Aldosterone

[Medgen]

I am a little confused about the purpose of aldosterone. From EK, I learned that it results in the re-absorption of Na, Cl, and the secretion of K, H+, and HCO3-. The net result is that it lowers the osmolarity of the filtrate. Then, from Kaplan, I learned that it has a role in regulating the blood pressure, as water re-absorption is a secondary effect. But does this water re-absorption happen in the kidneys? Moreover, since aldosterone has a role in the mechanism of the juxtaglomerular apparatus, which monitors the filtrate pressure, what exactly is the purpose of aldosterone? Is it supposed to regulate blood pressure or filtrate pressure? And when is it released? Low filtrate pressure (high osmolarity)? Thanks!

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[PiBond]

I am a little confused about the purpose of aldosterone. From EK, I learned that it results in the re-absorption of Na, Cl, and the secretion of K, H+, and HCO3-. The net result is that it lowers the osmolarity of the filtrate. Then, from Kaplan, I learned that it has a role in regulating the blood pressure, as water re-absorption is a secondary effect. But does this water re-absorption happen in the kidneys? Moreover, since aldosterone has a role in the mechanism of the juxtaglomerular apparatus, which monitors the filtrate pressure, what exactly is the purpose of aldosterone? Is it supposed to regulate blood pressure or filtrate pressure? And when is it released? Low filtrate pressure (high osmolarity)? Thanks!

Aldosterone is released in response to low BP (which triggers the renin-angiotensin II system). It causes the reabsorption of Na+ by the kidney and the secretion of K+ by the kidney. By reabsorbing Na+ the blood osmolarity increases and results in the secretion of ADH by the posterior pituitary. ADH inserts aquaporins into the collecting duct and allows water to flow out of the filtrate and back into the blood. The increased blood volume results in increased BP.

Does that help?

 

[Medgen]

Yep! But I just want to make sure that aldosterone is not released in response to a filtrate with a high osmolarity? Because EK says that the job of the juxtaglomerular apparatus is to regulate the filtrate pressure... but if that is the case, then it wouldn't really regulate the blood pressure would it? Or should I view the increase in blood pressure (increase in blood volume) as a secondary effect of aldosterone's primary role as a hormone involved in the regulation of filtrate pressure?

Thanks!!!

 

[muhali3]

EK might mean changes in hydrostatic pressure, not osmotic pressure...not 100% sure though.

 

[Chauvnet]

Yep! But I just want to make sure that aldosterone is not released in response to a filtrate with a high osmolarity? Because EK says that the job of the juxtaglomerular apparatus is to regulate the filtrate pressure... but if that is the case, then it wouldn't really regulate the blood pressure would it? Or should I view the increase in blood pressure (increase in blood volume) as a secondary effect of aldosterone's primary role as a hormone involved in the regulation of filtrate pressure?

Thanks!!!

Two ways aldoterone can be stimulated: decreased pressure (increases renin release and leads to increased aldosterone) and increased K concentration (direct effect). On the other hand, ADH responds to osmolarity (osmoreceptor in the hypothalamus) as well as pressure. Either ways, water is retained and pressure increases with increased volume. Overall, decreased BP=decreased filtrate pressure ---> aldosterone ----> increased volume ---> increased BP/filtrate pressure.

 

[Medgen]

Two ways aldoterone can be stimulated: decreased pressure (increases renin release and leads to increased aldosterone) and increased K concentration (direct effect). On the other hand, ADH responds to osmolarity (osmoreceptor in the hypothalamus) as well as pressure. Either ways, water is retained and pressure increases with increased volume. Overall, decreased BP=decreased filtrate pressure ---> aldosterone ----> increased volume ---> increased BP/filtrate pressure.

But how would you increase BP and filtrate pressure at the same time? You increase blood pressure by absorbing fluid from the filtrate and therefore the filtrate pressure should go down? Also, aldosterone acts to increase absorption of Na+ and Cl- and increases secretion of K+ and H+. As a result, it decreases the filtrate pressure by making it a hypotonic solution which leads to re-absorption of water. Am I missing something? Because I don't see how that would increase filtrate pressure. It seems as though the juxtaglomerular apparatus reacts to increased filtrate pressure and serves to decrease the filtrate pressure while at the same time increasing blood pressure... I guess that the hypothalamus can also increase aldosterone secretion and in this way increase the blood pressure even when the filtrate pressure is not particularly high in a mechanism that is independent of the juxtaglomerular apparatus.