where is it secreted from?

Answer 1

ADH (aka vasopressin)

secreted from posterior pituitary

Question 2

what does ADH do?

Answer 2

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

Question 3

what injury can cause a decrease in ADH secretion?

Answer 3

transection of pituitary stalk in head injury

diabetes insipidus

Question 4

how is urine concentration measured?

Answer 4

urine osmolality
high osmolality = concentrated urine
low osmolality = dilute urine

Question 5

…

Answer 5

sodium and (potassium?) pumped out of the loop of henle?

Question 6

what controls sodium balance?

Answer 6

steroids

• aldosterone (mainly)
• other steroids (e.g cortisol)

Question 7

what do steroids do to control sodium balance?

Answer 7

mineralocorticoid activity
- refers to Na+ reabsorption in renal tubules in exchange for K+/H+
too much mineralocorticoid activity = sodium gain
too little = sodium loss

Question 8

how do you calculate sodium concentration?

Answer 8

mmol Na+ / 1 L water

Question 9

how do intra and extracellular fluid differ/

Answer 9

intra = contains more fluid
intra = lower Na+ concentration

Question 10

can water move between all body compartments?

Answer 10

yes

means that if you lose water you loose it from everywhere

Question 11

can sodium move between all body compartment?

Answer 11

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

Question 12

how does water balance relate to sodium balance?

Answer 12

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

Question 13

what are the two possible causes for reduced concentration of sodium?

Answer 13

loss of sodium from the ECF (dangerous)
fluid retention (too much water dilutes sodium)

Question 14

what can cause increase in sodium concentration?

Answer 14

loss of water

increase in ECF sodium

Question 15

why is loss of sodium from ECF dangerous?

Answer 15

little fluid in ECF anyway
if sodium lost then water will follow causing reduction in fluid volume
will have severe symptoms

Question 16

what can cause sodium loss?

Answer 16

adrenal/kidney (adrenals not making steroid hormones so not reabsorbing sodium and water)
gut (diarrhoea and vomiting, fistulae)
skin (burns)

Question 17

what can cause fluid overload (too much water)?

Answer 17

reduced water excretion (SIADH)
increased intake (compulsive water drinking)

Question 18

what can cause increased sodium?

Answer 18

some IV medications
near drowning (consumption of high amount of sea salt water)
malicious (child been given salt by parent)

Question 19

what can cause loss of water?

Answer 19

diabetes insipidus (increased water loss)
reduced water intake

Question 20

how do you manage low sodium concentration?

Answer 20

if too little sodium - give sodium

if too much water - fluid restriction

Question 21

how do you manage increased sodium concentration?

Answer 21

too much sodium - remove sodium (i.e loop diuretic)

too little water - give water (as dextrose)

Question 22

what 3 fluids can be given and where does each affect?

Answer 22

dextrose = plasma, interstitial fluid and intracellular fluid compertment
saline = plasma and interstitial fluid
plasma/blood = plasma

Question 23

when is sodium inbalance serious?

Answer 23

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

Question 24

what are the 2 kinds of stimuli for ADH release?

Answer 24

osmotic (in health)

non-osmotic (in disease)

Question 25

give 3 examples of non-osmotic stimuli?

Answer 25

hypovolaemia/hypotension
pain
nausea/vomiting

Question 26

what is SIADH?

Answer 26

syndrome of inappropriate ADH

Question 27

2 opposing forces in capillary which control balance of water?

Answer 27

hydrostatic force (push water out)
oncotic force (pull water in)

Question 28

what can cause reduced oncotic force/increased hydrostatic force?

Answer 28

low proteins in the capillary (e.g low albumin)

Question 29

what is the impact of low blood volume on sodium?

Answer 29

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

Question 30

what does oedema signift?

Answer 30

effective circulating volume depletion

low blood volume

Question 31

how is oedema managed?

Answer 31

loop diuretics

cause loss of sodium and water

Question 32

what is pseudohyponatraemia?

Answer 32

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

Question 33

signs of pseudohyponatraemia?

Answer 33

if tests show very low sodium but patient has no symptoms

Question 34

what is the pathophysiology of addisons disease?

Answer 34

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

Question 35

what are the features of addisons disease and what causes these?

Answer 35

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

Question 36

how does water overload present?

Answer 36

patient often already in hospital
reduced [Na+]
unremarkable volume status
addisons test negative
often don't have symptoms of hyponatraemia

Question 37

why can volume status be clinically unremarkable in water overload?

Answer 37

as water retention is distributed all over body compartments

Question 38

how is ADH deficiency (e.g transection of pituitary stalk) treated?

Answer 38

desmopressin (exogenous ADH)