Regulation of osmolality

Question 1

What is ADH?

Answer 1

ADH (Vasopressin) = arginine vasopressin (AVP)

Polypeptide (9aas) synthesized in the supraoptic (SO) and paraventricular (PVN) nuclei of the hypothalamus in the brain.

Posterior pituitary hormone.

Question 2

What type of hormone is ADH?

Answer 2

Peptide hormone from posterior pituitary

Question 3

Where is ADH produced?

Answer 3

Polypeptide (9aas) synthesized in the supraoptic (SO) and paraventricular (PVN) nuclei of the hypothalamus in the brain.

Question 4

What is the half life of ADH?

Answer 4

Around 10mins

Question 5

How is ADH secretion controlled?

Answer 5

Plasma osmolarity

ECF volume

Question 6

Where do you find receptors that mediate thirst?

Answer 6

Lateral hypothalamus

Question 7

What is normal plasma osmolality?

Answer 7

280-290mOsm/kg H2O

Question 8

Where are osmoreceptors found?

Answer 8

Found in anterior hypothalamus, close to the SO and PVN

Question 9

A change in what property causes a change in ADH secretion?

Answer 9

Tonicity

Question 10

What is the function of ADH?

Answer 10

ADH ↑ the permeability of the collecting ducts to H2O, by incorporating H2O channels into the luminal membrane, (aquaporins) causing the increased reabsorption of water and the production of very concentrated urine

Question 11

Describe the movement of water at the collecting ducts without the presence of ADH

Answer 11

In the absence of ADH, collecting ducts are impermeable to H2O

Question 12

How does ADH influence urea movement in the collecting ducts?

Answer 12

The permeability of the late medullary CD to urea is enhanced by ADH.

Question 13

How does increased ECF volume affect ADH secretion?

Answer 13

↑ ECF volume → ↓ [ADH]

Question 14

How does decreased ECF volume affect ADH secretion?

Answer 14

↓ ECF volume → ↑ [ADH]

Question 15

What are low P receptors and where are they found?

Answer 15

• Low P receptors are located in the L and R atria and great veins.
• They are sometimes called “volume receptors” because they monitor the return of blood to the heart and the “fullness” of the circulation.

Question 16

What are high P receptors and where are they found?

Answer 16

• High P receptors are the carotid and aortic arch baroreceptors.

Question 17

What stimuli increases ADH secretion?

Answer 17

Increased Plasma osmolarity
Decreased ECF volume 
Pain, emotion, stress
Exercise
Nicotine
Morphine

Following traumatic surgery, inappropriate ADH secretion occurs, need to be careful about monitoring H2O intake.

Question 18

What stimuli decreases ADH secretion?

Answer 18

Decreased Plasma osmolarity
Increased ECF volume
Alcohol (suppresses ADH release)

Question 19

Describe the two types of Diabetes Insipidus

Answer 19

Central DI - the hypothalamic areas synthesizing ADH may become diseased due to tumours, or in meningitis. They may be “damaged” during surgery

Peripheral DI - the Collecting duct may be insensitive to ADH

Question 20

What is diabetes insipidus?

Answer 20

Caused by ADH deficiency

Patients are characterised by the passage of very large volumes of very dilute urine, generally > 10 l/day (polyuria). They drink large volumes of H2O (polydipsia).

Question 21

How is central diabetes insipidus treated?

Answer 21

Give ADH

Question 22

How is peripheral diabetes insipidus treated?

Answer 22

For Peripheral DI, importance of the thirst mechanism for survival, can’t give ADH. Usually 2° to hypercalcaemia or hypokalaemia so resolves when ion disorders corrected.

Question 23

What genes are associated with diabetes insipidus?

Answer 23

May arise as a genetic defect in the V2 (ADH) receptor or in gene for aquaporins (H2O channels).

Question 24

Define osmolarity

Answer 24

Osmolarity is the measure of solute concentration per unit volume of solvent. It’s not the same as tonicity! Osmolarity takes into account ALL of the solute concentrations, not just the ones that can’t cross the semipermeable membrane.

Question 25

Define osmolality

Answer 25

Osmolality is the measure of solute concentration per unit mass of solvent. You never measure osmolarity in practice, because water changes its volume according to temperature (but mass remains the same and so it is more convenient and consistent). Osmolality is the same in the ICF and ECF (285-290mOsm/kg)

Question 26

Define tonicity

Answer 26

Tonicity is the measure of osmotic pressure gradients between two solutions. Unlike osmolarity, tonicity is only influenced by solutes that cannot cross the semipermeable membrane, because there are the only solutes influencing the osmotic pressure gradient. Thus, you can have iso-osmolar solutions which are not isotonic