Regulation of osmolality Flashcards
Collecting duct is the site of … … under the control of what hormone
water regulation
ADH
Osmolarity v osmolality
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
List 3 factors which influence release of ADH, indicating the most significant physiologically
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
When oncotic pressure of the plasma increases, does ADH release increase or decrease
Increase
Discharge of ADH secreting neurons in the supraoptic and paraventricular nuclei is mediated by what receptors
Osmoreceptors in the hypothalamus
When plasma osmolarity increases
- effect on water movement
- effect on cell size
- effect on neural discharge of ADH secreting cells
- effect on ADH secretion
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
When plasma osmolarity decreases
- effect on water movement
- effect on cell size
- effect on neural discharge of ADH secreting cells
- effect on ADH secretion
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
End goal of ADH
Reduce diuresis - loss of water in urine
Normal plasma osmolality
280-290mOsm/kg H2O
Plasma osmolality is regulated tightly so small changes in either direction results in rapid changes in secretion of
ADH
Osmolarity v tonicity
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
Tonicity refers to the total conc. of what particles/solutes
non-freely penetrating particles/solutes
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
tonicity
What kind of particles/solutes don’t produce a change in tonicity
penetrating (i.e. freely permeable) ones
What does it mean if a solute is penetrating + name some
Can freely move across the cell membrane and has no effect on ECF tonicity
e.g. urea, O2, CO2
What does it mean if the solute is non-penetrating + name some
Can’t move across the cell membrane so does have an effect of ECF tonicity
e.g. Na+, K+
Tonicity depends on the concentration of what solutes only
non-freely penetrating solutes only
Effect on increased plasma urea on cell volume
No change as urea is a penetrating solute so can equilibrate across the cell membrane and pull water along with it
Ingesting hypertonic solutions like seawater causes dehydration - why
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
How does ADH result in reduced diuresis
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
In a patient in whom sodium intake exceeds sodium output, does ICF volume increase or decrease
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.
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.
equilibrate
efflux (i.e. out)
H2O
Maximum ADH conc. at the collecting duct produces what kind of urine (conc. and volume)
small volume of highly concentrated urine because lots of water is reabsorbed
In absence of ADH, collecting ducts are not as permeable to … so what kind of urine is produced (conc. and volume)
large volume of dilute urine
In the presence of ADH, movement of water out the collecting ducts greatly concentrates what penetrating (Free permeable) solute in the collecting duct
urea
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
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
In an anti-diuresis with high levels of ADH, urea is retained which causes what
uraemia (raised blood urea)
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
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
How does ECF volume affect ADH secretion
Increased ECF volume –> decreased ADH secretion
Decreased ECF volume –> increased ADH secretion
Where are low pressure baroreceptors (receptors in blood vessels that sense changes in BP) located (2)
Atrial walls
Large systemic veins
Where are high pressure baroreceptors located (2)
Carotid sinus (where carotids bifurcate) Aortic arch
How do the low pressure baroreceptors (stretch receptors) in the atrium react to a MODERATE decrease in ECF volume (not enough to affect MBP)
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
If MAP drops significantly, what receptors will respond to this and try to correct it
Aortic arch and carotid sinus baroreceptors
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
Pain, stress, exercise
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
Alcohol
Triggers of ADH release (3)
Increased plasma osmolarity >280mOsm
Decreased atrial stretch due to low blood volume (decreased ECF)
Decreased MAP
Diabetes insipidus is when you produce large amounts of dilute urine due to deficiency in
ADH
Central diabetes insipidus v peripheral diabetes insipidus
Central - damage to the ADH synthesising neurons in the hypothalamus
Peripheral - collecting duct insensitive to ADH produced
