Regulation of osmolality

Question 1

Collecting duct is the site of … … under the control of what hormone

Answer 1

water regulation

ADH

Question 2

Osmolarity v osmolality

Answer 2

Osmolarity is the number of osmoles of solute in a litre of solution.

while osmolality is the number of osmoles of solute in a kilogram of solvent

Question 3

List 3 factors which influence release of ADH, indicating the most significant physiologically

Answer 3

Change in plasma osmolarity - primary controlling factor

Change in ECF volume/atrial stretch (as increased/decreased blood volume stretches atria more/less)

Change in MAP

Question 4

When oncotic pressure of the plasma increases, does ADH release increase or decrease

Answer 4

Increase

Question 5

Discharge of ADH secreting neurons in the supraoptic and paraventricular nuclei is mediated by what receptors

Answer 5

Osmoreceptors in the hypothalamus

Question 6

When plasma osmolarity increases

• effect on water movement
• effect on cell size
• effect on neural discharge of ADH secreting cells
• effect on ADH secretion

Answer 6

increased osmosis out of cells so cells shrink

cell shrinkage sensed by stretch sensitive ion channels which triggers increased neural discharge of ADH secreting cells –> increased ADH release

Question 7

When plasma osmolarity decreases

• effect on water movement
• effect on cell size
• effect on neural discharge of ADH secreting cells
• effect on ADH secretion

Answer 7

water moves into cells from plasma so cell swells
cell swelling sensed by stretch sensitive ion channels which triggers decreased neural discharge –> decreased ADH secretion

Question 8

End goal of ADH

Answer 8

Reduce diuresis - loss of water in urine

Question 9

Normal plasma osmolality

Answer 9

280-290mOsm/kg H2O

Question 10

Plasma osmolality is regulated tightly so small changes in either direction results in rapid changes in secretion of

Answer 10

ADH

Question 11

Osmolarity v tonicity

Answer 11

Osmolarity takes into account the total concentration of penetrating solutes and non-penetrating solutes, whereas tonicity takes into account the total concentration of non-freely penetrating solutes only

Question 12

Tonicity refers to the total conc. of what particles/solutes

Answer 12

non-freely penetrating particles/solutes

Question 13

For an increase in plasma osmolarity to produce an increase in ADH secretion, plasma … also has to be increased, otherwise no osmotic drag is produced

Answer 13

tonicity

Question 14

What kind of particles/solutes don’t produce a change in tonicity

Answer 14

penetrating (i.e. freely permeable) ones

Question 15

What does it mean if a solute is penetrating + name some

Answer 15

Can freely move across the cell membrane and has no effect on ECF tonicity

e.g. urea, O2, CO2

Question 16

What does it mean if the solute is non-penetrating + name some

Answer 16

Can’t move across the cell membrane so does have an effect of ECF tonicity

e.g. Na+, K+

Question 17

Tonicity depends on the concentration of what solutes only

Answer 17

non-freely penetrating solutes only

Question 18

Effect on increased plasma urea on cell volume

Answer 18

No change as urea is a penetrating solute so can equilibrate across the cell membrane and pull water along with it

Question 19

Ingesting hypertonic solutions like seawater causes dehydration - why

Answer 19

because it’s got lots of solute which means more solute has to be excreted therefore increased diuresis –> dehydration as more water is required to excrete the excess solutes than the volume of seawater ingested

Question 20

How does ADH result in reduced diuresis

Answer 20

ADH circulates to the collecting duct and increases the permeability of it to water by binding to its receptor which triggers the recruitment of aquaporin 2 transporters to the membrane of the collecting duct which allows water to move out the duct into the vasa recta (blood), i.e. get reabsorbed into blood

Question 21

In a patient in whom sodium intake exceeds sodium output, does ICF volume increase or decrease

Answer 21

Decrease
Since the excess sodium is restricted to the ECF by the action of the Na+/K+-pump, water, which is freely permeable across cell membranes, water will move out of cells to equalise the osmolarity inside and outside the cell.

Question 22

If maximum ADH is present at the collecting duct, the contents of the duct … with that of the medullary interstitium via osmotic … of … and thus becomes highly … at the tip of the medulla.

Answer 22

equilibrate

efflux (i.e. out)

H2O

Question 23

Maximum ADH conc. at the collecting duct produces what kind of urine (conc. and volume)

Answer 23

small volume of highly concentrated urine because lots of water is reabsorbed

Question 24

In absence of ADH, collecting ducts are not as permeable to … so what kind of urine is produced (conc. and volume)

Answer 24

large volume of dilute urine

Question 25

In the presence of ADH, movement of water out the collecting ducts greatly concentrates what penetrating (Free permeable) solute in the collecting duct

Answer 25

urea

Question 26

Collecting duct membranes are quite permeable to urea, particularly towards medullary tips (i.e. very inner medulla) so as urea approaches the medullary tips, there's an increasing tendency for urea to do what

Answer 26

to move down its conc. gradient into the medullary intersitium which retains water along with it (as water will follow urea) therefore reinforcing the medullary gradient in the region of the ascending limb

Question 27

In an anti-diuresis with high levels of ADH, urea is retained which causes what

Answer 27

uraemia (raised blood urea)

Question 28

Although high levels of ADH cause urea retention from the collecting duct back into the surrounding interstitium (--> uraemia), it is important that it does this because if it didn't then ADH wouldn't be able to do its job efficiently - explain this

Answer 28

If urea wasn't retained then it would stay in the collecting duct and exert an osmotic effect which would hold water in the duct which is the opposite effect of what ADH intends to do so the conservation of water is more important than the associated retention of urea

Question 29

How does ECF volume affect ADH secretion

Answer 29

Increased ECF volume --> decreased ADH secretion Decreased ECF volume --> increased ADH secretion

Question 30

Where are low pressure baroreceptors (receptors in blood vessels that sense changes in BP) located (2)

Answer 30

Atrial walls | Large systemic veins

Question 31

Where are high pressure baroreceptors located (2)

Answer 31

``` Carotid sinus (where carotids bifurcate) Aortic arch ```

Question 32

How do the low pressure baroreceptors (stretch receptors) in the atrium react to a MODERATE decrease in ECF volume (not enough to affect MBP)

Answer 32

Usually the low pressure receptors in the atrium exert a constant inhibitory discharge of ADH secreting neurons but when they sense a decrease in blood, their inhibitory discharge on ADH secreting neurons is decreased so causing increased ADH release --> sodium and water retention

Question 33

If MAP drops significantly, what receptors will respond to this and try to correct it

Answer 33

Aortic arch and carotid sinus baroreceptors

Question 34

Change in plasma osmolarity (primary controlling factor) and change in ECF volume are the more important stimuli affecting ADH release What are some minor factors that increase ADH release

Answer 34

Pain, stress, exercise

Question 35

Change in plasma osmolarity (primary controlling factor) and change in ECF volume are the more important stimuli affecting increase/decrease of ADH release What is a minor factor that decreases ADH release

Answer 35

Alcohol

Question 36

Triggers of ADH release (3)

Answer 36

Increased plasma osmolarity >280mOsm Decreased atrial stretch due to low blood volume (decreased ECF) Decreased MAP

Question 37

Diabetes insipidus is when you produce large amounts of dilute urine due to deficiency in

Answer 37

ADH

Question 38

Central diabetes insipidus v peripheral diabetes insipidus

Answer 38

Central - damage to the ADH synthesising neurons in the hypothalamus Peripheral - collecting duct insensitive to ADH produced