Expert: John Locke - 5/2/2007

Question
Hello,

I am currently learning about the excretory system in my high school biology 20 course and am stuck on some homework. The question is as follows:

Marine fish, such as herring and cod, live in a hypertonic environment. These fish lose water through their gills by osmosis. To replace the water, the fish drink seawater. Because these fish excrete salt through their gills, kidney function is affected. Explain the effect on the volume of urine excreted and the concentration of solutes in the urine.

I know that having high salt content can increase blood pressure and therefore have increased blood flow to the kidneys, which would result in high levels of urine. As well, I believe that negatively charged molecules would follow the salt out of the nephron and back into the blood. If you could help me in any way, that would be wonderful!!

Thank you!!
Stephanie

Answer
Stephanie, thank you for using AllExperts and I'd be happy to help you answer your question.

To answer this, you must consider a few things: the direction of salt flow both in the kidneys and the fish overall, the direction of water flow in the kidneys and fish overall, and the direction of salt flow through the gills.

Let me give you some questions that, as you answer them, will help make the situation clear. If you are still unclear about what is going on, let me know, but only after you have made an honest attempt at these questions. The solution is really quite amazing.

In which direction is salt flowing with respect to the fish, and what is responsible for moving the majority of this salt?

In which direction is water flowing with respect to the fish? What organ(s) are responsible for moving the majority of this water?

"Because these fish excrete salt through their gills, kidney function is affected"; in light of your answers to the first two questions, how is kidney function affected--would it be increased or decreased? Remember that the gills and kidneys are distinct organs in fish.

In light of the answer to your last question, would the fish produce large or small amounts of urine? Given which organs are responsible for removing most of the water and salt, is it necessary for the kidneys to produce highly concentrated urine?

If you have further questions, let me know what you have come up with. Good luck.