RENAL PHYSIOLOGY: PRACTICE QUESTIONS
Renal Mechanisms -- Answers

ANSWER KEY TO RENAL MECHANISMS PRACTICE QUESTIONS

1. C. Autoregulation refers to the relatively constant rate of both RBF and GFR over a wide range of systemic arterial pressures (but it does not imply that RBF and GFR are always constant, since they can be influenced by sympathetic stimulation, hormones, etc.).

2. E. Although the sympathetic nerves that innervate the kidney influence several aspects of kidney physiology, they are not essential for normal renal function.

3. E. Both dilation of the afferent arteriole and constriction of the efferent arteriole would increase glomerular capillary hydrostatic pressure and thus increase GFR. Note that inulin (or creatinine) clearance is a measure of GFR.

4. A. Loss of the glomerular filtration barrier negative charge, characteristic of nephritis, leads to increased filtration of negatively-charged plasma proteins. If the protein filtered load exceeds the limited capacity of the renal tubules to reabsorb protein (Tm-Protein), the urine will contain plasma proteins.

5. B. Hydrostatic pressure decreases because of capillary resistance. Osmotic pressure increases because plasma protein concentration increases as fluid is lost by filtration. So net filtration pressure decreases as blood passes along the glomerular capillary, and may even reach zero (no net filtration force) by the end of the capillary.

6. D. Sodium is reabsorbed by active transport in all of the segments named, although it is not actively transported in either the descending or ascending thin limbs of the loop of Henle.

7. A. The proximal tubule is very permeable to water, so water reabsorption follows solute (dissolved particle) reabsorption.

8. C.

9. A.

10. D. Remember that creatinine is filtered but neither reabsorbed nor secreted (approximately), so creatinine concentration depends on the amount of water remaining. The volume of urine is less that the volume of fluid at any of the other sites listed.

11. C. Less urea would be cleared because less urea is be filtered. The reduced filtration would cause blood urea concentration to rise until the amount of urea excreted was equal to the amount of urea produced by protein metabolism even with the reduced plasma clearance.

12. C. Remembering that CK = UK x V / PK, so CK = 50 x 1.5 / 5 = 15 ml/min.

13. E. Since CK is less than GFR (80 ml/min, as measured by creatinine clearance in question 12), potassium must be filtered and reabsorbed.