What are the sizes of the different compartments containing fluid in the body?
ICF (intracellular fluids) are the biggest - 28L
ECF (extracellular fluid) = interstitial fluid and plasma
- Intersitial fluid = 10.5L
- Plasma = 3.5L
What is the difference in concentration of Na in the ICF and the ECF?
- Na conc high in ECF - roughly 140mmol/L
- Na conc low in ICF - roughly 4mmol/L
What is the principle relationship between sodium and water that you need to remember ?
That water follows sodium everywhere e.g. if your ECF volume is too high then the kidneys excrete more Na and thus you lose water with it.
If the ECF volume is too low, the kidneys ‘hang on’ to Na in an attempt to retain water and restore volume to normal
What are sodium levels controlled by ?
Controlled by mineralocorticoid activity - e.g. aldosterone (main one) and others e.g. cortisol
What does mineralocorticoids stimulate in terms of Na ?
The stimulate Na retention
What is water controlled by ?
ADH (anti-diuretic hormone)
What is the effect of ADH on water ?
ADH acts on renal tubules to cause:
- Water reabsorption and thus
- Antidiuretic effect (hence its name)
Increased ADH = concentrated urine
Decreased ADH = dilute urine
Describe what is meant by urine osmolality
- Concentrated urine = high urine osmolality
- Dilute urine low urine = osmolality
In general terms what can decreased sodium be due to ?
Too much water or too little Na
What are some of the causes of too much water in the body resulting in decreased Na levels ?
- Decreased excretion of water - SIADH (Syndrome of inappropriate antidiuretic hormone secretion, characterized by excessive release of antidiuretic hormone)
- Increased intake of water - compulsive water drinking
What could be the cause of decreased Na due to too little Na
Increased Na loss:
- Kidneys e.g. adrenal insufficiency (Addison’s disease)
Decreased sodium intake (rare)
In general terms what are the 2 main reasons for increased Na levels ?
- Too much water
- Or too much sodium
What are some of the causes of increased Na due to too little water ?
Increased water loss due to:
- E.g. Diabetes insipidus (problem with ADH secretion or action)
Decreased water intake:
- E.g. very young, elderly patients (insensible water loss continues)
What are some of the causes of increased Na due to too much sodium ? (this is rare)
- Some IV medications are given as sodium salts
- Near-drowning in sea
- Infants given high-salt feeds
What is the underlying problem in addisons disease ?
- Basic problem is adrenal insufficiency
- So can’t make enough steroids
- So doesn’t have enough mineralocorticoid activity
- So can’t retain enough sodium in the kidneys
- So loses sodium (and water with it) from ECF
- Decreased ECF volume means patient is clinically dehydrated
What are the 2 main reasons for ADH secretion ?
- Osmotic (in health)
- Non-osmotic (in disease) e.g. Hypovolaemia/hypotension, Pain, Nausea/vomiting
What is the underlying problem in diabetes insipidus ?
- Disruption of pituitary or pituitary stalk – so patient can’t secrete ADH from posterior pituitary
- Hence they cant concentrate there urine enough and prudce lots of dilute urine
Define what is meant by hypovolaemia
Implies water deficit.
Define what is meant by hypervolaemia
Water excess. It is most often seen as oedema
Define what hyponatraemia and hypernatraemia is
- Hyponatreamia condition caused by low Na
- Hypernatraemia is caused by high Na
What are the symptoms of hyponatreamia or hypernatraemia ?
altered consciousness, confusion, nausea, etc
If you suspect adrenal insufficiency then what would you measure ?
Cortisol and ACTH
What is the treatment of these Na and water problems ?