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Aldosterone, Blood Osmolarity, and Urine Concentration
#1 Posted : Saturday, August 28, 2021 5:16:44 AM
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1) On page 219 of the BioBiochem 2 Classroom companion, it states that aldosterone increases blood osmolarity. Wouldn't blood osmolarity remain the same? This is what I've read from other sources. Aldosterone increases sodium reabsorption, and water follows, so the osmolarity should remain the same, right?

2) The concentration of urine due to aldosterone would then remain the same (because sodium is reabsorbed and water follows the sodium), while ADH would concentrate the urine (because of water reabsorption) - is that correct?

Many thanks again!
#2 Posted : Friday, September 03, 2021 1:54:40 AM
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Hi James!

This can be a tricky concept to understand because things in the body are constantly in flux, and different resources may be talking about something that is happening at slightly different points in time. Technically, aldosterone will cause the retention of sodium which may increase osmolality very transiently (as the water isn't transported with the sodium so it isn't an instantaneous change) but water will quickly follow in order to maintain the serum (blood) osmolality -> i.e. the blood osmolality would remain the same overall. It is worth remembering that serum sodium levels (plasma osmolality) are kept constant through water reabsorption (not sodium) and that the triggers for sodium reabsorption are blood volume/pressure, NOT serum sodium levels.

For your second point, yes, ADH does concentrate the urine. The specific mechanisms and how aldosterone affects the urine (and where it acts) is slightly above the level of the MCAT, and something you'll learn a lot more about in medical school! For your purposes, it is correct to assume that the concentration due to aldosterone would remain relatively the same!

Hopefully that helps,
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