Quick Question about ADH

[ahmedqman]

My Kaplan book says in its homeostasis review that "A high solute concentration in the blood causes increased ADH secretion, while a low solute concentration in the blood reduces ADH secretion."

Now this doesn't make sense to me, if blood is high in solute concentration, doesn't that cause water to flow in to the vessels and create a rise in blood pressure? And if there is a rise in blood pressure, shouldn't there be a need to excrete water in the filtrate (thus lowering ADH secretion)? And if there is a low solute concentration, that means water is flowing out of the vessels, hence you want to reabsorb water in the blood (increase ADH secretion)?

What's wrong in my logic? Thanks.

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[killinsound]

ADH does not INCREASE the volume of your blood, it only stops you from losing any more water than you are already losing.

if you have a high osmolarity, you do not want to LOSE any more water from your blood

if you have high osmolarity, it is that current state that influences ADH release - that ADH release is what causes the water to be retained in the blood. the movement of water is a result of the ADH.

remember, if you are flowing water into a high osmolarity blood, the osmolarity lowers, so you dont have high osmolarity -> ADH secretion anymore.

 

[corbis11]

The key thing to remember about hormones is that they are release in response to a stimuli/changing condition.

A high solute conc. in the blood indicates that there has been water loss from blood(e.g. too much sweating) which our body responds to by secreting ADH to retain more water from the tubules so that the plasma osmolarity returns back to normal. ADH is aslo released when plasma volumes dip too low so that water is taken up and elevates plasma volumes back up to normal.

ADH isn't a big player in regulating blood pressure, that is the job of aldosterone and its other buddies renin-angiotensin

 

[Creightonite]

my kaplan book says that ADH increases water adsroption from the collecting duct of the nephron.

 

[ahmedqman]

ADH does not INCREASE the volume of your blood, it only stops you from losing any more water than you are already losing.

if you have a high osmolarity, you do not want to LOSE any more water from your blood

if you have high osmolarity, it is that current state that influences ADH release - that ADH release is what causes the water to be retained in the blood. the movement of water is a result of the ADH.

remember, if you are flowing water into a high osmolarity blood, the osmolarity lowers, so you dont have high osmolarity -> ADH secretion anymore.

Aha! That makes sense. Whatever the state of the blood is in the kidney, that's what ADH responds to. And blood osmolarity in the kidney is in its current state AFTER all other processes have occurred in tissues (blood pressure related stuff with regards to tissues), so all other factors that have occurred prior to blood entry into the kidney are irrelevant to whether ADH is secreted or not.