Body Fluid Osmolality

Question 1

Where are Osmoreceptors found? What do they sense?

What happens next?

Answer 1

• In hypothalamus
• Sense changes in plasma osmolarity

Signals secondary responses, mediated by 2 complimentary pathways;

• Urine concentration
• Thirst

Question 2

Describe the Synthesis of ADH (A peptide hormone)

Answer 2

• Made in Supraoptic nucleus of Hypothalamus

- Transported to Post. Pituitary gland and stored until release

Question 3

What triggers ADH release?

List 3 effects of ADH

Answer 3

• Increase in plasma osmolarity
• Reduce water excretion (V2 receptor mediated)
• Vasoconstriction (V1 receptor mediated)
• Increased urea recycling through UT1 transporters

Question 4

How does ADH reduce water excretion?

Answer 4

• Causes intracellular aquaporins to fuse with luminal membrane
• Binds to V2 receptors on Basal Membrane, causing fusion of inactive aquaporins
• Creates a channel through which water flows passively

Question 5

When is ADH secretion at zero?

What is diuresis?

How is ADH secretion regulated?

Answer 5

• Never, always secreted but more secreted when plasma osmolarity increases
• Loss of water
• Negative feedback via plasma osmolarity

Question 6

How does BP affect ADH release upon increase in plasma osmolarity?

Why is this done?

Answer 6

Low BP:
- More ADH released when osmolarity increases

High BP:
- Less ADH released when osmolarity increases

• More important to preserve volume than osmolarity

Question 7

What is Diabetes Insipidus?

What are 2 types of causes

Answer 7

• Inability to reabsorb water from the distal part of the nephron
• due to failure of Secretion/ Action of ADH
• Central
• Nephrogenic

Question 8

What are 3 symptoms of Diabetes Insipidus

Insipidus means tasteless

Answer 8

• Polyuria
• Polydipsia
• Low urine osmolality

Question 9

What happens in Central Diabetes Insipidus?

List 5 causes

Answer 9

• Impaired ADH synthesis/ secretion by Hypothalamus

Damage to hypothalamus/ pit gland due to;

• Brain injury (Basilar skull fracture)
• Tumour
• Aneurysm
• Encephalitis/ Meningitis
• Sarcoidosis/ Tuberculosis

Question 10

How is Central Diabetes Insipidus managed clinically and treated?

Answer 10

Treatment;
- Administering Desmopressin/ DDAVP (Artificial ADH)

Management;

• ADH injections
• ADH Nasal spray treatments

Question 11

What happens in Nephrogenic Diabetes Insipidus?

There is no treatment. How can it be managed?

Answer 11

• Acquired insensitivity of kidney to ADH

- Low salt, low protein diet to reduce urine output

Question 12

List 4 causes of Nephrogenic Diabetes Insipidus

Answer 12

• Mutations in gene coding for V2 receptors
• Chronic Pyelonephritis
• Polycystic kidneys
• Drugs such as Lithium

Question 13

In Nephrogenic Diabetes Insipidus, plasma ADH levels are normal.

Are there any treatments to correct the deficit?

Answer 13

No, but Thiazides can have some effect to increase Na excretion

Question 14

What is SIADH? (Syndrome of Inappropriate ADH secretion)

What are 3 characteristics?

Answer 14

• Excessive release of ADH from Post Pit gland or another source
• Dilutional hyponatreamia (Appears to have low sodium, but actually has too much water)
• Lowered plasma sodium concentration
• Increased total body fluid

Question 15

List 5 causes of SIADH

Answer 15

• CNS Disorders (stroke, abscess)
• Malignancy
• Lung diseases
• Drugs (opiates)
• Metabolic diseases (Hypothyroidism, Porphyria)

(Porphyria- Liver diseases where Porphyrins accumulate in body and affect CNS)

Question 16

List 3 symptoms of SIADH

Answer 16

• Hyponatraemia + Low plasma osmolarity
• Abnormal urine osmolarity
• Abnormal Na excretion (>20mM)

Consider diagnosis in hyponatraemic patients who DON’T have hypovolaemia/ oedema/ endocrine dysfunction/ renal failure/ drugs
(These can impair H2O excretion)