hard · MCAT bio-biochem
A researcher infuses a healthy volunteer with a large volume of isotonic saline. Over the next hour, plasma renin activity falls, and urinary sodium excretion rises substantially, even though plasma osmolarity and plasma Na^+ concentration remain essentially unchanged.
Which signal most directly triggers the natriuresis observed despite unchanged plasma osmolarity?
- Atrial stretch from the expanded plasma volume boosts atrial natriuretic peptide release, promoting renal Na^+ excretion regardless of osmolarity.
- Because plasma osmolarity is unchanged, ADH secretion rises to compensate, and the resulting water retention secondarily forces more Na^+ into the urine.
- The isotonic saline directly raises plasma Na^+ concentration, which osmoreceptors detect and use to suppress renin release.
- Reduced sympathetic tone from baroreceptor unloading directly increases the glomerular filtration rate enough to excrete the extra sodium load by filtration alone.
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