Sodium and water balance Flashcards
What does ‘mineralocorticoid activity’ refer to?
Sodium retention in exchange for potassium and/or hydrogen ions
What is the main steroid with mineralocorticoid activity?
Aldosterone
How does too much or too little mineralocorticoid activity affect sodium and water balance?
Too much mineralocorticoid activity = sodium retention, and thus water retention
Too little mineralocorticoid activity = sodium loss, and thus water loss
What is the effect of ADH action on the renal tubules?
Water reabsorption, giving an antidiuretic effect
What is urine osmololality?
The concentration of urine
Concentrated urine = high osmolality
Dilute urine = low osmolality
What are the causes of a low sodium concentration?
Too much water:
- reduced excretion of water e.g. syndrome of inappropriate ADH (SIAD)
- increased intake e.g. compulsive water drinking
Too little sodium:
- increased excretion from kidneys e.g. Addison’s, also from gut and skin
- reduced intake
What are the causes of too high a sodium concentration?
Too little water:
- increased excretion e.g. diabetes insipidus
- decreased water intake esp. in young and elderly
Too much sodium:
- some IV medications are given as sodium salts
- near-drowning in sea
- infants given high-salt feeds
Why does Addison’s disease cause low sodium and dehydration?
Not producing enough steroid so there is not enough mineralocorticoid activity
Therefore, less sodium can be retained in the kidneys
Sodium is instead excreted from the kidneys, with water following
This causes dehydration
What is the typical presentation for ‘too much water’?
Patient usually in hospital with other diseases
No symptoms specifically to low sodium and fluid status unremarkable
What are some examples of non-osmotic stimuli for ADH secretion in disease?
Hypovolaemia/hypotension
Pain
Nausea/vomiting
How does diabetes insipidus present?
Huge urine output
Huge water/fluid intake
May follow a trauma
Is sodium concentration high or low in diabetes insipidus?
High - reflects water defecit as water cannot be reabsorbed at the kidney
What is the pathology in diabetes insipidus?
Disruption of pituitary gland or stalk so ADH is not produced or there is a problem with ADH action
Water is not reabsorped at the kidneys and so all water is excreted
What is the term for water defecit?
Hypovolaemia
What is the cause of hypovolaemia with hyponatraemia?
Too little sodium
