mcat and the kidney

[meanderson]

I can't understand what's going on with one part of the kidney for the purpose of the mcat:

When it talks about filtrate osmoregularity, this just refers to the concentration inside the tubule right? high filtrate osmoregularity = high concentration?

What is the point of a lot of water leaving the filtrate in the descending loop of henle(increasing filtrate conc) only to have sodium leave in the ascending loop(decreasing filtrate conc). What is the general point of that?

When looking at all the diagrams, I don't see how the filtrate becomes so concentrated with urea? When does all this concentration occur and how?

Just for some reason the kidney is driving me crazy....and I don't know why. Every other mcat topic with the exception of circuits and some stereochemistry I've nailed cold.

Thanks for any help!

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[osar92]

im not sure if this answers your question or will help you out...
but we learned in physiology that the point of sodium leaving the ascending limb is to increase the sodium concentration in the interstitial fluid that is between the ascending limb and the collecting duct. This is good because this allows water to be reabsorbed from the collecting duct because it wants to balance out the high Na conc. in the interstitial fluid, and prevents us from excreting lots of water.

i dont know if it makes sense, but hope it helps a little

 

[Chronotropic]

Your about to find out why I would never go into teaching

When it talks about filtrate osmoregularity, this just refers to the concentration inside the tubule right? high filtrate osmoregularity = high concentration?

For the MCAT you can think of it that way.

What is the point of a lot of water leaving the filtrate in the descending loop of henle(increasing filtrate conc) only to have sodium leave in the ascending loop(decreasing filtrate conc). What is the general point of that?

The loop of henle is in the medulla and in order to concentrate urine the medulla must have high osmolarity. Water exits the decending limb in order to concentrate the FILTRATE. This creates an osmolarity difference between the filtrate and medulla. So solutes are able to difuse out of the the ascending loop of henle into the medulla. When the filtrate flows through the collecting duct water will flow into the medulla thereby concentrating the uring (if ADH is present).

When looking at all the diagrams, I don't see how the filtrate becomes so concentrated with urea? When does all this concentration occur and how?

Amonia is a byproduct of gluconeogenesis from proteins. This amonia is then converted to urea in the liver.

Hope that helps.