Describe the role of the kidneys in the excretion of non-volatile acid.
This is a complex but essential area of physiology for intensive care practice. It was expected
candidates would indicate that non-volatile acids are those not able to eliminated by the lungs
(lactate, sulphate, phosphate and ketone bodies). The kidney plays a central role via bicarbonate
(resorbing filtered bicarbonate and generating “new” bicarbonate = acid excretion). It was
expected candidates could detail the processes in various parts of the renal tubules and the role
or urinary buffers (dibasic phosphate and ammonia).
While a number of candidates were able to provide some of the details of ion transports in the
kidney, few showed understanding of the overarching concepts of the proximal bicarbonate
reabsorption being necessary to allow the distal acid excretion.
The college answer has a few problems. For instance, ammonia is not a buffer, as its pKa is 9.15, and at physiological pH 99% of it is present in the form of ammonium (NH4+). Also, it is unclear what was meant by "generating “new” bicarbonate = acid excretion". One becomes concerned with statements like this, because there is a real threat that the examiner marking your work may not be able to recognise a correct answer. With a pass rate this worrying (27%), it would have been interesting to see exactly what elements were considered essential in the marking rubric. Without any further complaining, here is an answer which would probably satisfy some of the examiners, even though it probably violates the maximum possible 10-minute word count of a person writing by hand:
Koeppen, Bruce M. "The kidney and acid-base regulation." Advances in physiology education (2009).
McNamara, J., and L. Worthley. "Acid-base balance: part I. Physiology." Critical Care and Resuscitation 3 (2001): 181-187.
Atherton, John C. "Role of the kidney in acid–base balance." Anaesthesia & Intensive Care Medicine 16.6 (2015): 275-277.
Weiner, I. David, Jill W. Verlander, and Charles S. Wingo. "Renal acidification mechanisms." Core Concepts in the Disorders of Fluid, Electrolytes and Acid-Base Balance. Springer, Boston, MA, 2013. 203-233.
Madias, Nicolaos E., and Horacio J. Adrogué. "Cross-talk between two organs: how the kidney responds to disruption of acid-base balance by the lung." Nephron Physiology 93.3 (2003): p61-p66.
Hamm, L. Lee, Nazih Nakhoul, and Kathleen S. Hering-Smith. "Acid-base homeostasis." Clinical Journal of the American Society of Nephrology 10.12 (2015): 2232-2242.