Regulation of Osmolality

Question 1

What hormone regulates water balance?

Answer 1

ADH (vasopressin)

Question 2

What is the half life of ADH?

Answer 2

Around 10 minutes

Question 3

What is the governing factor influencing ADH secretion?

Answer 3

Plasma osmolarity

When the osmotic pressure of the plasma increases, the rate of ADH-secreting neutron discharge increases in the sub optic and paraventricular nuclei, so ADH secretion from the PP increases and re-uptake increases in the kidney

Question 4

What happens to a cell when there is increased osmolarity?

Answer 4

Water leaves the cell and it shrinks (ADH released)

Question 5

What happens to a cell when the osmolarity is decreased?

Answer 5

Water enters the cell causing it to swell (ADH inhibited)

Question 6

What is the relationship between osmolarity and ADH release?

Answer 6

They are directly proportionate

Question 7

What is the role of the osmoreceptors when there is an osmotic change?

Answer 7

If there is increased osmolarity in the cell, the cell volume reduces as water leaves the cell. The osmorecpetors detect this and trigger an increases in ADH release - this is an extremely sensitive mechanism, the effect of which is multiplied many times over the strength of a stimulus
They are essential to maintain the resting membrane potential

Question 8

What is the normal plasma osmolarity?

Answer 8

300 mOsmoles/l

Question 9

What is the effect on osmotic drag and tonicity if a membrane is freely permeable to a molecule?

Answer 9

No effect

Question 10

What is the difference between effective and ineffective osmoles?

Answer 10

An effective osmole is one which causes a change in tonicity and therefore has and effect on ADH release

Question 11

Is urea an effective or ineffective osmole?

Answer 11

Ineffective

Question 12

Between the top of the ascending limb of the LoH and the top of the collecting duct, describer the osmolarity of the solution

Answer 12

Hypo-osmotic

Question 13

What is the role of aquaporins in relation to the collecting duct?

Answer 13

They influence its permeability to water, by incorporating H20 channels into the luminal membrane rather than the basal membrane

Question 14

Are collecting ducts permeable to water in the absence of ADH?

Answer 14

No

Question 15

What is the effect of high ADH on urea levels in the duct?

Answer 15

High ADH means that large amount of water leave the collecting ducts back into the interstitium. AS a result the urea is left in the ducts at very high concentrations. At the tip of medulla there is moderate permeability to the urea, so it moves out along a gradient, allowing it to be absorbed into the interstitium and reinforce the interstitial gradient in the thin ascending parts of the LoH

Question 16

What is the name for abnormally high levels of urea in the plasma?

Answer 16

Uraemia

Question 17

What are the two main factors governing ADH secretion?

Answer 17

Primary control is the plasma osmolarity (high osmolarity = high ADH secretion)

ECF volume - inversely proportional

Question 18

Describe the relationship between ECF volume and ADH secretion

Answer 18

They are inversely proportional

Question 19

Describe the effect of ECF volume on the secretion of ADH

Answer 19

When ECF volume is high, high P receptors in the carotid arch and aortic arch receptors detect this stretch and reduce ADH secretion, to allow more water to be excreted, lowering blood and ECF volume

When ECF volume is low, low P receptors in the atria and great veins detect the drop in stretch and compensate by increasing ADH secretion

Question 20

Compare the input of the atrial receptors with that of the carotid ones in small and large ECF volume changes

Answer 20

When there are small changes the atrial receptors mediate the correction

However if there are changes significant enough to affect the MBP, the carotid receptors step in and further contribute to the changes in ADH secretion

Question 21

What are some other factors which can stimulate ADH release?

Answer 21

Pain, emotion, stress, exercise, nicotine, morphine

Question 22

What is an unrelated factor which suppress ADH release?

Answer 22

Alcohol