water balance and vasopressin disorders Flashcards
What does the brain monitor to maintain water balance?
Body fluid osmolality (osmostat)
This monitoring leads to appropriate thirst and vasopressin release.
What is Vasopressin also known as?
Anti-diuretic Hormone (ADH)
It is made in the hypothalamus and released from the posterior pituitary.
What is the primary action of Vasopressin?
Reabsorb water and concentrate urine
This action is mediated through V2 receptors in the kidneys.
What are the two types of Vasopressin Deficiency?
- Vasopressin Deficiency (VpD) in pituitary/hypothalamus (Cranial Diabetes Insipidus)
- Vasopressin Resistance (VpR) in kidney (Nephrogenic Diabetes Insipidus)
What results from Vasopressin Deficiency or Resistance?
Deficient water reabsorption and excessive urine volume (polyuria)
This leads to dilute urine (insipid) and excess water wasting.
What does SIADH stand for?
Syndrome of Inappropriate ADH
It results in excessive water reabsorption and concentrated urine.
What are the clinical consequences of excessive Vasopressin action?
- Excess water retention
- Dilute body fluids
- Low plasma sodium (hyponatraemia)
What is the normal water intake for an adult in a temperate climate?
0.9 to 2-2.5 litres
This intake is necessary to match urinary losses.
What is a cause of excessive water loss leading to polyuria?
Vasopressin Deficiency or Resistance
Other causes may include primary polydipsia or osmotic diuresis.
What is the definition of polyuria?
Passing excessive urine volumes, defined as >2ml/kg/hr
For a 70kg person, this equates to >3.36 litres.
What are the key components needed for proper ADH function?
- Adequate ADH production
- Kidney responsive to ADH
- Effective feedback stopping ADH release
What can cause loss of thirst (adipsia)?
Hypothalamic damage
This condition can be difficult to treat.
What are the roles of V1 and V2 receptors for Vasopressin?
- V1a: Vasopressor effects, maintain blood volume
- V2: Appropriate water retention, maintain osmolality
What factors can stimulate non-osmotic release of Vasopressin?
- Low blood pressure
- Low extracellular fluid volume
- Pain
- Nausea
What is the effect of excessive water retention on plasma sodium levels?
It leads to low plasma sodium (hyponatraemia)
This can cause symptoms like confusion, drowsiness, and nausea.
What is the normal range for plasma sodium levels?
135-145mmol/L
A level of 140 ± 5 is considered normal.
What is the significance of the osmostat in water balance?
It drives thirst and ADH release in response to body fluid osmolality changes.
What is the action of Vasopressin on the distal nephron?
Increases water permeability and apical AQP2 expression
This enhances water reabsorption.
What is the clinical presentation of polyuria due to Vasopressin Deficiency?
Inappropriate high urine output and persistent thirst
This can lead to negative fluid balance.
What is the role of the thirst center in the hypothalamus?
It integrates signals to drive water intake when body water is low.
What physiological condition can lead to hypernatraemia?
Inadequate fluid intake due to loss of thirst
This is particularly dangerous in severe cases.
What are the consequences of high insensible losses of water?
Increased risk of hypernatraemia
This is especially a concern for individuals in extreme conditions.
What is the impact of renal impairment on polyuria?
It can lead to inappropriately high urine output
This is considered unusual.
What are the key steps in the investigation of polyuria?
- Accurate fluid balance and plasma [Na+]
- Check glucose, urea, eGFR, K+, Ca2+
- Distinguish between Vasopressin Deficiency/Resistance and primary polydipsia
