Physiology of thirst

Question 1

What are the 3 key determinants in water homeostasis

Answer 1

Antidiuretic hormone (ADH)
-osmotically stimulated secretion
Kidney
-wide variation in urine output
Thirst
- osmoregulated
- stimulates fluid intake

Question 2

What are osmoreceptors? where are they found?

how do they work? and what effect do they have

Answer 2

Group of specialised cells which detect changes in plasma osmolality

they are located in the anterior wall of the 3rd ventricle

osmoreceptors change to respond to the changes in plasma osmolality

they initiate neuronal impulses that are transmitted to the hypothalamus to synthesis ADH and to the cerebral cortex to register thirst.

Question 3

Outline ADH action in the kidneys

Answer 3

ADH action mediated via V2 receptors
aquaporins move and fuse with the luminal membranes
increases water permeability of the collecting tubules promoting water reabsorption
when ADH is cleared- water channels removed from the luminal surface (endocytosis) and returned to the cytoplasm.

Question 4

AVP and kidney relationship in high and low osmolality

Answer 4

low plasma osmolality

• AVP undetectable
• Dilute urine
• High urine output
• no thirst

High plasma osmolality

• High AVP secretion
• Concentrated urine
• low urine output
• increased thirst sensation

Question 5

What is the main lookout for polyuria and polydipsia

Answer 5

• Exclude diabetes mellitus first
• exclude hypercalcaemia and hypokalaemia
• 3 other main causes: cranial diabetes insipidus (lack of osmoregulated AVP secretion
• nephrogenic diabetes insipidus (lack of response to renal tubule to AVP
  -primary polydipsia
  habitually increased fluid intake, they need to relax

Question 6

How would one carry out a water deprivation test

Answer 6

Period of dehydration “overnight”
measure plasma and urine osmolalities and weight

injection of synthetic vasopressin
-DDAVP (Desmopressin)
measure plasma and urine osmolalities again.

Question 7

How would you treat cranial diabetes insipidus

Answer 7

DDAVP

overtreatment can cause hyponatraemia

Question 8

How would you treat nephrogenic diabetes insipidus

Answer 8

correction of cause (metabolic/ drug cause)

thiazide diuretics/ NSAIDs

Question 9

How would you treat primary polydipsia

Answer 9

Explanation/ persuasion

psychological therapy

Question 10

What are the symptoms of hyponatraemia

Answer 10

Low sodium levels, can have non-specific symptoms such as headache, nausea, mood change, cramps and lethargy

could be specific: confusion. drowsiness, seizures and coma.

maybe asymptomatic?

Question 11

How would you classify a patient with hyponatraemia

Answer 11

First exclude drug causes e.g. thiazide diuretics
exclude high concentrations of glucose, plasma lipids or proteins

classify by extracellular fluid volume status

hypovolemia: renal loss
normovolaemia: hypoadrenalism, hypothyroidism
hypervolaemia: renal failure, cardiac failure

Question 12

What are the symptoms of inappropriate ADH secretion (SIADH)

Answer 12

clinically euvolemic patient
low plasma sodium and low plasma osmolality
inappropriately high urine sodium concentration and high urine osmolality

assess renal, adrenal and thyroid function

many causes: neoplasias, neurological disorders, lung disease, drugs, endocrine (hypothyroid/ hypoadrenalism)

Question 13

How is SIADH treated

Answer 13

treat the underlying cause,
fluid restriction
demeclocycline- a drug that induces mild nephrogenic DI
Vasopressin antagonists