Regulation of Osmolarity Flashcards
what is water regulation controlled by?
controlled by ADH (Vasopressin) = arginine vasopressin (AVP)
what is vasopressin?
Polypeptide (9aas), synthesized in the supraoptic (SO) and paraventricular (PVN) nuclei of the hypothalamus in the brain.
Posterior pituitary hormone.
what is the half life of vasopressin?
10 minutes, so can rapidly be adjusted depending on the body’s needs for H2O conservation.
what is the primary control of adh secretion?
Primary control is plasma osmolarity
how is adh secretion controlled by plasma osmolarity?
When the effective OP of the plasma ↑, the rate of discharge of ADH-secreting neurones in the SO and PVN is ↑ → ↑ release of ADH from the posterior pituitary.
what are changes in neuronal discharge mediated by?
mediated by osmoreceptors in the anterior hypothalamus, close to the SO and PVN.
what mediates thirst?
Other receptors in the lateral hypothalamus mediate thirst.
what happens to osmoreceptors when osmolarity is increased?
↑ H2O out of cell
Cell shrinks/stretch sensitive ion channel activated
↑neural discharge
↑ADH secretion
what happens to osmoreceptors when osmolarity is decreased?
H2O enters cells
Cells swell
↓neural discharge
↓ ADH secretion
what effect does changes in osmoreceptors have on osmoreceptor discharge?
Changes in the volume of the osmoreceptors → changes in osmoreceptor discharge.
what is normal plasma osmolality?
80-290mOsm/kg H2O.
It is regulated very precisely.
what do small changes in normal plasma osmolality have on adh?
Small changes in either direction results in rapid changes in ADH. System has a very high “gain” a 2.5% ↑ in osmolality can produce a 10x ↑ in ADH.
what is an ineffective osmotic pressure?
An increase in osmolarity that does not cause an increase in tonicity is ineffective in causing an ↑ in [ADH].
describe osmolality versus tonicity?
what is the concentrating ability of the human kidney limited by?
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.
what effect does drinking seawater or hypertonic solutions have on an individual?
Ingestion of hypertonic solutions, such as seawater, increase the solute load to be excreted and ∴↑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?
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.
describe the mechanism of action of ADH on a collecting duct cell?
This ↑ the permeability of the collecting ducts to H2O, by incorporating H2O channels into the luminal membrane, (aquaporins).
if ADH is present what happens to H20?
If ADH is present then H2O is able to leave the collecting duct.
when H20 leaves the CD what happens?
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 if maximum ADH is present?
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 much urine is produced with maximum ADH?
With maximal [ADH] produce a small volume of highly concentrated urine, which contains relatively less of the filtered H2O than of solute, ∴ 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.
what happens 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 ∴ a large volume of dilute urine is excreted, compensating for H2O excess.
