Biochemistry- Na+ and water Flashcards
Does water follow sodium or does sodium follow water?
Water follows sodium.
What hormones regulate sodium and therefore regulate water?
Mineralocorticoids- e.g. aldosterone (main one)
Cortisol
What does a mineralocorticoid do?
Activity means sodium retention occurs and therefore water is retained.
What are the consequences of too little mineralocorticoid activity?
Little/no sodium is retained therefore water is lost.
Which hormone controls water? Where is it released from?
ADH- released from posterior pituitary.
What is the function of ADH?
It acts on renal tubules to cause
- water reabsorption
- antidiuretic affect
If there is an increase in ADH, what will the urine be like?
concentrated.
If there is a decrease in ADH, what will the urine be like?
Diluted.
How can ADH’s affect be assessed?
Look at urine osmolarity.
A high urine osmolarity- urine is concentrated
A low urine osmolarity- urine is dilute.
How can disorders to do with sodium concentration be classified?
Decreased Na- Too much water
-Too little sodium
Increased Na- Too little water
-too much sodium.
What are the causes of too much water in the body?
Increased intake of water
Decreased urine output e.g. SIAD.
What are the causes of too little sodium in the body?
Increased sodium loss- kidneys e.g. in Addisons disease
Gut
Skin
Or decreased sodium intake (rare).
What are the causes of too little water in the body?
Diabetes insipidus (problem with ADH secretion) Decreased water intake.
What are the causes of too much sodium in the body?
Some IV medications (given as sodium salts)
Near- drowning in the sea
Infants given high salt feeds
A 24 year-old student presents with a six month history of malaise, tiredness, poor appetite and one stone weight loss. She has developed a craving for salty foods – crisps in particular. She has had a number of dizzy spells particularly while in warm places.
She is thin. She has low BP which falls further on standing. You have the impression that she is tanned, and you find increased pigmentation in her mouth and hand creases.
Her bloods show a low sodium [122] and a high potassium [5.8].
Addisons disease.
What is Addison’s disease?
Basically primary adrenal insufficiency meaning you can’t make enough mineralocorticoid. This means she can’t retain sodium and therefore can’t retain water.
What symptoms will someone with Addisons disease have?
She will be clinically dehydrated- causing symptoms of dizziness (due to postural hypotension)
Also get hyper pigmentation (as the pituitary produces lots of ACTH to try and stimulate the adrenals- ACTH contains a sequence for melanocyte stimulating hormone in it).
Typical presentation of too much water
Patient often in hospital with other illness
Routine biochemistry shows decrease in [Na]
On examination, volume status usually unremarkable
Patient usually has no symptoms specifically due to low sodium
Investigations for causes of sodium loss e.g. Addison’s negative
Patient presumed to have too much water
Most of these patients have ‘inappropriate’ ADH secretion
What is SIAD?
Syndrome of innapropriate antidiuresis.
What are the non-osmotic stimuli for ADH release?
Hypovolaemia/hypotension
Pain
Nausea and vomiting.
Analysing the example of too much water:
ADH is secreted to a non-osmotic stimuli. This causes water retention- often this occurs slowly and the water is distributed over all body compartments- therefore patients volume status will be unremarkable clinically.
It will first present when U&E’s are done and sodium is low
Diagnosis of water excess is a diagnosis of exclusion once they’ve excluded that sodium isn’t too low as in not enough is being secreted (Addisons).
A 29 year-old man is admitted to ITU following a cycling accident in which he sustained a severe head injury. During his ITU stay his urine output is in excess of 12 litres daily; his IV fluid requirement is correspondingly large.
Serum sodium is 167 mmol/L on admission to ITU and slowly falls as fluid replacement ‘catches up’. He is in addition commenced on desmopressin (exogenous ADH) which produces a sharper fall in sodium.
A huge head injury transected his pituitary stalk. His posterior pituitary is no longer connected and ADH cannot be excreted.
Therefore water is not reabsorbed in its usual way.
He is essentially peeing out pure water.
Giving him exogenous ADH will help to restore water balance (desmopressin).
Sodium level is high reflecting the water deficit.
What does hypovolaemia and low sodium imply?
Sodium deficit is extremely low (too little sodium)
What clinical sign shows hypervolaemia?
Most often seen as oedema.
