Care for some nephrology?

[LapisLazuli]

---

Members do not see this ad.

Alright, I have this question here, which might be easy to you guys... but its really tripping me up. (still undergrad)

Hyponatremia (reduced plasma Na+) occurs sometimes in individuals whose effective circulating volume is decreased. Which of the following factors contributes to the development of hyponatremia in this situation?

A. Impaired ability of the kidneys to excrete solute free water
B. Elevated levels of atrial natriuretic peptide (ANP)
C. Increased excretion of Na+ by the kidneys
D. Decreased levels of ADH
E. Decreased levels of aldosterone

I would choose B. Is C wrong because Na excretion only changes ECF volume, and not plasma Na conc.?
Also, in this case, what are some common causes for the atria to be secreting too much ANP?

Help!

 

[ckent]

The correct answer is A. Usually, hypo or hypernatremia is a problem of ADH and free water excretion. When the body becomes extremely hypovolemic, the hypovolemia will overrule the body's concern of retaining a normal sodium concentration (better to keep blood flow to the brain and worry about coma later) and ADH will be secreted, impairing the kidney's ability to secrete free water like it would like to do with a low sodium concentration. ANP is secreted when the body is volume overloaded (secreted by the atria in response to stretch receptors), and helps the kidney get rid of volume. C. is wrong because you would expect aldosterone levels and sodium retention levels to be high in the setting of hypovolemia.

 

[Arginine]

Originally posted by LapisLazuli
Is C wrong because Na excretion only changes ECF volume, and not plasma Na conc.?

I totally agree with ckent about A being the correct choice. FYI, plasma and interstitial fluid are both considered part of ECF volume.

 

[LovelyRita]

Good question b/c I have a quiz on this stuff tomorrow.

To me, the question and the answer options are phrased oddly. The first sentence jumps out as corresponding with Addison's disease, which is a complete loss of aldosterone secrection leading to the inability of the kidney to reabsorb Na+, therefore dumping the Na+ out in the urine. This makes A correct.
And besides, the word "sometimes" is used, suggesting an abnormal situation, such as a disease.

E could be correct too, based on the first sentence. I think A and E are both correct unless the question is worded wrong or the answers are worded wrong.

Just curious, what is the source of the question?

M.