Question 43

[kbshah6109]

If it is a very hot day, so the body would try to retain as much water as possible, thus ADH secretion would increase and the water will be reabsorbed from the DCT and collecting duct. So, because of this, wouldn't there be a high solute concentration in the urine. Doesn't high solute concentration mean that the urine will be HYPOtonic to the blood plasma. And what distinguishes whether the amount would be small or large.

As I was writing my question, this thought came to me, which might help answer my question - the blood plasma is ALWAYS has a higher solute concentration that the urine, so this is why water flows down its gradient when ADH is released. So, on a hot day, the urine will still be hypertonic to the blood plasma, but it will be more concentrated than before. Thus, there is only a small amount of hypertonic solution.

Is that right?


Hypertonic/hypotonic solution always confuse me. Do you have an easy way to understand them or remember which way water flows. I always remember it as: hypertonic - water flows out
hypotonic - water flows in.
but it gets confusing when the wording is different - like the blood plasma is hypotonic to the urine or the urine is hypertonic. to the blood plasma.

[srraghav1542]

I think you have it switched around. Hypertonic- water flows in and hypotonic- water flows out. For example, if blood plasma is hypertonic in relation to the urine, then water flows from the urine (hypotonic) to the blood to decrease the concentration. Thus, in your example of a hot day where ADH is secreted to prevent dehydration, the urine will be hypertonic to the blood, not hypotonic. So the nephron works to absorb water into the blood, which makes the urine more concentrated. Remember, tonicity is always in relation to something else.
If, on the other hand, a person ingests too much water, ADH secretion will be halted and the water will flow from the blood to the nephron. In this case, since the blood is hypotonic to the filtrate (hypertonic), water flows into the kidney in order to prevent the blood from becoming too dilute. A secondary effect of ingesting too much water will be a rise in Blood Pressure if water does not flow into the kidney.

Okay, just wanted to offer my two cents on this to clear things up. Also, just a correction to the part about ADH. The blood plasma is not always higher solute concentration than the urine. Think about, if this were the case, then ADH would be secreted from the pituitary all the time, which we know is not true. Also, in the 2nd example of drinking too much water, if plasma was always higher solute concentration, then water will be taken up due to the presence of ADH as you suggest. This is quite dangerous to the body if this were to occur. However, increase in plasma osmolarity is a trigger for ADH release as when you ingest salt.

[kbshah6109]

The two website I've found say that in a hypertonic solution - water flows OUT. and in a hypotonic solution - water flows IN.

I'm still confused on the question from the exam. Not sure why this concept is really not clicking.
So if ADH is secreted on a hot day to prevent dehydration, the water will flow from the urine to the blood plasma, which means that now the urine will have more electrolytes and thus a higher solute concentration. Wouldn't that mean that after ADH was secreted and water flows out of the urine, that now the urine is hypotonic because it contains more solute concentrations.

The link below shows the concepts of osmosis that I have been looking over.
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Thanks!

[srraghav1542]

I think the reason why you are confused is because you need to think about what the solution is hypertonic to. I looked at the website you provided and it is consistent with what I mentioned earlier. That is, if the solution is hypertonic to the cell, then the cell itself is hypotonic. Therefore, water will flow out of the cell and into the solution to dilute the high concentration of the solution. On the other hand, if the cell is hypertonic to the solution (E.g. red blood cell in distilled water), then the solution itself is hypotonic. Thus water will flow from the solution to the cell and the cell will lyse. Everything in that you said is right, up until the part about the urine being hypotonic to the blood after water flows out into the plasma. If there is a higher solute concentration in the urine, it is hypertonic, not hypotonic. Remember, the urine and the blood will always be opposite to each other in terms of tonicity. If the urine is hypertonic, then the blood is hypotonic because water has been taken up by the plasma. And if the urine is hypotonic, then the blood is hypertonic because water has been moved from the plasma to the filtrate. Hopefully this makes sense, let me know if you have any more questions

[kbshah6109]

makes complete sense now.
thanks!!

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