Describe the physiological mechanisms by which the kidney is able to maximally concentrate urine.
Answering this question well required the demonstration of understanding of the concept of achieving maximum urinary concentrating ability. Answers required a description of the usual concentrating processes and the changes that would occur in circumstances where maximally concentrated urine would be made. A key concept was the creation of a medullary concentration gradient to allow water reabsorption independent of solute reabsorption. This required an explanation of the contribution of the loop of Henle, the vasa recta, and urea cycling in the creation and maintenance of this gradient, along with the impact of ADH. More detailed explanation of each contribution was required as overarching statements were not sufficient to attract all marks for each section. Answers that focused solely on the counter-current exchange and multiplier process were insufficient on their own to achieve a passing mark. The examiners commented that a significant proportion of candidates excluded the role of urea in their answers.
This requires the candidate to explain the countercurrent mechanism, the role of urea, and the effects of vasopressin, all in 10 minutes.
Schafer, James A. "Renal water reabsorption: a physiologic retrospective in a molecular era." Kidney International 66 (2004): S20-S27.
McDonald, Keith M., et al. "Hormonal control of renal water excretion." Kidney international 10.1 (1976): 38-45.
Pallone, Thomas L., et al. "Countercurrent exchange in the renal medulla." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 284.5 (2003): R1153-R1175.