4. Body Fluid Osmolality Flashcards
Where are osmoreceptors located?
In hypothalamus, specifically in organum vasculosum of lamina propria
How do osmoreceptors work?
Fenestrated leaky endothelium exposed directly to systemic circulation (on plasma side of blood brain barrier)
Sense changes in plasma osmolarity
Signal responses which are mediated via 2 pathways leading to 2 outcomes: concentration of urine, thirst
What triggers ADH to be released?
Increase in plasma osmolality
What does ADH release lead to?
Reduced water excretion
Blood vessel vasoconstriction
Increased urea recycling
How does ADH work?
Binds to V2 receptors on basal membrane
Causes intracellular aquaporins to fuse with the luminal membrane
What are the 2 things that happen when change in plasma osmolarity is sensed?
- ADH released, affects kidney, causes renal water excretion
- Causes thirst, brain communicates drinking behaviour, water intake
What happens if there is a reduced ECV?
Set point is shifted to lower osmolarity values and the slope of the relationship is steeper
What happens if there is an increase in blood pressure?
The set point is shifted to higher osmolarity and the slope of the relationship decreases
How does the body compensate for thirst?
Large deficits in water or an increase in salt only partially compensated for in the kidney
Ingestion of water is the ultimate compensation
What is diabetes isipidus?
Too little ADH
Inability to reabsorb water from the distal part of the nephron, due to failure of secretion or action of ADH
What are the symptoms of diabetes insipidus?
Polyuria
Polydipsia
Low urine osmolality (dilute urine)
What are the causes of diabetes insipidus?
Nephrogenic
Central
What is central diabetes insipidus?
Impaired ADH synthesis or secretion by the hypothalamus, due to damage to hypothalamus or pituitary gland
What can damage the hypothalamus or pituitary gland?
A brain injury A tumour Sarcoidosis or tuberculosis An aneurysm Some forms of encephalitis or meningitis
How is central diabetes insipidus treated?
By administering ADH (desmopressin)
Why does central diabetes insipidus cause a large quantity of urine?
Water is inadequately reabsorbed from the collecting duct
What is nephrogenic diabetes insipidus?
Acquired insensitivity of kidney to ADH
How is nephrogenic diabetes insipidus managed?
Low-salt, low-protein diet reduces urine output
Difficult to manage clinically
What can cause nephrogenic diabetes insipidus?
Mutations in gene coding for V2 receptors
Chronic pyelonephritis
Polycystic kidneys
Drugs such as lithium
What is too much ADH called?
Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
What is SIADH characterised by?
Excessive release of ADH from the pituitary gland or another source
-dilutational hyponatremia, total body fluid is increased
What can SIADH be due to?
CNS disorders (stroke, abscesses) Malignancy Lung diseases Drugs (opiates) Metabolic disease (porphyria, hypothyroidism)
What are the symptoms of SIADH?
Hyponatremia and low plasma osmolality
Inappropriate urine osmolality
Inappropriate Na+ excretion
