Hypertonic Urine?

[betterfuture]

The primary function of the nephron is to produce urine which is hypertonic to the blood. This is to conserve water in the body.

My question is, why is it conserving water? Wouldn't the nephron only concentrate urine if say, ADH or aldosterone was secreted, that way the body would increase reabsorption of water if one was say dehydrated or such. Why would it function to conserve water under normal circumstances?

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[Astra]

If you did not conserve water under normal circumstances, then you would have to drink much more water than normal. Evolutionary, this would make us weaker as a species.

 

[betterfuture]

So what are they assuming under normal conditions? Does normal conditions mean body is dehydrated?

Because it would make more sense that there are special conditions that would cause urine to be concentrated or dilute.

For example, if one is hydrated (too much water), then urine will be hypotonic to blood because there is more water than solutes in the urine. This is because body wants to get rid of water so it is excreted out of the body. If one is dehydrated, the hypothalamus will detect this and try to conserve water in the blood by signaling for release of hormones that will increase water level in the blood, thus more reabsorption of water occurs. Here, urine will be hypertonic to the blood, because there is less water than solutes aka concentrated urine. Right?

 

[yeasureyoubetcha]

So what are they assuming under normal conditions? Does normal conditions mean body is dehydrated?

I would think there would always be some basal level of Aldosterone and ADH in the system to conserve a "normal" amount of water and maintain homeostasis. Normal conditions would be neither deydrated or overhydrated

 

[betterfuture]

So why is urine hypertonic to blood if normal implies neither dehydrated or overhydrated.

 

[yeasureyoubetcha]

So why is urine hypertonic to blood if normal implies neither dehydrated or overhydrated.

If there's a basal level of aldosterone and ADH present, there will always be some water retained which would make urine hypertonic to blood. I feel like when we say dehydrated or overhydrated we're talking about the entire body. When we look at the blood vs. nephron we care about the contents of the circulatory system vs. the collecting duct and DCT

 

[betterfuture]

Oh, I missed that part. Just reread it.

So basically hormones are at play here.

 

[yeasureyoubetcha]

Oh, I missed that part. Just reread it.

So basically hormones are at play here.

Indeed

 

[betterfuture]

Alright, thanks!

 

[EverStriving]

Think of it like a tank with a pipe going in, and three drains at the bottom. The water you drink during the day is the pipe going in. The three drains at the bottom are your urine, the water that evaporates from your skin, and the water you breathe out. The water level in the tank is your hydration level. If your urine drain is too big, then you could become dehydrated no matter how much you drank. At baseline functioning, it is better to keep the drains small, to keep the water level high while minimizing the required flow from the inlet. So it is a good thing that your kidneys conserve water at baseline, because that minimizes the time and effort needed to find water to maintain the required hydration level, not to mention the risks of running into predators at sources of water.

 

[betterfuture]

Think of it like a tank with a pipe going in, and three drains at the bottom. The water you drink during the day is the pipe going in. The three drains at the bottom are your urine, the water that evaporates from your skin, and the water you breathe out. The water level in the tank is your hydration level. If your urine drain is too big, then you could become dehydrated no matter how much you drank. At baseline functioning, it is better to keep the drains small, to keep the water level high while minimizing the required flow from the inlet. So it is a good thing that your kidneys conserve water at baseline, because that minimizes the time and effort needed to find water to maintain the required hydration level, not to mention the risks of running into predators at sources of water.

Very wonderfully explained!