13. body fluid osmolality

Question 1

What helps maintain maintain medullary hyper-tonicity?

Answer 1

slow flow of vasa recta

urea recycling

Question 2

Where are osmoreceptors located?

Answer 2

hypothalamus - Specifically in the OVLT (Organum Vasculosum of the Lamina Terminalis)

Question 3

Where is ADH produced? Where is it released from?

Answer 3

Supraoptic nucleus of hypothalamus. Released posterior pituitary gland

Question 4

What type of hormone is ADH?

Answer 4

peptide hormone

Question 5

what triggers ADH release?

Answer 5

Increased osmolality

Question 6

What does an increase in ADH result in?

Answer 6

• Reduced water excretion (V2 receptor- mediated)

* Blood vessel vasoconstriction (V1 receptor-mediated)

Question 7

How does ADH increase H20 reabsorption

Answer 7

• ADH causes intracellular aquaporins to fuse with the
luminal membrane.
• ADH binds to V2 receptors on the basal membrane.
• G-protein-coupled receptors, when activated, cause fusion of inactive aquaporin 2 vesicles with the membrane.
• Creates a channel through with water can
pass
• Passive flow of water

Question 8

What are the 2 response pathways of osmoreceptors?

Answer 8

Concentration of urine and Thirst

Question 9

summarise the two pathways of regulation of osmolality

Answer 9

Changes in Plasma osmolarity → Hypothalamic Osmoreceptors→(1) ADH → reduced Renal Water Excretion

Changes in Plasma osmolarity→Hypothalamic Osmoreceptors →(2) Thirst → increased Water intake

Question 10

WHat does decrease in osmolarity do to ADH release?

Answer 10

• ↓ osmolarity inhibits ADH secretion =
diuresis
• Negative feedback loops that begin within
the anterior hypothalamus
• Result is a feedback loop which stabilizes
osmolarity

Question 11

Describe the role blood pressure has on ADH

Answer 11

Increase in BP: shallow slope - needs a larger increase in osmolarity to initiate ADH release

Decrease in BP: steep slope - allows a smaller increase in osmolarity to initiate ADH release

Question 12

Which is more important in a haemorrhage (ie volume drops) - correcting volume or osmolarity? How does ADH respond to this?

Answer 12

Volume because don’t want to have to wait for osmolality to increase fo r ADH to be naturally released (to increase H20 reabs + therefore blood volume).

Question 13

Kidney can’t fully compensate for decreased water intake or increased salt intake so how does the body ensure we restore fluid levels?

Answer 13

Aldosterone simulates thirst centres in brain, which induce thirst by increasing plasma osmalality. We feel thirsty and drink water until we’ve consumed enough to bring plasma osmalality back to normal (stop feeling thirsty).

Question 14

What is diabetes insipidus?

Answer 14

Failure to secrete ADH or respond to ADH, leading to inability to reabsorb water from distal part of the nephron.

Question 15

what are the 2 types of diabetes insipidus?

Answer 15

• Nephrogenic

* Central

Question 16

Symptoms of diabetes insipidus?

Answer 16

polyuria, polydipsia, low urine osmolality (ie dilute urine)

Question 17

What is central diabetes insipidus?

Answer 17

Low plasma ADH due to damage to hypothalamus or pituitary gland.
• Impaired ADH synthesis or secretion by the hypothalamus

Question 18

What could cause central diabetes insipidus?

Answer 18

• a brain injury, particularly a fracture of the base of the skull
• a tumour
• sarcoidosis or tuberculosis
• an aneurysm
• some forms of encephalitis or meningitis

Question 19

how is central diabetes insipidus treated?

Answer 19

• Treated by administering ADH (desmopressin)

Managed clinically by ADH injections or by ADH nasal spray treatments

Question 20

What is nephrogenic diabetes insipidus?

Answer 20

Acquired insensitivity of the kidney to ADH

Question 21

how is nephrogenic diabetes insipidus treated?

Answer 21

• Difficult to managed clinically but a low-salt, low-protein diet reduces urine output.
• No current treatment to correct the deficit
• Thiazides can have some effect to ↑Na+ excretion

Question 22

What could cause nephrogenic diabetes insipidus?

Answer 22

Mutations in gene coding for V2 receptors, chronic pyelonephritis, polycystic kidneys or drugs such as lithium

Question 23

What is SIADH?

Answer 23

syndrome of inappropriate antidiuretic hormone

- excess release of ADH from PP or ectopic source

Question 24

What is SIADH characterised by?

Answer 24

• Dilutional hyponatremia
• In which the plasma sodium concentration is lowered
• Total body fluid is increased

Question 25

what can cause SIADH?

Answer 25

• CNS disorders e.g. stroke, abscesses
• Malignancy
• Lung diseases
• Drugs e.g. opiates
• Metabolic disease e.g. porphyria, hypothyroidism

Question 26

what are the symptoms of SIADH?

Answer 26

• Hyponatremia and low plasma osmolality
• Inappropriate urine osmolalitym (concentration higher than normal)
• Inappropriate Na+ excretion (urinary [Na+] greater than 20mmol/L despite decrease in plasma Na+ concentration

Question 27

when should SIADH be considered?

Answer 27

The diagnosis should be considered in hyponatremic patients in the absence of hypovolemia, oedema,
endocrine dysfunction, renal failure and drugs, all or which can impair water excretion