Osmoregulation Flashcards
What are the two ways to change concentration of solution?
Add/remove solute
Add/remove solution
How is the osmolality of ECF adjusted?
By adding/removing solute
Mechanism of ADH
- Vasopresson binds to membrane receptor
- Receptor activates cAMP second messenger system
- Cell insers AQP2 wate pores into apical membrane
-Water is absorbed via osmosis
What is the normal range of ECF osmolarity
285-295 mOsm/kg
What are changes in ECF osmolarity detected by?
Osmoreceptors in anterior hypothalamus
What is the threshold for ADH release?
280-285 mOsm/kg
Where is ADH released from?
- PVN and SON neurons release ADH from their axon terminals in posterior pituitary
Thirst?
- Changes detected by osmoreceptors in anterior hypothalamus
- Project to centres mediating thirst, drinking
- Strong desire to drink when plasma osmolality ≥295 mOsm/kg
- Oropharyngeal and upper gastrointestinal receptors reduce thirst on drinking
- Thirst is also stimulated by
- Large (10-15%) drops in blood volume/pressure
- Angiotensin 2 acting on hypothalamus
What is the main cation in ECF?
Na+
What happens in a water deficit?
- ECF osmolality increases (hyperosmolality)
- Hypernatremia (Na > 145)
What are the two causes of Hypernatremia?
- Gain of sodium (rare)
- Loss of water (common)
Cause - Gain of sodium (rare)
- Iatrogenic
- Excess ingestion (rare)
- Excess mineralocorticoid activity
- e.g., primary hyperaldosteronism (Conn’s)
- Hypernatremia, if present, is usually mild
Cause - Loss of water (common)
Extra-renal losses
- Dehydration
- Infection (increased losses via skin and lungs)
Renal losses
- Osmotic diuresis
- Diabetes insipidus
What is diabetes insipidus?
Renal water loss (inability to concentrate the urine)
Lack of effective ADH, either
- Central (failure of secretion)
- Nephrogenic (lack of renal response)
Presents with polydipsia and polyuria
- Thirst mechanism alone is normally enough to prevent significant hypernatremia
- But hypernatremia will rapidly develop if access to water is restricted
Hypoosmotic hyponatremia?
it signifies water excess
What can hyponatremia cause?
- CNS damage/disease
- Ectopic ADH production by tumour
What role does ADH have in hypovolemic state
in an emergency – impending circulatory collapse – it takes on another role as a last line of defence against volume depletion, temporarily ignoring the osmoregulatory function.
