Explain the counter-current mechanism in the kidney.

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College Answer

Higher scoring candidates described the counter-current multiplier mechanism, the counter-current exchanger and the contribution of urea cycling to the medullary osmotic gradient. Detailing the mechanisms as to how they may be established, maintained and or regulated. Descriptions of the multiplier (LOH) alone did not constitute a passing score. Values for osmolality at the cortex & medulla and within the different parts of the LOH was required. A description of the counter-current exchanger system where inflow runs parallel to, counter to and in close proximity to the outflow was expected. This could have been achieved by describing the anatomical layout of the loop of Henle and the vasa recta.

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