Where is ADH secreted from ??
the posterior pituitary in the brain
What is the volume of ECF determined by and what is it regulated by??
Since the osmolarity (i.e. concentration) of ECF is tightly controlled,
the volume of the ECF is determined by the total quantity of solute (mainly NaCl),
so regulation of ECF volume is all about Sodium Balance
Body directly controls osmolarity and volume of ECF in vascular system.
This affects the osmolarity and volume of the other compartments.
Normally osmolarity is maintained at expense of volume.
Osmoreceptors control H2O excretion and intake, how?
i) renal excretion by altering ADH release.
ii) intake by altering thirst.
Na+ content is largely regulated by kidney by controlling ??
ii) Sodium Reabsorption
Little evidence for regulatory “Na+ appetite” in humans.
Multiple regulatory pathways.
Tubular fluid NaCl concentration receptors within macula densa.
Pressure receptors in central arterial tree.
Pressure receptors in renal afferent arterioles (intrarenal baroreceptors).
Volume receptors in cardiac atria and intrathoracic veins.
Renal sympathetic nerves (stimulate renin release)
Renin/angiotensin II/aldosterone (stimulate Na+ reabsorption)
Atrial Natriuretic Peptide (causes natriuresis, inhibits Na+ reabsorption)
Direct pressure effect on kidney
Increased NaCl to the macula densa will lead to an increased formation rate of adenosine (ADO) which through A1 receptors causes an increase in calcium that contracts the afferent arteriole and inhibits renin release.
Renin may be released by ??
drop in sodium delivery to Macula Densa
drop in wall tension in the afferent arteriole (e.g. a drop in mean or pulse pressure)
increase in sympathetic activity
(e.g. baroreceptor response to low BP)
i.e. can be caused by Low Blood Volume (Hypovolemia)
Increases Na+ reabsorption in DCT and CD
as well as from :
– sweat glands
– salivary glands
Secreted by zona glomerulosa in adrenal cortex. Due to:
(a) Angiotensin II
(b) increased plasma K+ concentration.
Aldosterone, like other steroids, binds to nuclear receptors and stimulates protein synthesis, so its actions are slow, compared with, say, ADH. Its main action is on the principal cells of the collecting duct where it stimulates Na+ reabsorption by stimulating production of Na+:K+ pumps and ENaC Na+ channels. Water follows Na+.
It also acts on the principal cells of the collecting duct to stimulate K+ secretion.
effect of a drop in plasma volume?
Activation of sympathetic nervous system. ??
Stimulates renin release.
Vasoconstricts afferent arteriole. decrease in GFR. and increase in blood pressure
Reduces surface area of filtration barrier via mesangial cells. decrease in GFR.
Another volume sensor.... The Atria.
Action of ANP ??
Atrial Natriuretic Peptide (ANP) released when cardiac atria
stretched (due to increase in blood volume)
Actions of ANP
ANP acts on collecting duct cells to inhibit Na+ entry through ENaC channels, it also increases GFR by dilating afferent arterioles, and inhibits renin release and aldosterone production.