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namf
Forum Elite Topics: 80 Posts: 310 |
Hi all, I just listened to Goljan on BUN/creatinine ratios, and was confused by his CHF example where he says BUN will increase relative to creatinine. Why this would be so boils down to the kidney secreting both creatinine and urea, but only reabsorbing the urea (I think this is what he says). Therefore, if--in the ex of CHF--your kidneys secrete renin, GFR goes down (right?), and somehow this results in BUN increasing relative to creatinine!? Why, if your kidneys are underserved by blood (decreased GFR right?), would you be able to absorb more urea and raise your BUN/creatinine level??? Please as always, explain as to a basic beginner. Thank you very much!! 
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mdwannabe
| Forum Guru Topics: 37 Posts: 1131
OK... so if you are bleeding, you are loosing volume, you need to reatain as much fluid as possible. So you need a lot of urea to be retained in kidney medulla, so it can drag more free water in collecting duct, under the influence of ADH. However, your afferent ateriole is more dilated than efferent (R-A axis and Aldo effect, so GFR is maintained, or actually slightly increased) hence creatinine concentration stays the same but BUN goes up. CHF situation is basically the same, since the kidney percieves the decrease in flow as hemorhage situation and responds in the same way. In case of ARF or CRF the problem is with the glomerulus, so creatinine and urea are not being filtered out effeciently, so they are both going up. Thats the scoop.
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namf
| Forum Elite Topics: 80 Posts: 310
Thank you very much mdwannabe. In summary, are you saying that increased reabsorption of urea facilitates increased reabsorption of water to keep volume up (ex: bleed or CHF)? Then what is the mechanism by which this works? I'll see if I can find it in Costanzo as well. Thanks again.
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namf
| Forum Elite Topics: 80 Posts: 310
Seems I found the answer, Costanzo's paperback txtbk of physio, p. 267: Regarding urea recycling, when ADH levels are high, urea permeability is increased in the distal/cortical collecting tubules, so that urea can pass from out of the urine/filtrate into the interstitial fluid of the inner medulla. I guess that from that point on, the urea can be reabsorbed into the blood vessels more readily as well. Sound good?
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mdwannabe
| Forum Guru Topics: 37 Posts: 1131
Yep you got it right. :-) In hypovolemic stress ADH will increase the number of water transposters in DCT (aquaporins recruited to mebrane from cytosol) and increase urea transposrt in Ascending limb. Moreover Aldo will increase expression of Sodium pumps to increase sodium gradient in DCT so that ENaCs can trasport more Na and more H2O. Good job, namf!
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