Physiology 4 - Osmotic Regulation

Question 1

Describe the half-Life of ADH?

Answer 1

~10 mins

So it can be rapidly adjusted based on H2O need

Question 2

What does ADH do?

Answer 2

It determines the water solubility of the collecting duct, more ADH = more water resorped = More concentrated urine

Question 3

What controls ADH?

Answer 3

Osmoreceptors
ECF volume

Pain/emotion/stress/exercise/nicotine/morphine/traumatic surgery all increase ADH
- Alcohol suppresses ADH

Question 4

Where are osmoreceptors found?

Answer 4

Anterior Hypothalamus

Question 5

How do osmoreceptors work?

Answer 5

They use stretch sensitive ion channels to detect movement of water in and out the cell
They trigger more ADH release when water moves out of them and they shrink

Question 6

What does it mean to say osmolarity receptors are a “high gain” system?

Answer 6

only a 2.5% increase in osmolarity will increase ADH 10x

Question 7

How do tonicity/osmolarity affect osmoreceptors?

Answer 7

Tonicity is the relevant one.

Only molecules that can’t cross the cell membrane (and therefore can’t equalize) will affect the movement of water. E.g. Na+ but not urea

Question 8

What effect does ECF volume have on ADH?

Answer 8

More ECF volume decreases ADH and vice versa

Question 9

What detects ECF volume?

Answer 9

Low P receptors in Atria & Great Veins (aka volume receptors)

High P receptors in carotid & aortic arch baroreceptors

Question 10

How do Low P receptors in atria/great veins work?

Answer 10

They have a base tonic inhibitory effect on ADH secreting neurons via the vagal nerve.
When ECF volume decreases, they fire less and more ADH is released so more fluid is retained)

(Important in hemorrhage)

Question 11

How does ADH increase Water reabsorption from the collecting ducT?

Answer 11

Acts on a an ADH receptor

• -> Activates a cAMP 2* messenger
• -> Causes storage vesicles to express aquaporin-2 water pores on the luminal surface of the cell membrane

These increase water permeability

Question 12

What causes water to move out the collecting duct?

Answer 12

Once ADH makes the duct cells permeable to water it travels out thanks ot the high medullary interstitial osmolarity created by the loop of henle (from there into the blood)

So it gets more concentrated the further down the collecting duct as the interstitial osmolarity increases as it goes

Question 13

What is the role of urea in water conservation during anti-diuresis?

Answer 13

The tip of the medullary tube is partially permeable to Urea.
ADH makes the collecting more permeable to urea

When water leaves it concentrates urea so it then follows.

There it reinforces the interstitial gradient and reserves water (if it stayed in the tubule more water would need to be excreted with it)

Question 14

What is Diabetes Insipidus?

Answer 14

A condition where ADH fails and you cant concentrate urine. Instead you excrete a large amount of hypotonic urine

Question 15

What are the signs of DI?

Answer 15

Polyuria (>10L/day) and consequent Polydipsia

Question 16

What are the types of DI?

Answer 16

Central DI:
- Disease of ADH synthesizing part of Hypothalamus e.g. tumour/meningitis/surgery

Peripheral DI:
- Desensitization of the Collecting duct to ADH
E.g. genetic defects in V2 (ADH) receptor or in the aquaporins

Question 17

How do you treat DI?

Answer 17

Peripheral DI relies on thirst mechanism for you to survive. Its usually down to hypercalcaemia or hypokalemia and will resolve when the ion disorders does.

Central DI needs exogenous ADH

Question 18

How much fluid passes through the bowmens capsule, proximal tubule, end of loop of henle adn end of collecting duct per day?

Answer 18

Bowmans capsule filters 190l/day (300mOsM)

End of proximal tubule sees 54L/day (the rest is reabsorped) (300mOsM)

End of Loop of henle sees 18L/day (rest is secreted into interstitium and taken up by vasa recta) (100mOsM)

End of collecting duct sees 1.5L/day of final urine (Avg) (50-1200mOsM depending on ADH)

Question 19

which receptors are involved in a small change of ECF and which in a big change?

Answer 19

small change: atrial

Large change: aortic/carotid baroreceptors detect changes which affect MBP

Question 20

What happens when there is NO ADH

Answer 20

very dilute urine
o The collecting duct is impermeable to water so it cannot move out of the collecting duct
o This causes a large amount of urine with low concentration to be released : 30-50mosmole
o This makes up for XS water in the system

Question 21

what happens where there is a large amount of ADH

Answer 21

o The contents equilibrate with the medullary intersistitium via the osmotic effect of H20
o Small amount of highly concentrated urine
o Becomes highly concentrated at the tip
o Water released into the ECF and then reabsorbed by the high oncotic pressure of the vasa recta

Question 22

What is considered to be a high osmolarity?

Answer 22

280mosmol/l