Effect of ADH on urine output?

[animasian]

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I have a question about ADH's effect on urine output. If there are higher-than-normal concentrations of ADH in the blood, would urine output increase or decrease?

At first glance, it seems like urine output would decrease, since ADH increases water reabsorption from the tubules. However, wouldn't this reabsorption also increase blood pressure, therefore increasing the glomerular filtration rate and ultimately increasing the urine output?

Or do both of these things (reabsorption/decrease urine output & increased GFR/increased urine output) occur simultaneously until an equilibrium is reached with a net decrease in urine output?

 

[orangetea]

That's a really good question and here's how I was thinking about it but there could be a better answer I am sure 🙂

Since ADH is released due to low blood pressure and works on the collecting duct it would do as you said, increase water re-absorption and increase blood pressure. But there are osmoreceptors in the hypothalamus(master regulator ) and so once everything is back to "normal" the aquaporins in the collecting duct start reducing in number and you have your "Regular" urine output amount back. I don't think the effect will last long enough for you to have increased urine output.

I really do have some of the same similar questions after reviewing homeostatis/endocronology because I feel like when I read content it's presented so black and white when in reality there are multiple pathways that communicate with each other and you kind of start losing grasp of the bigger picture. Anyway this was a great way for me to start looking at the bigger picture 😉

 

[Sed8&Intub8]

I have a question about ADH's effect on urine output. If there are higher-than-normal concentrations of ADH in the blood, would urine output increase or decrease?

At first glance, it seems like urine output would decrease, since ADH increases water reabsorption from the tubules. However, wouldn't this reabsorption also increase blood pressure, therefore increasing the glomerular filtration rate and ultimately increasing the urine output?

Or do both of these things (reabsorption/decrease urine output & increased GFR/increased urine output) occur simultaneously until an equilibrium is reached with a net decrease in urine output?

Be careful. You can really mess yourself up if you do what you are doing. I did this in physiology and did poorly even though I knew the material and answer.

If you have a question that is asking you "There is higher than normal ADH concentration in the blood, what will happen to urine output?" The answer is it will decrease. It is only asking you what will happen next. Urine output will decrease because of water reabsorption in the collecting duct. That's it.

Yes, that will increase BP which will then increase filtration, which will increase urine output. But it isn't asking you that, because it's a cycle. You could continue, you could then say, OK, now that urine output has increased, BP will go down, so urine output will decrease again, and so on and so forth. It's all trying to maintain homeostasis. ADH was higher because BP was low so overall some of the fluid has to go to the blood to increase the BP to bring it up. There was a net decrease in urine output. If it increased reabsorption then increased excretion by the same amount it would have accomplished nothing and the hormone was useless.

 

[sillyjoe]

That's a really good question and here's how I was thinking about it but there could be a better answer I am sure 🙂

Since ADH is released due to low blood pressure and works on the collecting duct it would do as you said, increase water re-absorption and increase blood pressure. But there are osmoreceptors in the hypothalamus(master regulator ) and so once everything is back to "normal" the aquaporins in the collecting duct start reducing in number and you have your "Regular" urine output amount back. I don't think the effect will last long enough for you to have increased urine output.

I really do have some of the same similar questions after reviewing homeostatis/endocronology because I feel like when I read content it's presented so black and white when in reality there are multiple pathways that communicate with each other and you kind of start losing grasp of the bigger picture. Anyway this was a great way for me to start looking at the bigger picture 😉

@orangetea is exactly right. Your body will only regulate current conditions. In this scenario, your hypothalamus has a "set point" for blood pressure and if the blood pressure gets below the set point, ADH will be released and vice versa if blood pressure goes up.

Think of it this way. In the winter we use our heat to heat the house. How is that regulated? By a thermostat/regulator and you create the set point at which you want the thermostat to maintain the temperature. If we set it at 72 and it goes below 72 the thermostat/regulator will trigger the heater to go on. Once the temperature reaches 72 it will shut off.

Your hypothalamus = thermostat regulator and when the blood pressure reaches the set point set by the hypothalamus it will shut off the release of ADH.

EDIT: @SwedishMD2B makes an important point. Always think of the immediate response, not the response to the response.

 

[Evisju7]

I really love your thought process (I'm NOT being sarcastic). Unhelpful on a test, but brilliant in application

 

[animasian]

Thanks everyone for your genuine replies! I think the important thing is to recognize that what happens in real life is not as "Step 1 Step 2" or "black-and-white" as presented in the textbook, but simultaneously to not overthink these multiple choice questions that are testing simple concepts!

 

[neurodoc]

ADH, as its name suggests, does decrease free water excretion by the kidneys, and for that reason an increase in ADH would result in decreased urine output (diuresis). The feedback (homeostatic) circuitry for ADH secretion involves physiologic receptors that monitor several parameters, including: blood volume, BP, and blood electrolytes (especially sodium levels).

Understanding ADH physiology can be very important in clinical practice, especially when you need to know what's going on with patients who present with "electrolyte disorders" such as hyponatremia.

Is such a patient's hyponatremia due to excessive ADH or to something else (like "salt wasting" or excessive ingestion of H2O...e.g. "polydipsia")? SIADH is a fairly common cause of hyponatremia, often due to the side-effects of medications. If you understand renal physiology, and especially how ADH works, you will be able to diagnose what is causing a patient's hyponatremia. You'll need to determine the patient's serum and urine osmolality as well as the electrolyte content of the patient's urine.

As regards the OP's question, as others have said, the best answer is that an increase in ADH would result in a decrease in urine output.