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2.2 T1: The endocrine system > T1 L17: Fluid and thirst balance > Flashcards

T1 L17: Fluid and thirst balance Flashcards

1
Q

How do osmoreceptors work to influence plasma osmolality?

A

They alter their own volume by a transmembrane flux of water in response to plasma osmolality which initiates neuronal impulses that signal for the hypothalamus to release ADH and to the cortex to register the thirst

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2
Q

What is the human form of ADH?

A

Arginine vasopressin (AVP)

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3
Q

How does ADH work?

A

Via V2 receptors, aquaporins normally stored in the cytoplasmic vesicles fuse with the luminal membrane to increase the permeability of renal collecting tubules promoting water reabsorption

When ADH is cleared, the aquaporins are endocytosed back into the cytoplasm

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4
Q

What is polyuria?

A

Excessive urine

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5
Q

What is Polydipsia?

A

Excessive thirst

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6
Q

What causes cranial (central) diabetes insipidus?

A

Lack of osmoregulated AVP secretion causing polyuria and polydipsia

Can be idiopathic or genetic Eg. Familial mutation of AVP gene or DIDMOAD

Secondary causes: Post-surgical, traumatic, rare causes Eg. Tumours, meningitis

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7
Q

What causes nephrogenic diabetes insipidus?

A

The renal tubules are resistant to AVP causing polyurea and polydipsia

Caused by mutations to V2 receptor gene, metabolic high [Ca2+] or low [K+], drugs like lithium, or chronic kidney disease. Can be idiopathic

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8
Q

What is primary polydipsia?

A

Psychogenic polydipsia caused by excessive consumption of fluids because they think they should be drinking more

Causes lower plasma osmolality, supressed AVP secretion, low urine osmolality and polyurea, loss of renal interstitial solute because it’s all being washed out

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9
Q

What are the symptoms of hypothalamic syndrome?

A
  • Disordered thirst
  • Disordered appetite (hyperphagia)
  • Disordered temperature regulation
  • Disordered sleep rhythm
  • Hypopituitarism
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10
Q

What is ‘Evian potomania’?

A

Excessive drinking of ‘Evian’ water

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11
Q

What should be investigated if someone presents with polyuria and polydipsia?

A
  • Medical history
  • Exclude diabetes mellitus
  • Document 24 hour fluid balance
  • Exclude hypercalcaemia or hypokalaemia
  • Do a water deprivation test
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12
Q

What is a water deprivation test?

A

You challenge the body by dehydrating it to see what happens

Plasma and urine osmolalities are measured. Then desmopressin is injected and the osmolalities are measured again to see how the body responds

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13
Q

What would the water deprivation test results look like in a healthy individual?

A

Normal plasma osmolality, high urine osmolality

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14
Q

What would the water deprivation test results look like in someone with cranial diabetes insipidus?

A

Poor urine concentration after dehydration and a rise in urine osmolality after desmopressin because the kidneys can still react to desmopressin the brain just can’t produce it

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15
Q

What would the water deprivation test results look like in someone who has nephrogenic diabetes insipidus?

A

Poor urine concentration after dehydration. No rise in urine osmolality after desmopressin because the kidneys can’t respond to AVP

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16
Q

What is the treatment for Cranial diabetes insipidus?

A

Desmopressin

Over-treatment can cause hyponatraemia

17
Q

What is the treatment for nephrogenic diabetes insipidus?

A

Correction of the cause and then thiazide diuretics or NSAID’s

18
Q

What is the treatment for primary polydipsia?

A

Explanation or persuasion to stop drinking so much. Therapy may be needed to stop

19
Q

What is the definition of hyponatraemia?

A

sodium <135mmol/l

Severe: sodium <125mmol/l

20
Q

What are the symptoms of hyponatraemia?

A

Can be asymptomatic. Symptoms depend on the rate of fall because the brain can adapt if it’s chronic

Non-specific symptoms: Headache, nausea, mod change, cramps, lethargy

Severe/sudden symptoms: Confusion, drowsiness, seizures, coma

21
Q

How is syndrome of inappropriate ADH (SIADH) diagnosed?

A
  • Clinically euvolaemic patient
  • Low plasma sodium and low plasma osmolality
  • Inappropriately high urine sodium concentration and high urine osmolality
22
Q

What is the treatment for SIADH?

A
  • Fluid restriction
  • Demeclocycline can be given which induces mild nephrogenic diabetes insipidus
  • Vasopressin antagonists can be given that induce water diuresis
23
Q

How is severe hyponatraemia treated?

A

Correct severe hyponatraemia slowly because rapid correction risks oligodendrocyte degradation and CSH myelinolysis

Alcoholics and malnourished patients are most at risk of it

2.2 T1: The endocrine system (23 decks)

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