4. Body Fluid Osmolality

Question 1

Where are osmoreceptors located?

Answer 1

In hypothalamus, specifically in organum vasculosum of lamina propria

Question 2

How do osmoreceptors work?

Answer 2

Fenestrated leaky endothelium exposed directly to systemic circulation (on plasma side of blood brain barrier)
Sense changes in plasma osmolarity
Signal responses which are mediated via 2 pathways leading to 2 outcomes: concentration of urine, thirst

Question 3

What triggers ADH to be released?

Answer 3

Increase in plasma osmolality

Question 4

What does ADH release lead to?

Answer 4

Reduced water excretion
Blood vessel vasoconstriction
Increased urea recycling

Question 5

How does ADH work?

Answer 5

Binds to V2 receptors on basal membrane

Causes intracellular aquaporins to fuse with the luminal membrane

Question 6

What are the 2 things that happen when change in plasma osmolarity is sensed?

Answer 6

1. ADH released, affects kidney, causes renal water excretion
2. Causes thirst, brain communicates drinking behaviour, water intake

Question 7

What happens if there is a reduced ECV?

Answer 7

Set point is shifted to lower osmolarity values and the slope of the relationship is steeper

Question 8

What happens if there is an increase in blood pressure?

Answer 8

The set point is shifted to higher osmolarity and the slope of the relationship decreases

Question 9

How does the body compensate for thirst?

Answer 9

Large deficits in water or an increase in salt only partially compensated for in the kidney
Ingestion of water is the ultimate compensation

Question 10

What is diabetes isipidus?

Answer 10

Too little ADH

Inability to reabsorb water from the distal part of the nephron, due to failure of secretion or action of ADH

Question 11

What are the symptoms of diabetes insipidus?

Answer 11

Polyuria
Polydipsia
Low urine osmolality (dilute urine)

Question 12

What are the causes of diabetes insipidus?

Answer 12

Nephrogenic

Central

Question 13

What is central diabetes insipidus?

Answer 13

Impaired ADH synthesis or secretion by the hypothalamus, due to damage to hypothalamus or pituitary gland

Question 14

What can damage the hypothalamus or pituitary gland?

Answer 14

A brain injury
A tumour
Sarcoidosis or tuberculosis
An aneurysm
Some forms of encephalitis or meningitis

Question 15

How is central diabetes insipidus treated?

Answer 15

By administering ADH (desmopressin)

Question 16

Why does central diabetes insipidus cause a large quantity of urine?

Answer 16

Water is inadequately reabsorbed from the collecting duct

Question 17

What is nephrogenic diabetes insipidus?

Answer 17

Acquired insensitivity of kidney to ADH

Question 18

How is nephrogenic diabetes insipidus managed?

Answer 18

Low-salt, low-protein diet reduces urine output

Difficult to manage clinically

Question 19

What can cause nephrogenic diabetes insipidus?

Answer 19

Mutations in gene coding for V2 receptors
Chronic pyelonephritis
Polycystic kidneys
Drugs such as lithium

Question 20

What is too much ADH called?

Answer 20

Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

Question 21

What is SIADH characterised by?

Answer 21

Excessive release of ADH from the pituitary gland or another source
-dilutational hyponatremia, total body fluid is increased

Question 22

What can SIADH be due to?

Answer 22

CNS disorders (stroke, abscesses)
Malignancy
Lung diseases
Drugs (opiates)
Metabolic disease (porphyria, hypothyroidism)

Question 23

What are the symptoms of SIADH?

Answer 23

Hyponatremia and low plasma osmolality
Inappropriate urine osmolality
Inappropriate Na+ excretion