Lecture 9 - Regulation of Extracellular Fluid Volume and Osmolarity Flashcards
6 factors affecting sodium reabsorption?
- GFR
- Aldosterone
- Peritubular physical forces
- Medullary blood flow
- Sympathetic nerves
- Hormones
How does GFR affect sodium reabsorption? What is this called?
As GFR increases, Na+ filtration rate and absorption rate all go up BUT = there is a mechanism to buffer changes in GFR so that Na+ reabsorption is adequate (not matched 100% though so there is a small increase in excretion rate): glomerular-tubular balance => if the GFR increases or decreases the reabsorption will as well so that the excretion rate is not affected
How does aldosterone affect sodium reabsorption? Slow or fast acting?
Acts on cells of cortical collecting duct to bind to intracellular receptors to cause changes in transcription/translation to ultimately:
PRINCIPAL CELLS:
1. Stimulate Na+ reabsorption by increasing Na+ conductance across lumenal membrane causing the Na+/K+-ATPase to pump more Na+ out on basolateral suface => further Na+ reabsorption
- Stimulate K+ secretion by direct hormone effect on conductance without passing by nucleus causing the Na+/K+-ATPase to pump more K+ in and more Na+ out on basolateral suface => further Na+ reabsorption
INTERCALATED CELLS:
3. Stimulate H+ secretion via H+ ATP-ase pump insertion on lumenal membrane causing more Na+ reabsorption on the lumenal surface => causing the Na+/K+-ATPase to pump more Na+ out on basolateral suface => further Na+ reabsorption
Slow acting (hrs)
What 4 factors regulate aldosterone secretion? Main one?
- RAA system***
- Plasma K+ (rise stimulates it)
- ACTH (rise enhances the effects of 1 and 2)
- ANP (rise inhibits it)
What experiment was done to figure out what factors had an effect on sodium reabsorption in the kidney?
At the time, we thought only aldosterone and GFR regulated Na+ reabsorption, so the following experiment was done on a dog:
- Force the two factors to conserve Na+: drop perfusion pressure so that GFR is very low and then administer aldosterone so that the very little Na+ that is filtered is reabsorbed
- Infuse saline to see if the body is going to continue to accumulate Na+, which will increase the ECF and lead to pulmonary edema and death
RESULTS:
Na+ excretion actually INCREASED => proposal for natriuretic hormone that is facilitating its excretion
How do peritubular physical forces affect sodium reabsorption?
In the peritubular capillaries, net oncotic pressure is high (due to filtration of 20% of the plasma) and net hydrostatic pressure is low, which facilitates net reuptake
SO, anything that decreases oncotic pressure or increases hydrostatic pressure will decrease net reuptake in the peritubular capillaries, and vice versa AND anything that increases interstitial hydrostatic pressure will decrease net reuptake because the proximal tubule is so leaky
What can raise oncotic pressure in the peritubular capillary? Effect on reuptake?
Increase in efferent arteriole resistance => increase filtration + decrease hydrostatic pressure => increase in peritubular capillary reuptake
What can decrease oncotic pressure in the peritubular capillary? Effect on reuptake?
Decrease in efferent arteriole resistance => decrease filtration + increase hydrostatic pressure => decrease in peritubular capillary reuptake
How does medullary blood flow affect sodium reabsorption?
- Increase in medullary flow rate => more water exiting the vasa recta more solute reuptook and the interstitium would be diluted => less water reabsorbed in the descending limb of the loop of Henle => less solute reabsorbed in the thin ascending limb of the loop of Henle => increase in amount of solute excreted
- Decrease in medullary flow rate => less water exiting the vasa recta and less solute reuptook so the interstitium would be concentrated => more water reabsorbed in the descending limb of the loop of Henle => more solute reabsorbed in the thin ascending limb of the loop of Henle => decrease in amount of solute excreted
How do sympathetic nerves affect sodium reabsorption and ECF volume?
- Constriction of arterioles => decrease RPF and GFR + decrease of medullary blood flow => decrease Na+ excretion
- Stimulation of renin production => angiotensin II => decrease GFR + increase in aldosterone => decrease Na+ excretion + increase in renal vascular resistance
- Direct stimulation of Na+ reabsorption in the proximal tubule and thick ascending limb of the loop of Henle
What stimulates SNS to increase ECF volume?
Decrease in extracellular fluid => decrease in firing of low pressure baroreceptors => increase renal sympathetic nerve activity
How do hormones affect sodium reabsorption?
INTRARENAL HORMONES:
- ANP
- Angiotensin II
- Prostaglandins
- Kallikrein-kinins
+ EXTRARENAL
List the 7 actions of angiotensin II that cause Na+ retention and directly or indirectly increase BP.
- Vasoconstriction of renal vessels causing increased GFR/RPF => increased peritubular oncotic pressure => increased reuptake
- Stimulates aldosterone secretion by suprarenal glands to increase Na+ reabsorption in cortical collecting duct
- Stimulation of ADH secretion by hypo to increase water/Na+ reabsorption
- Decreases medullary blood flow, which stimulates Na+/H2O reabsorption in thin descending and ascending limbs of the loop of Henle
- DIRECTLY stimulates Na+ reabsorption in proximal tubule by directly stimulating the Na+/H+ lumenal exchanger and the Na+/K+-ATPase, in distal tubule by stimulating Na-Cl thiazide sensitive carrier, and in collecting duct by stimulating Na+ channels
- Stimulates sympathetic CNS outflow
- Stimulates general peripheral vasoconstriction
Describe the release and modes of action of atrial natriuretic peptide.
Atrial myocytes stretch and function as mechanoreceptors => release ANP (as a peptigen, which is later cleaved) in bloodstream:
- Increased kidney natriuresis (= excretion of Na+) and diuresis at the medullary collecting duct
- Vasodilation of renal arteries => increases GFR and RPF
- Inhibits the synthesis and release of aldosterone by the adrenal medulla
- Decreases renin secretion by the kidney
- Dilates peripheral veins => decrease in peripheral resistance => decrease in BP
- Opposes the effects of angiotensin on vasopressin release in the brain
What are the pressure sensors that allow the body to regulate the ECF volume?
- Low pressure baroreceptors in the atria
- Low pressure baroreceptors in the hepatic circulation
- High pressure baroreceptors in the juxtaglomerular apparatus
- High pressure baroreceptors in the aortic arch
- High pressure baroreceptors in the carotid sinus
