water and sodium homeostasis Flashcards
how is water distributed in body
total water body is 60% of body weight
40% of body weight in intracellular fluid
20% of body weight in extra cellular fluid
what is ECF made of
intravascular fluid
Interstitial fluid
how does water move
from low osmolality to high osmolality to make an isotonic (equal) solution
water is freely permeable through ECF and ICF
what is the main contributor to ECF
sodium is main contributor to ECF osmolality and volume
also anions chloride and bicarbonate because sodium is negative so these are positively charged
also glucose and urea contribute to osmolality
what is the main contributor in ICF
predominant cation in cells is potassium
what is interstitial fluid
surrounds the cells but does not circulate
what is plasma
circulates as the fluid component of the blood
how is plasma osmolality determined
by sodium and associated anions
how do you estimate plasma osmolality
2(Na) + 2(K) + urea + glucose mmol/L
what is end result for osmolality
intra and extra cellular osmolality are equal
change in plasma osmolality pulls or pushes water across cell membranes
where do we get water from
food and drink
metabolism
where do we lose water
skin
lugs
urine
faeces
skin and lungs are insensible water loss - can’t be measured
should water output and Input balance
yes
what are IV fluids (intravenous fluids)
liquids given to replace water, sugar and salt that you might need if you are ill or having an operation, and can’t eat or drink as you would normally.
given straight into a vein through a drip
why don’t we give water intravenously
it is hypo- osmolality/ hypotonic vs cells
water enters blood cells causing them to expand and burst : haemolysis
however this only occurs in the vicinity of the intravenous cannula
if you could achieve instantaneous mixing it wouldn’t occur
describe ECF osmolality
it is very tightly regulated
changes in ECF osmolality lead to a rapid response
normal plasma osmolality is 275-295 mmol/kg
water deprivation or loss will lead to a chain of events
how is ECF osmolality resotred
1) water deprivation / dehydration
2)
describe ECF volume changes
slower response compared to osmolality
Causes of water depletion in body
-reduced intake as we get older
- vomiting / diarrhoea / diuresis/ diuretics
- sweating
symptoms of dehydration
thirst
dry mouth
in elastic skin
sunken eyes
raised haematocrit
weight loss
confusión - brain cells
hypotension
how does water excess occur
when given too many fluids in hospital or just drinking too much water
what happens in water excess
water goes from ECF to ICF
ECF osmolality decreases
inhibition of ADH from posterior pituitary
no stimulation of thirst centre in hypothalamus
increased urine volume.
risk of cerebral over hydration if acute excessive intake ie water intoxication
consequences of excess water
hyponatraemia
cerebral overhydration
- headache
- confusion
- conclusions
define hydrostatic pressure
pressure difference between plasma and interstitial fluid
water moved from plasma into interstitial fluid
define oncotic pressure
pressure caused by the difference in protein concentration between the plasma and interstitial fluid
water moves from interstitial fluid into plasma
what is oedema
excess accumulation of fluid in interstitial space
disruption of the filtration and osmotic forces of circulating fluids
obstruction of venous blood or lymphatic return
inflammation increases capillary permeability
loss of plasma protein
what is serous effusion
excess water in a body cavity
pathogenesis of oedema and serous effusion
increased fluid leakage into interstitial spaces
OR
impaired reabsorption of fluid
inflammatory
venous
lymphatic
hypoalbuminaemic
how much fluid in normal pleural space
around 10 ml
what are pleural effusions
the build-up of excess fluid between the layers of the pleura outside the lungs
what are pleura
thin membranes that line the lungs and the inside of the chest cavity and act to lubricate and facilitate breathing
how much fluid in normal pleural space
about 10mL of fluid
what causes pleural effusions
disruption of balance between
hydrostatoc and oncotic forces in the visceral and parietal pleural vesses
AND
lymphatic drainage
what is transudate
fluid pushed through the capillary due to high pressure within the capillary
transudates have low protein content
what is exudate
fluid that leaks around the cells of the capillaries caused by inflammation and increase in permeability of pleural capillaries to proteins
exudates have high protein content
why is plural fluid protein measured
to differentiate between exudative and transudative effusions
what is normal range of plasma sodium
135- 145 mmol / L
concentration is a ratio, not a measure of total body content
what usually causes high or low sodium
gain or loss in water
not sodium
what are clinical effects of changes in plasma sodium
they are on the brain due to constrained volume ( skull)
rate or change is more important than absolute levels
what happens when too much sodium
hypernatraemia
water deficit
- poor intake
- osmotic diuresis
- diabetes insípidus
sodium excess
- mineralocorticoid (aldosterone )
salt poisoning
what happens when too little sodium
hyponatraemia
artefactural
sodium loss
- diuretics
- addison’s disease
excess water
- IV fluids (iatrogenic)
SIADH
excess water and sodium
- oedema
effects of too much sodium
cerebral intra cellular dehydration
tremors, irritability , confusion
effects of too little sodium
cerebral intra cellular overhydration
headache, confusion, convulsions
what are IV fluids (intravenous fluids)
liquids given to replace water, sugar and salt that you might need if you are ill or having an operation, and can’t eat or drink as you would normally.
given straight into a vein through a drip
describe steps of water homeostasis
- osmoreceptors detect water conc
- hypothalamus sends signal to posterior pituairy gland
- pituairy gland secretes ADH which targets the kidney responsible for maintaining water levels
- ADH alters the tubules of kidney to become more/less permeable
- if more water required in blood stream –> high conc of ADH make the tubules more permeable
- if less water required in blood stream –> low conc of ADH make the tubules less permeable
- kidneys conserve water if you are dehydrated and they can make urine more dilute to expel excess water if necessary
what is anti diuretic hormone
maintains blood pressure, blood volume and tissue water content by controlling the amount of water and hence the concentration of urine excreted by the kidney.
what is the renin - anigiotension - aldoesterone system
regulates blood volume and systemic vascular resistance, which together influence cardiac output and arterial pressure.
where is renin released from
mainly kidneys
what does renin do
stimulates the formation of angiotensin in blood and tissues, which in turn stimulates the release of aldosterone from the adrenal cortex.
what stimulates release of renin
- sympathetic nerve activation (acting through β1-adrenoceptors)
- renal artery hypotension (caused by systemic hypotension or renal artery stenosis)
- decreased sodium delivery to the distal tubules of the kidney.
what is hyponatraemia
when the concentration of sodium in your blood is abnormally low.
why does body need sodium
fluid balance, blood pressure control, as well as the nerves and muscles.
