Describe the renal handling of bicarbonate and the changes in urine pH along the nephron. (80% of marks)
How is this affected by hypoventilation? (20% of marks)
This question sought knowledge of an important and basic area of physiology that applies to many
circumstances encountered in daily intensive care practice. For a good answer candidates were
expected to mention that bicarbonate is freely filtered, it’s fate along the nephron and that it is not
normally found in the urine. Mechanisms by how it is reabsorbed and generated along the different
segments of the nephron were expected to be described in some detail. The last part of this
question relating to hypoventilation was poorly answered. For a good answer candidates were
expected to mention that Hypoventilation results in an increase in arterial PCO2 that readily
diffuses into tubular cells resulting in increased intracellular H2CO3 and subsequently bicarbonate,
that is reabsorbed, and H+ that is secreted.
Syllabus: D1 Renal Physiology, 2e, 2k
References: Ganong, Review of Medical Physiology, Ch 39 and 40
What the examiners here say about the mechanism of renal compensation for chronic hypercapnia is interesting, because it is derived from the official college textbook, but not especially well supported in the nephrology literature. Moreover it is not clear what the college mean by "hypoventilation" - do they mean acute or chronic? Surely chronic, as acute (minute by minute) changes in respiratory CO2 handling basically go unnoticed by the kidneys. Certain changes do take place with longer exposure, and that's what is included in the answer,
Renal handling of bicarbonate
Changes in pH along the nephron
Response to hypoventilation
Barker, E. S., et al. "The renal response in man to acute experimental respiratory alkalosis and acidosis." The Journal of clinical investigation 36.4 (1957): 515-529.
Adrogué, Horacio J., and Nicolaos E. Madias. "Renal acification during chronic hypercapnia in the conscious dog." Pflügers Archiv 406.5 (1986): 520-528.
Cogan, Martin G. "Chronic hypercapnia stimulates proximal bicarbonate reabsorption in the rat." The Journal of clinical investigation 74.6 (1984): 1942-1947.
Madias, Nicolaos E., Charles J. Wolf, and Jordan J. Cohen. "Regulation of acid-base equilibrium in chronic hypercapnia." Kidney international 27.3 (1985): 538-543.
Boron, Walter F. "Acid-base transport by the renal proximal tubule." Journal of the American Society of Nephrology 17.9 (2006): 2368-2382.