Regulation of Circulating Volume (Na+ Amount) Flashcards
How is [Na+] (CONCENTRTION) regulated?
By adding or getting rid of water
Your body easily senses changes in [Na+] (osmolality) and induces ADH or thirst to change water uptake/output
How is Na+ AMOUNT regulated?
Excretion of Na+ is regulated by volume stimuli
Baroreceptors sense change and signal to aldosterone, angiotensin II, renal sympathetic nerve firing, etc
When people are forced to eat a lot of salt, what happens to [Na+] and what happens to Na+ amount?
[Na+] is really well regulated by our bodies via thirst and ADH, so [Na+] doesn’t change if you’re allowed to drink
Na+ amount intake is increased hugely starting on day 0, and the body tries to get rid of as much as it can, but only gets rid of half the first day, then 75% the second day, and it is not until day 5 that output = input of Na+. This is because the body excretes Na+ based upon VOLUME STIMULI, and it takes a few days for volume to gradually increase enough to match that increased Na+
Why does it take so long for Na+ amount in the body to level out?
Na+ excretion is regulated by circulating volume, and it takes days for volume to increase enough to change aldosterone and other control signals enough to increase output to equal input
You begin responding right away by increasing water retention and increasing your Na+ output
However, you really need to make sure [Na+] remains constant so you don’t want to keep enough [Na+] that volume can’t keep up and you get hypernatremic
Volume sensors
Cardiopulmonary receptors: atrial stretch receptors and other sensors in pulmonary and systemic veins (feedback to kidney via sympathetic nerve)
Arterial baroreceptors (feedback to kidney via sympathetic nerve)
Afferent arteriolar stretch and macula densa cells of JGA
Liver
Brain
Aldosterone
Diffuses into nucleus of principal cells of collecting tubule and enhances transcription and synthesis of Na+ channels so allow reabsorption of Na+
Increases Na+ reabsorption of the last 1-2% of Na+ that was originally filtered
RAAS
Liver and other organs secrete angiotensinogen –> kidneys and other organs secrete renin (rate-limiting step) –> renin converts angiotensinogen to angiotensin I –> angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II –> angiotensin II causes secretion of aldosterone from adrenal cortex
What causes increased RAAS activation (and thus increased Na+ and volume reabsorption)?
1) Decreased afferent arteriole stretch
2) Decreased Na+ sensed at macula densa of JGA
3) Increased beta1 stimulation
How do thiazide diuretics decrease BP?
At first, they decrease your volume by making you excrete salt and lose water with it
After a week or so, you don’t lose any more water but you’re in a lower volume state where Na+ amount has been decreased so volume has been decreased
Mechanisms that decrease this diuresis after a few days are stimulated by depleted ECF: RAAS, angiotensin II, increased peritubular oncotic pressure (?), autonomic innervation of PCT
Things other than aldosterone that regulate Na+ balance
Angiotensin II
ANP, BNP, urodilatin
Renal nerves (sympathetic afferents)
ADH
Peritubular starling factors
Natriuretic hormone (EDLS)
How does renal nerve (sympathetic) firing affect Na+ balance?
Low frequency firing of sympathetic nerves: increases renin release from JGA via beta1
Intermediate frequency firing of sympathetic nerves: increased Na+ reabsorption from PCT via alpha1 (mediated by stimulation of Na/H exchanger and Na/K ATPase)
High frequency firing of sympathetic nerves: increased afferent arteriole (and efferent arteriole) tone to decrease RBF and GFR via alpha1
How do natriuretic peptides affect Na+ balance?
Respond to increased volume state by increasing excretion of Na+ and increasing GFR
Slow Na+ reabsorption by aldosterone in principal cells
Homeostatic role only important when you have increased volume
ANP released from atria, BNP released from cardiac ventricle, urodilatin released and acts in kidney
