Explain the role of urea in the body.
This question invited candidates to describe the Urea Cycle in the liver. Urea is a
waste product derived from deamination of amino acids and the detoxification of
NH3. Many candidates did not outline the major steps in the biochemical process.
The kidneys excrete up to 60% of the filtered urea load. The counter-current
exchange mechanism in the renal medulla traps urea in the interstitium and
generates a concentration gradient, essential for reabsorption of water. The
handling of urea by the Loop of Henle and collecting duct, as well as the effect of
ADH, should have been discussed.
Many candidates did not include sufficient detail in their answers.
Syllabus: D1 2c,g. I2a.
References: Textbook of Medical Physiology, Guyton p 795, 319.
Renal urea handling:
Klein, Janet D., Mitsi A. Blount, and Jeff M. Sands. "Urea transport in the kidney." Comprehensive Physiology 1.2 (2011): 699-729
Weiner, I. David, William E. Mitch, and Jeff M. Sands. "Urea and ammonia metabolism and the control of renal nitrogen excretion." Clinical Journal of the American Society of Nephrology 10.8 (2015): 1444-1458.
Bankir, Lise, and Baoxue Yang. "New insights into urea and glucose handling by the kidney, and the urine concentrating mechanism." Kidney international 81.12 (2012): 1179-1198.
Bankir, Lise, et al. "Direct and indirect cost of urea excretion." Kidney international 49.6 (1996): 1598-1607.