Regulation of Osmolality Flashcards
water retention is contorlled by what? where is it made? and what sits half-life?
Water Regulation controlled by ADH (Vasopressin) = arginine vasopressin (AVP)
Polypeptide (9aas), synthesized in the supraoptic (SO) and paraventricular (PVN) nuclei of the hypothalamus in the brain
Posterior pituitary hormone
Half-life around 10 minutes, so can rapidly be adjusted depending on the body’s needs for H2O conservation
what is the primary control of ADH secretion?
plasma osmolarity
When the effective OP of the plasma increases, what happens to the rate of discharge of ADH?
the rate of discharge of ADH-secreting neurones in the SO and PVN is increases = increased release of ADH from the posterior pituitary.
Changes in neuronal discharge of ADH are mediated by what?
Changes in neuronal discharge are mediated by osmoreceptors in the anterior hypothalamus, close to the SO and PVN
Other receptors in the lateral hypothalamus mediate thirst
if Osmolarity ↑, what happens to osmoreceptors?
if Osmolarity ↓, what happens to osmoreceptors?
Water flows inside as it follows its gradient and the cells swell
Changes in the volume of the osmoreceptors = changes in osmoreceptor _________ (Stretch-sensitive ion channels)
discharge
Normal plasma osmolality is 280-290mOsm/kg H2O
how do changes in osmolarity affect ADH levels?
It is regulated very precisely
Small changes in either direction results in rapid changes in ADH. System has a very high “gain” a 2.5% increase in osmolality can produce a 10x increase in ADH
10 times increase in [ADH] for a 2.5% increase in osmolality
Means it’s a very high gain system and very sensitive
why effective osmolarity pressure?
An increase in osmolarity that does not cause an increase in tonicity is ineffective in causing an increase in [ADH]
Remember the difference between osmolarity and tonicity!
Solutes that can penetrate membranes move together with water and don’t produce any “osmotic drag” or tonicity
Urea is freely permeable on these osmoreceptors so it doesn’t matter and it moves freely and doesn’t create any osmotic drag
what does the amount of urine produce depend on?
The concentrating ability of the human kidney is relatively limited and the amount of urine produced depends not only on the [ADH] but also on the amount of solute to be excreted
If the amount were 2400mOsm, then even with maximally concentrated urine (1200-1400mOsm/l), this would mean excretion of 2 l of urine
what does the ingestion of hypertonic solutions cause?
Ingestion of hypertonic solutions, such as seawater, increase the solute load to be excreted and therefore increased urine flow = dehydration, because more H2O is required to excrete the solute load than was ingested with it
Shipwrecked sailors die if they drink seawater
what is the site of water regulation? what is responsible for it?
The site of water regulation is the Collecting duct, whose permeability is under the control of ADH = Anti-Diuretic Hormone (Vasopressin)
Whether or not the dilute urine delivered to the distal tubule is concentrated and to what extent depends on the presence or absence of the posterior pituitary hormone, ADH
How does ADH increase water permeability in order for it to be reabsorbed?
This increases the permeability of the collecting ducts to H2O, by incorporating H2O channels into the luminal membrane, (aquaporins)
Mediated by aquaporins and can be stored as vesicles
ADH = exocytosis of vesicles to membrane so increases permeability of the membrane for water and the water pass on to the interstitium
H20 channels are mainly recuted by ADH on what side of the membrane?
On luminal side there is a pretty constant amount of aquaporin and little on the basolateral side
what happens to the collecting duct if ADH is present?
If ADH is present then H2O is able to leave the collecting duct (CD). That means that the cortical CD becomes equilibrated with that of the cortical interstitium ie 300 mOsm/l. The CD then passes through the hypertonic medullary interstitial gradient, created by the countercurrent multiplier of the loop of Henle
What happens to the contents of the CD if maximum AD is present?
If Maximum ADH is present then the contents equilibrates with that of the medullary interstitium via osmotic efflux of H2O and thus becomes highly concentrated at the tip of the medulla
how does maximal [ADH] compensate for a water deficit?
With maximal [ADH] produce a small volume of highly concentrated urine, which contains relatively less of the filtered H2O than of solute, therefore compensating for water deficit
Effectively adds pure H2O to the ECF
H2O is reabsorbed by the oncotic P of vasa recta, which will be even greater then usual in the presence of the H2O deficit
Equilibrate with the maximum amount of 1200 mOsmol
Describe what happens to the collecting ducts and urine if ADH is not present?
In the absence of ADH collecting ducts are impermeable to H2O, so that the medullary interstitial gradient is ineffective in inducing H2O movements out of the CD and therefore a large volume of dilute urine is excreted, compensating for H2O excess
Since further ions are reabsorbed from the CD, urine osmolarity can fall to 30-50 mOsm/l
In an anti-diuresis with high levels of ADH, what happens to urea in the collecting duct?
In an anti-diuresis with high levels of ADH, urea is retained in order to save water and reinforce medullary gradient in region of thin ascending limb of LoH
Uraemia occurs
summary diagram showing the different release mechanisms and action of ADH
summary of volume and osmolarity of fluid
