Physiology of Thirst and Fluid Balance Flashcards
Where are osmoreceptors located?
Located in the anterior wall of 3rd ventricle
-Fenestrations in the blood-brain barrier allow circulating solutes (osmoles) to influence brain osmoreceptors
How does your brain register thirst?
Osmoreceptors
Located in the anterior wall of 3rd ventricle
Fenestrations in the blood-brain barrier allow circulating solutes (osmoles) to influence brain osmoreceptors
Osmoreceptor cells alter their volume by a transmembrane flux of water in response to changes in plasma osmolality
This initiates neuronal impulses that are transmitted to the hypothalamus to synthesise ADH, and to the cerebral cortex to register thirst
Cranial (central) diabetes insipidus (DI)
-description
-Lack of osmoregulated AVP secretion
idiopathic,genetic, secondary
-Excess solute-free renal water excretion
=polyuria
Nephrogenic diabetes insipidus (DI)
- description
Lack of response of the renal tubule to AVP
Hypothalamic syndrome
-description
Disordered thirst and DI Disordered appetite (hyperphagia) Disordered temperature regulation Disordered sleep rhythm Hypopituitarism
Water deprivation test
-Normal response to dehydration
Normal plasma osmolality, high urine osmolality
Water deprivation test
- Cranial diabetes insipidus
- Treatment
- Poor urine concentration after dehydration
- Rise in urine osmolality after desmopressin
- treat with DDAVP (desmopressin)
Water deprivation test
- Nephrogenic diabetes insipidus
- Treatment
-Poor urine concentration after dehydration
-No rise in urine osmolality after desmopressin
-Treatment: Correction of cause (metabolic / drug cause)
Thiazide diuretics / NSAIDs
SIADH
- Clinically euvolaemic patient
- normovolemic hyponatraemia
- Low plasma sodium and low plasma osmolality
- Inappropriately high urine sodium concentration and high urine osmolality
High levels of ADH so fluid is not lost, this dilutes levels of electrolytes in the blood
Osmoregulation
-Low plasma osmolality
Comment on AVP, urine concentration and volume, thirst
AVP undetectable
dilute urine
high urine output
-no thirst
Osmoregulation
-High plasma osmolality
Comment on AVP, urine concentration and volume, thirst
High AVP secretion
Concetrated urine
Low urine output
-increased thirst
Psychogenic polydipsia
-increased fluid intake
- low plasma osmolality
- low ur
Hyponatraemia
sodium below 135
severe-below 125
-non-specif symptoms or sudden/severe
Treatment of SIADH
-Fluid restriction
-Demeclocycline
(drug that induces mild nephrogenic, inhibits arginine vasopressin)
-Vasopressin (V2 receptor) antagonists
