Control of Plasma Osmolarity

Question 1

If water intake is less than water excretion, what happens to the plasma osmolarity?

Answer 1

Increases

Question 2

If water intake is greater than water excretion, what happens to the plasma osmolarity?

Answer 2

Decreases

Question 3

What is normal plasma osmolarity?

Answer 3

280-310 mOsm/Kg

Question 4

What senses changes in plasma osmolarity?

Answer 4

Hypothalamic osmoreceptors

Question 5

What are the two different efferent pathways in response to changes in plasma osmolarity?

Answer 5

ADH -> kidney -> affects renal water excretion

Thirst -> brain -> changes in drinking behaviour

Question 6

Where are osmoreceptors located?

Answer 6

OVLT of hypothalamus

Question 7

What causes an increased release of ADH from posterior pituitary?

Answer 7

Conditions of predominant loss of water osmoreceptors in hypothalamus

Question 8

What does the secretion of ADH do to renal water excretion?

Answer 8

Decreases renal water excretion

Question 9

Does decreased osmolarity stimulate or inhibit ADH secretion?

Answer 9

Inhibits ADH secretion

Question 10

If blood volume collapses, is volume or osmlarity more important?

Answer 10

Volume is more important

Question 11

What induces drinking?

Answer 11

Increases in plasma osmolarity or decreases in ECF volume

Question 12

What is diuresis?

Answer 12

Increased or excessive production of urine

Question 13

Does low plasma ADH lead to diuresis or anti-diuresis?

Answer 13

Diuresis

Question 14

What is central diabetes insipidus caused by?

Answer 14

Plasma ADH levels are too low

Damage down to hypothalamus/pituitary gland

Question 15

What is nephrogenic diabetes insipidus caused by?

Answer 15

Acquired insensitivity of the kidney to ADH

Question 16

What happens in diabetes insipidus?

Answer 16

Water is inadequately reabsorbed from the collecting ducts so a large quantity of urine is produced

Question 17

What characterises SIADH?

Answer 17

Syndrome of inappropriate ADH

Characterised by excessive release of ADH from PP gland o another source

Question 18

What happens in dilutional hyponatremia?

Answer 18

Plasma Na levels are lowered and total body fluid is increased

Question 19

Where are AQP1 channels found?

Answer 19

PCT, descending limb

Question 20

Where are AQP2 channels found?

Answer 20

Apical membrane of distal DCT and apical vessels in collecting duct, principal cells expressed in the presence of ADH

Question 21

Where are AQP4 channels found?

Answer 21

Basolateral membrane of collecting duct principal cells (potential exit pathways for water entering via AQP2)

Question 22

Describe how decreases in plasma osmolarity results in diuresis

Answer 22

• No ADH stimulation means no AQP2 in apical membrane, no AQP3/4 on basolateral membrane only of the latter DCT and CD
• Limited water reuptake in latter DCT/CD -excrete lots of urine
• Tubular fluid rich in water passes through the hyperosmotic renal pyramid with no change in water content
• Loss of large amount of hyposmotic (dilute) urine

Question 23

What happens if plasma osmolarity increases?

Answer 23

Release of ADH causes insertion of AQP2 into apical membrane

Water moves out of CD into hyperosmotic environment if there are AQPs in both the apical and basolateral epithelia

Kidney must absorb as much as water as possible so produces hyperosmotic urine

Question 24

What nephrons allow us to create the concentration gradient?

Answer 24

Juxtamedullary

Question 25

Where does ADH act?

Answer 25

DCT and CD

Question 26

What happens to the permeability of DCT and CD cells to water when there is an increase in plasma osmolarity?

Answer 26

ADH release means that permeability increases - insertion of AQP2 channels

Question 27

What is diabetes insipidus characterised by?

Answer 27

This condition is characterised by excessive thirst and secretion of copious amounts of dilute urine.

Question 28

How come the water absorption in the descending limb is passive?

Answer 28

This is because only ions are reabsorbed in the ascending limb so this creates a ‘salty’ medullary insterstitium and forms a concentration gradient

Question 29

Where is the maximum osmolarity in the loop of henle?

Answer 29

At the tip

Question 30

Why does osmolarity of the filtrate increase down the descending loop of henle?

Answer 30

This is because the descending limb is permeable to water but impermeable to Na+

Na+ remains in the descending limb and filtrate concentration increases (as water leaves as you go down)

Question 31

Why does the osmolarity of the filtrate decrease in the ascending limb of the loop of henle?

Answer 31

Ascending limb actively transports NaCl out of lumen into interstitium

IMPERMEABLE to water

NaCl leaves, H2O remains - osmolarity decreases and the fluid leaving the ascending limb has a low osmolality of 100 mOsm/kg

Question 32

What happens to the concentration of the interstitial fluid surround the loop of henle as Na+ is actively transported out of the ascending limb?

Answer 32

Increases (see counter current multiplication)

Question 33

What is an example of an effective osmole?

Answer 33

Urea

Question 34

Why is urea recycling ADH dependent?

Answer 34

It travels through aquaporins

Question 35

How does vasa recta conserve the concentration gradient?

Answer 35

Flow in vasa recta is in opposite direction to fluid flow in the tubule so the osmotic gradient is maintained

Question 36

What is the interstitial concentration at the bottom of the medulla?

Answer 36

1200 mOsm

Question 37

What is the interstitial concentration at the top of the medulla?

Answer 37

300 mOsm

Question 38

What is the osmolarity at the top of the ascending limb?

Answer 38

100 mOsm