Control of Plasma Osmolarity

Question 1

What is osmolarity a product of and what is plasma volume a product of

Answer 1

Osmolarity is a product of water

Plasma volume is a product of salt

Question 2

What is the relationship between solute concentration of urine and volume of urine

Answer 2

Inversely proportional

Question 3

What detects changes in omsolarity and what two pathways do they stimulate

Answer 3

Osmoreceptors found in the OVLT in the hypothalamus

They stimulate thirst or ADH release

Question 4

Describe the secretion of ADH - including how it is controlled

Answer 4

ADH release is stimulated after a 1% increase in osmolarity

ADH release is controlled by -ve feedback loop

Decreased osmolarity inhibits ADH release

Renal response very sensitive to changes in ADH

Question 5

How do changes in blood volume/pressure have an effect on the response to changes in osmolarity

Answer 5

Decreased ECV causes there to be more ADH released at lower plasma osmolarities. Body only looks at volume when ECV decreases -> kidneys continue to conserve fluid even though it will decrease osmolarity

Increased ECV causes the less ADH to be released with increasing osmolarity and ADH is released at higher osmolarities than normal. Makes sure the volume cannot increase anymore and blunts response to ADH

Question 6

How is the thirst response stimulated and what does it do

Answer 6

Thirst response is stimulated by large deficits in water

Drinking is stimulated by increased osmolarity or decreased ECV - if ADH not sufficient

Thirst increases intake of free water

Question 7

What is diabetes insipidus and what are the two types

Answer 7

Diabetes insipidus is where the body produces a large amount of urine and does not conserve water correctly

Central diabetes insipidus - results from plasma ADH levels being too low

Nephrogenic diabetes insipidus - acquired insensitivity of kidney to ADH

Manage via ADH injections or ADH nasal spray

Question 8

What is SIADH

Answer 8

Syndrome of inappropriate ADH secretion

It is characterised by excessive release of ADH or ADH-like peptide from the PP gland or another source

Results in dilutional hyponatremia - plasma Na levels decrease while total body fluid increases

Urine osmolarity and urine Na osmolarity increase due to water retention

Question 9

What effect does ADH have in the collecting duct

Answer 9

ADH causes the insertion of AQP2 channels into the apical membrane of the CD principal cells

If ADH is low, AQP2 channels are pulled away from the apical membrane

If ADH present, AQP2 inserted into membrane

There is a fast turnover of AQP2 channels -> can quickly increase number of AQP2 channels in apical membrane

Question 10

What factors causes urine to be concentrated

Answer 10

Juxtamedullary nephrons - help establish vertical osmotic gradient

Vasa recta - maintain osmotic gradient

Collecting ducts - use gradient along with ADH to produce urine of varying concentrations

Urea - helps in forming osmotic gradient

Question 11

How does blocking NKCC2 transporters with a loop diuretic cause copious urine to be produced. What is a consequence of blocking these NKCC2 transporters

Answer 11

By blocking NKCC2 transporters, the medullary interstitium becomes isosomtic and copious urine is produced as no water leaves the DL of the loop of Henle

Blocking NKCC2 transporters means there is no way for K to be reabsorbed after it leaves the cells by the ROMK channels - ROMK is not blocked by the diuretics

This results in lots of K being excreted in urine

Question 12

Why does urea act as an effective osmole in the kidney but not in the rest of the body

Answer 12

Urea has transporters in the rest of the body which facilitate urea diffusion across most cell membranes

If solutes can pass across a membrane, they are ineffective at exerting an osmotic force across the membrane

Therefore urea is not an effective osmole in the rest of the body

In the kidney, there are no urea transporters -> acts as an effective osmole

Question 13

Describe urea recycling

Answer 13

Urea leaves CD cells through AQP channels along with water

Once in the interstitium, urea increases the omsolarity of the interstitium so more water is drawn out of the CD and PCT

Urea then diffuses back into the filtrate in the ascending limb

Urea recycling is proportional to [ADH]

Question 14

Describe the counter current flow seen in the nephron

Answer 14

Salt is pumped out of the thick AL into interstitium to create osmotic gradient between DL and interstitium

This causes water to leave interstitium until osmotic gradients are equal

There is a counter-current multiplication along the loop of Henle which helps to reabsorb water as it means the interstitial fluid’s osmolarity increases down the DL so water continuously moves out of the DL into the intersitium

Interstitial fluid osmolarity is lowest at the top of the loop and is greatest at the bottom of the loop -> allows more water to be reabsorbed

Question 15

Why is the vasa recta able to mimic its surroundings

Answer 15

The vasas recta has no active transporters

It relies solely on diffusion of solutes and osmosis of water

This means it will take up fluid/salt until it has the same osmolarity as its environment

Question 16

Describe how the blood flow through the vasa recta helps it to maintain the osmolarity of the interstitium

Answer 16

Blood flow through vasa recta is opposite to the flow of filtrate through the loop of Henle -> it flows down the AL first and takes up salt until its osmolarity is the same as the interstitial fluid’s osmolarity

The vasa recta then ascends close to the DL of the loop where it takes up water due to the vasa recta having a high osmolarity

This means the water removed from the filtrate is taken up by the vasa recta and does not remain in the interstitium where it can decrease the osmolarity and disrupt the osmotic gradient