Thirst and fluid balance Flashcards Preview

203: The endocrine system > Thirst and fluid balance > Flashcards

Flashcards in Thirst and fluid balance Deck (26)
Loading flashcards...
1

Osmoreceptors
- Function
- Location
- Mechanism

Receptors that detect changes in plasma osmolarity
- Especially in changes to Na+

Located in the anterior wall of the third ventricle.
- Contains leaky blood vessels close to the hypothalamus.

Mechanism:
When changes in osmolarity occurs---> Alters the volume of the cell
- Changes water influx into cell.

Triggers the secretion of ADH [hypothalamus] and directs thirst [cortex]

2

ADH
- Synthesis and secretion
- Action

Peptide hormone made in hypothalamus nuclei
- Supraoptic
- Paraventricular

Secreted into the posterior pituitary gland via secretory granules.

Action
- Binds to V2 receptors on the basolateral membrane of tubule cells in collecting duct.
- Triggers the insertion of aquaporins [AQP2 in lumen, 3+4 in basal membrane]

3

Physiology in low plasma osmolarity
- ADH
- Urine
- Thirst

ADH negligible.

Urine dilute, large volume.

No thirst

4

Physiology in high plasma osmolarity
- ADH
- Urine
- Thirst

ADH secreted, high levels.

Urine concentrated, low volume.

Thirst triggered.
- Drinking suppresses ADH secretion to prevent overshooting.

5

Plasma osmolarity ranges

285-295 mosmol/kg

This is maintained under normal physiology using
- Kidneys
- ADH secretion
- Thirst

6

Diabetes insipidus
- Definition
- Causes

Diabetes= urine
Insipidus= insipid [lacking flavour, watery]

Characterised by polydipsia and polyuria.

Causes
- Cranial/ Central: brain
- Nephrogenic: Kidney
- Secondary

7

Cranial/ central diabetes insipidus.
- Description
- Causes
- Treatment

Polyuria and Polydipsia caused by inadequate synthesis/ secretion of ADH.

Causes:
- Idiopathic [27%]
- Genetic [<5%]
- Secondary [most common]

Treatment
- Administering artificial AVP [desmopressin]

8

Genetic causes of cranial diabetes insipidus

Mutations to do with AVP:
- Familial mutation of the AVP gene-
- Wolfram syndrome [DIDMOAD]---> Autosomal recessive, incomplete penetrance.

9

Secondary causes of cranial DI

Most common cause of cranial DI.

- Post-surgical brain surgeries, esp. pituitary

- Brain trauma

- Tumours

- Infections [TB, encephalitis, meningitis]

- Autoimmune

- Sarcoidosis

- Histiocytosis

10

Nephrogenic diabetes insipidus
- Description
- Causes [6]
- Treatment

Polydipsia and polyuria caused by the kidneys inability to respond to AVP.

Causes:
- Idiopathic
- Genetic
- Metabolic [hypercalcaemia, hypokalaemia]
- Drugs: lithium
- CKD.

Treatment
- Treat underlying causes
- Thiazide diuretics
- NSAIDs

11

Hypothalamic syndrome

Damage to the hypothalamus
- Affects the HPA axis.

Consequences
- Disrupted thirst, DI
- Hyperphagia
- Abnormal temperature regulation
- Hypopituitarism
- Sleep rhythm disruptions.

12

Primary polydipsia
- Consequences
- Treatment

Excessive intake of water despite plasma osmolarity being normal.
- Causes high urine output and low plasma osmolarity.
- Can be caused by psychotic disorders [delusions, hallucinations]


Consequences
- Suppresses AVP secretion
- Low plasma osmolality
- Low urine plasma osmolality
- High urine output
- Loss of renal interstitial solute

Treatment
- Psychological therapy
- Explaining the reason

13

Investigations into polyuria and polydipsia.

Medical history
- Familial?
- Water intake?
- Exclude diabetes mellitus

24 hour fluid balance documented
- Urine out put
- Water intake
- Weight day and night

Exclude hypercalcemia and hypokalemia

Water deprivation test

14

Water deprivation test

Diagnostic test to dehydration.

1. After a period of dehydration
- Plasma and urine osmolarity measured
- Weight measured

2. Desmopressin is administered

3. Plasma and urine osmolality measured again

15

Water deprivation test results
- Normal person

After period of dehydration
- Plasma osmolality increases
- Urine osmolality increases

16

Water deprivation test results
- Cranial DI

After period of dehydration
- Plasma osmolality increases
- Urine osmolality decreases

After desmopressin administered
- Urine osmolality increases

17

Water deprivation test results
- Nephrogenic DI

After period of dehydration
- Plasma osmolality increases
- Urine osmolality decreases

After desmopressin administered
- Urine osmolality does change

18

Hyponatraemia
- Definition
- Symptoms

Plasma sodium levels < 135 mmol/L


Symptoms
- Can be asymptomatic
- Headaches
- Nausea, cramps
- Lethargy
- Mood change

When sudden/severe
- Confusion, drowsiness,
- Seizure
- Coma

19

Causes of hyponatraemia [13]
- Blood
- Drugs
- Losses
- Endocrine
- Organ

Drugs
- Thiazide diuretics

Hyperglycaemia
Hyperlipidaemia
High protein levels.

Losses
- Renal
- From the skin [burns, sweating]

Endocrine
- Hypoadrenalism
- Hypothyroidism
- SIADH

Organ failure
- Renal
- Cardiac
- Liver/ cirrhosis

XS IV dextrose [fluids]

20

Hypovolaemic causes of hyponatraemia [5]

Renal loss

Diarrhoea

Vomiting

Burns

Sweating

21

Normovolaemic causes of hyponatraemia

Hypoadrenalism

Hypothyroidism

SIADH

22

Hypervolaemic causes of hyponatraemia

Renal failure

Cardiac failure

Cirrhosis

XS IV dextrose

23

SIADH
- Description
- Presentation

Syndrome of inappropriate ADH- Too much ADH

Presentation
- Euvolaemic hyponatraemia
- Low plasma osmolality
- High urine osmolality
- High urine sodium


24

Causes of SIADH

Neoplasms

CNS disorders

Lung disease [infections, asthma, CF]

Drugs

Hypothyroidism, Hypoadrenalism

25

SIADH treatment

Treat underlying cause

Restriction of fluid

Demeclocycline
- Induces mild nephrogenic DI

V2 antagonists [Vaptans]

26

Hyponatraemia treatment

Correct underlying cause

Severe hyponatraemia corrected slowly unless risk of:
- Oligodendrocyte degeneration
- CNS myelinolysis
Especially in alcoholics and malnourished