Session 5 - Control of Plasma osmolality

Question 1

How is water intake and urine balanced in the body?

Answer 1

Amount of water intake and urine output to maintain osmolarity of plasma.
Urine osmolarity can vary to account for different water intake.

Question 2

What is normal plasma osmolarity?

Answer 2

280-310 mOsm/Kg

Question 3

How is change in plasma osmolality detected and signalled?

Answer 3

1) Hypothalamic osmoreceptors sense change in osmolarity.
Then.
a) ADH hormone secretion is changed to affect renal water excretion at the kidney.
b) Thirst (behavioural) is increased, to increase water intake.

Question 4

Where are osmoreceptors found in the brain?

Answer 4

In the Hypothalamus in the OVLT.

Sense change in plasma osmolarity.

Question 5

How does ADH affect plasma osmolarity?

Answer 5

Under loss of water, ADH release increased.
ADH causes increased expression of AQP in DCT/ collecting duct to reabsorb more water.

Decreased osmolarity inhibits ADH release.

Question 6

How does blood pressure affect conservation of H20 by kidneys?

Answer 6

Even in circulatory collapse the kidneys will conserve H20 to maintain circulatory volume, even if the plasma osmolality is low (overrides)

Question 7

What is central diabetes insipidus?

Answer 7

When plasma ADH levels are too low, causing production of large quantities of urine.
-Damage to pituitary
- Brain injury
- Tumour
(treated with ADH injection/ nasal spray)

Question 8

What is nephrogenic diabetes insipidus?

Answer 8

From an acquired insensitivity of the kidney to ADH.

Water inadequately reabsorbed, so large quantities of urine produced.

Question 9

What is SIADH, syndrome of inappropriate antidiuretic hormone secretion? What happens?

Answer 9

• Increased ADH secretion, from Posterior pituitary or another source.
• Dilutional Hyponatraemia, plasma sodium levels lowered, but body fluid increased.

Question 10

Which aquaporins are found on which membranes?

Answer 10

AQP 2 = Abundant on APICAL

AQP3 AQP4 = Basolateral

Question 11

When a decreased plasma osmolarity is detected how does feedback rectify it?

Answer 11

• No ADH stimulation
• No aquaporin 2 on apical membrane
• Limited water reuptake in DCT.
• Loss of large amounts of hypo-osmotic urine
  = diuresis.

Question 12

How does the body respond if a plasma osmolarity increase is detected?

Answer 12

• Increase in ADH secretion.
• Insertion of AQP 2 on apical membrane.
• More water moves out of collecting duct into cells, then into interstitium (AQP3 + 4)
• Produced concentrated urine, by reabsorbing more water.

Question 13

Why is patient urine output initially very high after kidney transplant?

Answer 13

Because it takes time for the countercurrent system to develop which creates the medullary gradient required for reabsorption of water in the CD.

Question 14

What is the medullary countercurrent system?

Answer 14

The system responsible for causing increasing vertical osmotic gradient in the kidney medulla.

• Juxtamedullary nephron - long loop of henle to establish the gradient.
• Vasa recta - helps maintain the gradient.
• Urea helps in urine concentration mechanism.

Question 15

How is the medullary gradient established?

Answer 15

• Thick ascending limb of loop of henle
• Diluting action on the filtrate:
  Removes solute without waterm therefore increasing osmolarity of interstitium.

Question 16

What effect to loop diuretics have on the medullary countercurrent system and the production of urine?

Answer 16

Block NaKCC channel.
Medullary interstitium becomes isosmotic (less Na reabsorbed)
Therefore copious amounts of dilute urine produced.

Question 17

What is the osmolarity of the interstitium at the corticomedullary border and at the papilla?

Answer 17

Corticomedullary = 300 mOsm/Kg

Papilla = Up to 1200 mOsm/Kg

Question 18

What is an effective/ ineffective osmole?

Answer 18

Urea
Urea is ineffective osmole when the membrane allows it to freely pass.
In the Kidney, urea is an effective osmole because the different parts of the nephron have different permeabilites to it.

Question 19

How is urea used as an effective osmole when cycled?

Answer 19

When ADH is present:
- It follows water out of the lumen of the medullary collecting duct. (via AQP)
- It increases the concentration in the interstitium (effective osmole)
- It then moves back into the loop of henle.
(can cycle round, as long as ADH present)

The cortical collecting duct cells are IMPERMEABLE to urea.

Question 20

What is counter current multiplication?

Answer 20

The creation of a concentration gradient, caused by water moving out of the descending limb into the interstitium, and sodium and chloride being moved out of the ascending limb (to create a 200mOsmol gradient between intersitium and lumen) of the loop of henle.

Question 21

What is counter current exchange?

Answer 21

• Blood in the vasa recta moving in the opposite direction to fluid in the loop of henle creating a counter current which maintains the gradient.
  ——————————————————————-
  Descending limb of vasa recta
  -Isosmotic blood in vasa recta enters hyperosmotic
  milieu of the medulla ( high conc. Na+ ions, Cl- ions + urea)
  -Na+, Cl- + urea diffuse into the lumen of vasa recta
  -osmolarity of blood in vasa recta increases as it reaches tip of hairpin loop.
  ——————————————————————–
  Ascending limb of vasa recta
  -Blood ascending towards cortex will have higher solute content than surrounding interstitium.
  -Water moves in from the descending limb of the loop of Henle

See diagram for detailed mechanism of system.